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To BMS or not to BMS

150K views 943 replies 62 participants last post by  mons2b 
#1 ·
Aloha, I am trying to plan my Lithium purchase. Probably 50 Calb cells @200ah. But I am hearing conflicting heated, (even namecalling) arguments about weather to use a bms or not. So will the person with the most Lithium-use-hours experience stand up and give me the "skinny"? thanks
Francis
 
#2 ·
I'm not sure if hours has anything to do with it, otherwise that might well be Jack R. one of the no-bms guys. But he does have a few cars, all BMS free and what he reports as working perfect. Others use a BMS and are working perfect, I'm kind of in the middle, I would love to go BMS free as I think it is the best solution, however it might not be the right solution. I've decided to charge individual cells instead of a series string eliminating the need for a BMS during charging. (I will have monitoring during discharging).

To all others, I hope this is a nice discussion even though there probably isn't an answer to this question. It's your choice to make, and your responsibility to research on your own and make the choice that's right for you.
 
#3 · (Edited)
I guess I should first ask "What is the function of a BMS? Does it regulate down a charge for each individual cell attached to it from a series string of cells? Or does it actually boost a charge coming from a series of charging cells? In my mind individual chargers would be the go, but using one for each cell (50) would start to get $ and heavy as it needs to put out to charge a 200ah cell. Maybe get a good 12v charger and group 4 cells??

***also what is the best instrument brand name that shows the state of charge of each cell simultaneously? ie can any show 50 cells at the same time?

francis
 
#4 ·
The function and even meaning of a BMS can vary wildly. "Battery Management System" or "Battery Monitoring System" are two entirely different things. Then there is Shunting, or non-shunting, and some that can re-distribute power to reduce wasted power. Some trigger chargers to turn off with a HVC, some trigger controllers to reduce power with a LVC.

The main thing to remember is safety, anything can be dangerous. Nitrous in street racers can explode and destroy cars, garages, houses. The same goes for anything that you attach to you high voltage pack. All wiring must be safe, ideally fused and properly secured.

I've seen a car burn to the ground from an improperly installed car stereo using a single 12v battery, it can happen to anyone.
 
#5 ·
I'm going free when mine arrives. For one, it will a large chunk to your battery cost and a lot of time to install. The costs can be over $1000 even 2000.

After watching this and this you'll have a good idea of what you want to do.

My plans are to not discharge to the edge of the cliff and not charge to the hilltop. Discharge to say 90-95% and charge up to within 5-10% of max. You will need to educate yourself if you plan to DIY or you may toast $12000 worth of batteries.

That guys philosophy is that in order to know how deep to discharge you MUST have an amp-hour counter installed. So if you have a 200Ah cell you can discharge the pack down to within 10Ah or so of being dead. However to do so, you MUST bottom balance the cells in the pack so they are at the same state of charge at bottom.

Jacks testing shows that using a BMS to "Top Balance" cells to get them charged to the MAX will cause them to likely be out of balance at the bottom (discharge end) thus when you're running the pack low you risk one cell bottoming before the rest and if that happens it's toast!

Yes IF, and that's a big IF, the BMS works correctly it should prevent that from happening. But doing so in the first place is placing a needless burden on the batteries if you want them to last longer. Discharging cells very low shortens their life as does over charging them. However if you stop before getting too low and before getting too high, you'll get longer life anyway.

Here's a bit of my reasoning for staying away from a BMS. My career for about two decades was in electronics and circuit board repair. Have you ever known electronics to fail? I have seen MANY. Electronics, no matter how well designed do fail from time to time. Lightning striking a tree NEARBY gives off enough EMI to destroy electronics from a distance, even in your garage.

Here's a scenario. You're asleep and a storm comes through, strikes a tree nearby and the EMI radiated from it torches a circuit board on one of your cells. If that cell shorts across the battery who knows what it could take out, the BMS controller it's attached to, wiring between the two, could fry the harness starting a fire etc. Who knows what could happen with a failure but there have been many fires documented due to BMS board failures. I don't want anything capable of failing and shorting a battery in my car while I sleep or while I drive!
 
#6 ·
there have been many fires documented due to BMS board failures
Actually there have been ZERO fires documented due to BMS failure. Lets not propagate baseless stories here. In every reported fire there were a dozen of things that could have gone wrong not even counting the BMS.

I am biased and I am not promoting my BMS or any others, but let's stick to facts and leave the fiction where it belongs, in TV shows and books.

Yes, electronics fail, but so do all other things. Based on your logic you should live in a cave and never step outside since you can be killed by lightning. I'm sure many people who are hit by lightning every year regret leaving the house that particular day, but it doesn't turn them or most others into hermits.

My point is there are 2 sides to every story. Even if I wasn't selling MiniBMS but just made one set for my own use, I would not regret it. Even though you have a solid theory of how not to damage your pack you have not had any practice yet. I have been using mine every day for 2 years. let's compare our notes after you get 2 years from your BMSless pack. Including how much time you spend caring for your pack and how much your time is worth to you.

I'd be the first one to tell you that its certainly possible to go without BMS if several other conditions are met. However, just because all these conditions are met by one person's project, it does not give him the right to yell all over the world that everyone else SHOULD BE FINE because he has shown that HE IS FINE. Almost every EV conversion has some unique combination of parts and you can't judge one from looking at another.

I wish that every one who supports BMS'less approach had guts to come back and tell the story when something goes against their theory, few people do that. The judgement is still out which way works better.
 
#7 · (Edited)
Wait, Didn't Jack Rickard ruin an entire pack of thundersky because there was nothing to stop the charger from overcharging? Had he had a cell-level HVC installed (one form of BMS) he'd have saved the pack. Not something you hear much about on the forums.... Very interesting. Not someone I'll be taking advice from.

IMHO, you ABSOLUTELY need monitoring on a cell level. You need something to look for overvoltage while charging (not on the pack level, on the cell level). If it senses a high cell, it turns off the charger. Then you need to look for overdischarge (LVC) and throw an alarm and reduce the throttle somehow.

In regards to cell balancing, I don't care if people want to top balance, bottom balance, coulomb count, individually charge. There's no RIGHT way. They all work. My point is, and I think most agree, you should monitor the cells individually. Even if you're using a cheap Cell-Log 8 with the alarms optoisolated and linked together or have a fully fledged BMS balancing/monitoring system.


So what am I using? Elithion. Why? Because I could afford it at the time and because I worked at EVComponents providing support for that system. Now I work for Davide as a consultant and own his BMS book, which is a great read. Not only does Elithion balance, it monitors temperature, voltage and SOC and gives me a TON of useful information back from my pack, which I intend to publish reguardless of results. I want the data and thats why I bought it. It'l be logged via Canbus through bluetooth on an Android device.
 
#12 ·
Dimitri I haven't studied and read exhaustively the details of purported BMS fires but I've read others writings about them and it coincides with my experience and knowledge of electronics, enough so that I'm convinced that BMS is not needed and is IMO a liability. Unlike you, I have no dog in this fight. I'm only answering the OP's call from my perspective.

I'm glad we agree that electronics do fail. As I said, I've seen lots of failures in 20 years of electronics, some dangerous, some benign. That said my charger could fail and torch the pack left to it's own devices but that is easily rectified without a BMS. Actually BMS circuitry could be modified or configured in a circuit to kill the charger at a predetermined voltage. But again, nothing is certain but death and taxes.

My point, as is Jacks to the best of my knowledge is this. One can buy your BMS and install 50 electronic circuit boards assembled of the finest components with the greatest care, spend hours wiring 5 pounds of cabling running through the vehicle to the dash and elsewhere, confident it is the holy grail of battery protection and will prevent battery failure.

Or one can buy a meter with Ah counting capacity, set the low levels to notify the driver at 20% remaining and again when the tank is empty and I'm about to destroy my pack, even have it shut down the vehicle or take it to limp mode. I'll have the charger set to charge to a voltage less than full which will give me more life according to the OEM. All I need to do is bottom balance and keep from bottoming out a cell. Even that may be unnecessary, not real sure. I can also install redundant protection on the pack to kill the charger should it decide to fail. Simple and inexpensive.

Should I take your advice and one of your quality boards fail shorted, that cell is dead and come hell or high water you (the driver) nor your BMS is likely going to stop it. No, Lifepo4 isn't going to catch fire from what I've read but don't throw me under the bus because I haven't consulted an expert directly nor have I proven to myself that it can't. Again that's what I've read. And one or two cells is likely less than your BMS system if somehow one of them does go south.

You guys hocking your BMS wares certainly have reason to bash those of us who rationally decide NOT to send you our money and chose another route, a route that is much less potentially harmful and blatantly simple. So go ahead if it makes you feel better brother!
 
#13 ·
The only reason I responded to your post is this statement you made, which is as blatant lie as it gets, with very serious consequences to people involved in these issues.
there have been many fires documented due to BMS board failures
I know you did not make this up yourself, you just repeat blindly what you hear on EVTV.

I have full respect for your plans to go without BMS and I am not pushing anything to anyone, I just try to stop lies from propagating, which seems a futile effort since everyone likes drama more than facts. Pros and cons of BMS have been discussed to death on this forum, no reason to repeat everything again, so I stay away except when lies are posted as facts.

As for your plan, it sounds great. I only ask one thing, come back 2 years from now and honestly share how real life experience differs from your plans and how much time/money you spent avoiding to use BMS.
 
#14 ·
If 1000 cars had Lithium packs, of which 900 had BMS and 100 did not and there were 5 fires reported, all of which had BMS in them. Does that say anything about BMS causing it or simply that vast majority had it to begin with, hence larger chance of fire in a car with BMS than without one.
Sure, you can correct for unequal sample sizes. The difficulty would be in getting unbiased data. Many people don't like to admit their mistakes.

Did Jack really destroy an entire pack? I don't recall hearing about that.
 
#15 ·
Did Jack really destroy an entire pack? I don't recall hearing about that.
No, he just killed a couple cells, not a huge deal. This "killing whole pack" is another urban legend that just keeps spreading itself. I can't imagine how you can kill the whole pack at once, other than having BMS set it on fire :D

You would kill the weakest cell in each incident, replace it, then kill the next weakest cell in the next incident, etc etc, but not the whole pack at once.
 
#16 ·
Aloha, all. Best to keep an open mind on the bms as there is a lot of testimony coming out for pros and cons, so best to take it all in Objectively and decide on the merits. Jack R has a lot of good info and so do the pro guys. I think some sort of balancing will win out, (either top or probably bottom) as I see cells wandering off over time. (at least in AGM). So far I think charging to 90-95% and discharging to no less than 35-40% is in order, so I think it pays to buy bigger ah than absolutely needed and treat them with care. Using a good individual cell monitor and culling out a cell that is out of range is a good idea. For my agm setup I see the suggestion of using this homemade cheap "BMS" is a great idea. http://www.evdl.org/pages/hartregs.html (since I bought my car with AGM from a guy that only series charged and never balanced, and was only getting 5 miles on his pack. I balanced and am up to about 15 miles and hope to get to 20 miles after a little more TLC on the batteries) But lets wait and see.
Francis
 
#17 ·
Here's something I would love to see: Run a BMS on half the pack, and go BMS free on the other. May the best half of the pack win!

Also, test for yourself! You can buy Headways for ~$20 each, an RC charger for ~$75, and a load tester with Voltmeter and Ammeter for $50 at Harbor Freight. Get an IR thermometer, a voltmeter, and a timer, and you are ready to rock.

Here's my limited experience:

Lead acid: On an Optima pack, it would go very quickly out of balance with weak regens. Doing a small number of 500+ Amp regens during a drive keeps the batteries well balanced, within about +-0.02 V.

Lithium: Once bottom balanced, 4 China HiPower 10 Ahr cells stayed well balanced over about 20 severe cycles, without BMS. It appears Jack was right for this small number of cycles.

I pushed them until 2 of them vented. How's this for an idea, discharge one cell and fully charge the other. Then do gentle charge/discharge cycles, and see if they'll grow into balance like Jack claims. I think this would be a good test for if a BMS is needed -- do cells naturally balance? Anyone else out there game to test this idea and see if we get the same results?

I just got some Headways and have more China HiPowers on order, the testing continues. I'll post up details, good or bad, when I'm done.
 
#20 ·
How's this for an idea, discharge one cell and fully charge the other. Then do gentle charge/discharge cycles, and see if they'll grow into balance like Jack claims. I think this would be a good test for if a BMS is needed -- do cells naturally balance? Anyone else out there game to test this idea and see if we get the same results?

I just got some Headways and have more China HiPowers on order, the testing continues. I'll post up details, good or bad, when I'm done.
I've been thinking about a test almost like the one you propose:
Take three matched batteries. Discharge them all in parallel to 2 volts. Put 10% of the capacity into one, 20% into another and leave the third one at zero.
Connect in series and then repeatedly charge and discharge at 1C, stopping when one battery reaches 3.8 volts on charge and 2 volts on discharge. See if they drift closer to balance or not.
At the end of many cycles, remeasure the capacity to find out which is more damaging, running near the bottom or near the top.
 
#18 ·
I don't use a "BMS" in my car.
Why? I haven't found one that monitores my LiFePo-pack the way I want to a price that I could afford and with the safety issues I need.

Do I monitor the whole pack? Yes, that's neccessary I think.
Whole voltage, amphours, amps, but no tiny boards or "spaghetti"-wiring.

After ~40 - 50 charges (Zivan NG3), all cells (SkyEnergy/CALB 121AHA) are still close together in voltage.

there have been many fires documented due to BMS board failures
I have read about only one car that burned to the ground because the owner overcharged his pack dramaticly.
Every other case of fire I read (six I guess) was a vehicle with a mounted "BMS" on it.
Thats my personal statistic. Fell free to widen it :)
 
#929 ·
I don't use a "BMS" in my car.
Why? I haven't found one that monitores my LiFePo-pack the way I want to a price that I could afford and with the safety issues I need.

Do I monitor the whole pack? Yes, that's neccessary I think.
Whole voltage, amphours, amps, but no tiny boards or "spaghetti"-wiring.

After ~40 - 50 charges (Zivan NG3), all cells (SkyEnergy/CALB 121AHA) are still close together in voltage.


I have read about only one car that burned to the ground because the owner overcharged his pack dramaticly.
Every other case of fire I read (six I guess) was a vehicle with a mounted "BMS" on it.
Thats my personal statistic. Fell free to widen it :)
Hello. I have the same charger. Two groups of Chevy volt cells configured for 144v. Running in parallel. I've had a suggestion to run charger at 151 volt. Do you concur?
 
#19 · (Edited)
When you have thousands of dollars/pounds tied up in your pack, a fire isn't the only problem that can ruin your EV day.
I've run a conversion business for years, and tested a huge quantity of cells and system setups, and I'd like to say that I'd never run a Lipo or LifePo4 (especially LifePo4) pack without a BMS for more than 2 or 3 cycles. You can take that advice or leave it, I have written this only because I am so alarmed that so many would advise a BMS is not nesaccary.
It cost many hundreds worth of cells to come to this conclusion, over a large number of EV's (no fires however). You don't have to replace many battery's to cover the cost of a BMS, not to mention the inconvenience of stripping down a pack to replace cells (not all packs are as easily built as cheap thundersky's!).

Steve

PS, for Gottdi, I've lost 4 banks of cells due to BMS failures (mainly during a prototyping and testing period), this in no way compares to the hundreds of cells I have sitting around that were damaged by NOT being managed, or the several customers who have homebuilt EV's rotting in their garage because they believed they would not need a BMS and lost thousands worth of cells, 3 users did indeed lose the whole pack (or more than 70% therof), 2 by overdicharging and 1 by overcharging.
 
#21 ·
Not sure anyone said cells can't be damaged so I'm not using a BMS. I said I was planning on going a different route. Only an idiot would stick the batteries in their car and just drive with no monitoring of any type, that or a high roller! I'm neither.

In addition to my planned monitoring of the entire string, I'm going to initially monitor the entire pack manually for the first few cycles to get initial data about the cells. Once I have that I'll send the charger off for custom programming.

Fortunately I have a background in electronics and am designing something so far I've not found, a device to override the charger should it malfunction.

For you BMS proponents, don't be shy, here's your chance to make a sale! Describe what your BMS will do for this scenario and if you handle it without a cell level monitoring system. If so I'm open to it but I don't think you can do it for anywhere near what it will cost me.

Say someone is running a pack at 96V using a Zivan NG5 wired on 240V charger for quick charging like I have mine. It is programmed to stop charging at 3.6V/cell or 108V max. This charger is capable of putting out 190V, probably more.

So Joe Sixpack comes home & rolls into the garage, plugs in and goes inside for the evening, entrusting his home, vehicle and battery pack to his charger and your BMS. When the pack hits the point to go into CV mode, it malfunctions and continues in the CC mode, the voltage continues to climb, applying 200VDC at 25A to the pack. What does your BMS do? Now you're at 6.66V per cell.
 
#49 ·
For you BMS proponents, don't be shy, here's your chance to make a sale! Describe what your BMS will do for this scenario and if you handle it without a cell level monitoring system. If so I'm open to it but I don't think you can do it for anywhere near what it will cost me.

Say someone is running a pack at 96V using a Zivan NG5 wired on 240V charger for quick charging like I have mine. It is programmed to stop charging at 3.6V/cell or 108V max. This charger is capable of putting out 190V, probably more.

So Joe Sixpack comes home & rolls into the garage, plugs in and goes inside for the evening, entrusting his home, vehicle and battery pack to his charger and your BMS. When the pack hits the point to go into CV mode, it malfunctions and continues in the CC mode, the voltage continues to climb, applying 200VDC at 25A to the pack. What does your BMS do? Now you're at 6.66V per cell.
As old Sargent Carter used to tell Gomer Pyle... "I CAN'T HEAR YOUUUUUU!" So what's the deal? Can you guys handle this or not? Well if you can't do it you should certainly find a way to make it happen. I think if you don't offer it soon you're certainly hosing your customers who've spent thousands on your product to protect their pack only to have the tops blowing off like popcorn.
 
#22 ·
The bare minimum a BMS needs to do is monitor for HVC and LVC as well as temperature and be able to independantly cut off charge and discharge.
Better still, it could be able to balance and limit charge current to balance current.

The non BMS folk keep pointing out that all you need to do is count AH into and out of the pack, just how do you manage that (without ever losing count, or forgetting to reset it etc)? This is exactly what happened to one of my customers who now has £8k of dead LifeBatt cells. He used the vehicle daily for 6 months or so with no problems, then one day the Cycle Analyst farted (or he forgot to reset it) and thought he had 20 AH more than he actually had. He found out that when the pack is low, cells drop below voltage and go into reversal within seconds. The real irony here was, that he had an LVC circuit installed, but didn't bother hooking it up to the throttle! (like this one, that costs around £3 per series cell, http://endless-sphere.com/forums/viewtopic.php?f=31&t=21695&start=0)

Another customer assumed charging to an average of 3.55v per cell on a pack would never overcharge. One day whilst using my charging facilities in the yard on his way past, a passerby came in to tell us that the car outside was "popping". 1 bank of cells had reached full charge and the voltage had skyrocketed to 4.4v or so, they then vented and went 0v, causing even more voltage to be available to overcharge the next cell.

Running without a BMS is an experiment in luck and mechanical sympathy.

I would want to see a LOT of such experiments end well before publicly telling new users to try without a BMS!

I believe Kendrick (see the link above) is planning to produce a version with HVC also, this would offer the basic protection required for Lipo and LifePo4 at absolutely minimul cost and complexity. No excuse not to run with at least these installed!

Steve
 
#23 ·
He found out that when the pack is low, cells drop below voltage and go into reversal within seconds.
Can you explain how this can happen? I asume they were top balanced, and prety much out of range regarding capacity?

1 bank of cells had reached full charge and the voltage had skyrocketed to 4.4v or so, they then vented and went 0v, causing even more voltage to be available to overcharge the next cell.
And can you explain this situation too? How was it possible a whole bank went to 4.4 and the charger kept measuring an average below 3.55?
 
#25 ·
After 6 months without a balance ANY pack will be out of balance..

As for overcharging, 144v pack of 48 LifePo4 cells. He charged to 170v (3.55 per cell). If 47 of those cells have not peaked yet, they will still be at 3.4v or so, 47 x 3.4v = 159.8. So his charger had a good 10v in hand to give the 48th cell that had peaked before the rest (and this would have happened sooner of course if the cells had been bottom balanced).
So, this overcharged cell peaks out, goes POP and reads suddenly 0v. When the next cell peaks there is an even higher potential to damage further cells.

A good point of note here, is that in many many charge cycles with a BMS, I don't think I've ever seen all the cells reach the "knee" simultaneously (talking LifePo4 here, LiPo are actually a lot easier to work with in this respect, since they dont run away at the end of charge).

I've not got all day to argue this, it's up to the vehicle operator to decide if he is happy risking his pack (or perhaps his vehicle or even his home and family), I just wanted to make sure that anyone reading this thread has complete information in order to make his/her decision.

Since I already pointed out in the previous thread that minimum protection to make your pack live a long happy life need cost only 1-3% of the price of even a small pack, whats the point in arguing about it?
It's a bit like deciding not to fit a contactor to your EV to save pennies...

Steve
 
#27 ·
By bank, I meant cells in parallel not series.

Other assumptions? you mean your comment that the cells were poorly balanced or top balanced?
As I said above, ANY pack that hasn't been balanced for 6 months should be considered unbalanced.
In this case, before construction all the cells in the pack had been connected to a 3.65v powersupply and trickled (many in parallel at a time) until less than 100ma was required for them to maintain voltage.

One point for those who ARE cycling within boundries of 20%-80%, is that it is not possible to verify the SOC of a LifePo4 cell by any other means than charging or discharging it until the voltage knee is reached. You could be just one more charge away from over/undervoltang the weakest cell and not even have an idea. Just how many cycles has anyone managed using this approach without needing to rebalance or service in any way?
The customers mentioned above all had less than 25 cycles on their packs before learning thier lesson, though I can think of one person I know who ran a pack for many years with no BMS. That person however, is one of the best known motor invertors of this generation, and perhaps the most mechanically sympathetic guy I know..

Steve
 
#28 ·
By bank, I meant cells in parallel not series.
Ah, ok. A whole parallel bank was at 4.4 volts, while all others where not near their knee.

Other assumptions? you mean your comment that the cells were poorly balanced or top balanced?
My other assumption was, that the pack was top balanced and the cells where very different in capacity. Because within seconds the weak cells went in reversal. That's fast.

As I said above, ANY pack that hasn't been balanced for 6 months should be considered unbalanced.
That's the question. There's not much data to be sure.
 
#29 ·
This is not directed towards anyone in particular,

In my work with Lifepo4 at 2 different companies, I've heard of more issues of people without BMS than with them (actually have only heard about problems with wiring, or dead boards). I've heard of no fires or issues with the two different that I've sold (Elithion and Manzanita Micro). I've helped well over 50 customers, none of which have had any issue. I have heard of some of the cheap chinese bike BMS having issues, but one was definately user error, the other was undetermined and the customer never contacted us further after digging more. There were no fires, just sparks and magic smoke. I would not qualify those for use in a vehicle larger than a bike, so I can safely say, I've not heard of any fires due to a failed BMS in a car or motorcycle. That is my direct experience, and I've also built over 10 battery packs of varying capacity.

If you guys want to do things your way, fine, no one is stopping you. But since you're entitled to your opinions, so are we. We're sharing our extensive knowledge about batteries and BMS and the issues that we've seen over the years that we've been involved. I started off as an enthusiast (like jozzer) and then worked as a consultant. I've learned a lot through trial and error, and so have many customers.

I've come to the following conclusion:
You absolutely need a cell-level monitoring system.
(notice I don't mention balancing. Because I think top balancing, bottom balancing, individual charging and self-balancing are all valid ways to keep a pack balanced. I'm not here to argue which is better. I have a couple preferences actually)

With regards to HVC:
Jozzer has pointed out what happens when you don't have HVC, and so have I. Without cell level monitoring, the charger only sees pack voltage. It doesn't know if a cell with different IR went high before the others (which it absolutely will, I've NEVER seen a pack simultaniously reach their knee at the same time, Especially if it was bottom balanced before assembling the pack). So if you're at 170V on the charger, all cells except one are normal, but that one is overvoltaged, it shorts out and fails. Now your voltage drops by 4V or whatever. Well, now your charger sees 166V and maybe goes back into constant current mode, with a higher current. So the next battery that was high gets higher, higher and then overcharged, shorts and the pack now sees 4V less. Rinse and repeat. This can happen very quickly, but without monitoring, you'd never know until its too late, and there's nothing to shut off the charger.

With respect to cell reversal, read this:
http://en.wikipedia.org/wiki/Rechargeable_battery#Reverse_charging

I've seen it happen several times and it completely kills the battery. It's not only for lead acid. It can happen with any type of battery put in a series string.



Now a personal request, since we're all adults.....please don't discount people like myself, Jozzer and Dimitri with loads of experience in this realm. Many people talking about BMS have never put together a pack, although some have put together small packs. We have built many more than that and are sharing our experience.

And as far as Jack Rickard's pack, He may have only ruined a couple cells beyond use, but that is not what I heard from more than one trusted source. If its untrue, appologies. My point was, an HVC and LVC circuit would have completely protected this from happening.
 
#30 ·
just a comment, since I don't know the specifics of all the BMS systems out there, but everyone assumes they are going to fail, yet everyone assumes that their AH meter or "Alternate" protection to a BMS will never fail. Odd?

It's also assumed (or atleast mentioned frequently) that a BMS will fail short circuit and drain a cell, I find that hard to believe since anything close to short circuit on the terminals of a battery will vaporize pretty quick (IE blow like a fuse, not start a fire). However if I was a BMS designer I would do everything I could to have it fail "open" and at the same time trigger some sort of fault HVC/LVC/anything you just need to know it's broken, be blunt ask the manufacturer how/why it can fail, ask if they've tested it, ask what it does when it fails and if they considered that in the design, if they haven't move on.

I think "spagetti" wiring is the worst excuse not to use a BMS that anyone has ever come up with. Use some common sense, wire it properly and safely, secure and protect the wiring. If you aren't capable of that you shouldn't be building an EV in the first place, and that is no fault of a BMS.

On the other hand I hope my system will allow me to go BMS free, or atleast BMS disabled unless I want to bring it online once a month to balance the cells.

Don't tell others what to do, tell others what you did and let them make their own choice. BMS makers seem pushy at making you think you need a BMS but those against BMS are way more forceful with their "opinion" that a BMS is going to burn your car to the ground. Tell the facts, share your experience and let the user decide, it's their car not yours, it's their choice not yours.
 
#31 ·
Not even gonna touch the BMS-discussion with a ten feet pole, that topic is way too infected for me to even bother with and you seem to be quite capable of pissing each other off without my help. :D

However:

I think "spagetti" wiring is the worst excuse not to use a BMS that anyone has ever come up with. Use some common sense, wire it properly and safely, secure and protect the wiring. If you aren't capable of that you shouldn't be building an EV in the first place, and that is no fault of a BMS.
I once worked for the Swedish Telephone company. All of the old electro-mechanical telephone stations had racks of hundreds or thousands of relays, all the wires were painstakingly bundled together with some kind of waxed string in perfect, pedantic order! Every single rack had hundreds of single wires running straight as arrows, cut just long enough to reach their terminal and between the racks ran thousands and thousands of wires, all color coded, bundled together and extremely easy to trace despite the sheer number of them.

Spaghetti wiring is nothing but sloppy craftsmanship. Period.
 
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#32 ·
Like Frodus said, this is not focused on any one person.

Now you see, this is the issue with this ongoing debate. All the professionals from what I have been reading ONLY state this is what I have heard from.... Holy crap guys. I hear that crap all the time. My brothers cousins sisters brothers, friend said...........WOW, it must be true.
I've heard of more issues DIRECTLY from the customers. Not from this person who told me that, who heard it from someone. I've actually helped these particular people deal with more issues without a BMS than not.
 
#33 ·
I'm going to run on a bit so be prepared.:D

I'll throw this out again on the pro / con BMS debate and let you all decide if it is a BMS and if it's worth looking at during planning your system

Right now using off the shelf stuff we (Myself, RWaudio and a few others) are actually building this.

We are going at it with a top balancing, limited voltage charge protocal and LVC or LVW.

What we plan is using, one, reletivly inexpensive, isolated DC to DC converter attached to each battery (large format prismatics) or battery group (small formats other then prismatics) to charge it.

The DC to DC is set to be able to only charge to a specific set voltage. The DC to DC then CAN NOT/WILL NOT over charge the cell. Since many of the DC to DC units can have a trim pot each unit can be adjusted to bring your cells in to perfect top balance.

So for top end charging we set the voltage of the DC to DC to a safe 90% for the cells. Now the DC to DC units I am using (thanks RWaudio) can pass 25 amps at 3.65 volts and will accept any voltage between 35 and 76 volts. They can be turned on and off individually and have over and undervolt and over temp protections and a lot of others I don't understand.

Chargeing power can be handled by any of a number of inexpensive units with whatever voltages within the range of the DC to DC units because of the DC to DC units built in protection protection.

Then for LVC or LVM we use a simple voltage device like the Cell Log8M to monitor each cell (large format prismatic) or battery group (small formats other then prismatics). The Cell Logs have a warning system that can be set up to monitor each cell and trigger an alarm that could be used as an audible warning or to trigger an event such as power cut back, main contactor disconnect or whatever.

So for bottom end we set the Cell Logs for some safe limit voltage say 80-90% DOD. What happens here depends on your set up It could shut you down until you fix/charge or with the proper controller back off power each time a cell log hits low voltage.

This should keep you in the fat/flat area of the charge/discharge curve and do wonders for your pack life.

As long as you plan your pack size for the middle 80-90% of your pack this shold work pretty well.

Let's face it, when you consider the above it's not unlike filling a gas tank. You should never fill a tank all the way (100%), If you fill the expansion space you're eventually going to get a boom when things heat up. Most people never use their reserve fuel, if the do they'er in trouble eventually.

Remember this is all off the shelf stuff so it's going to be a bit crude. and have some holes in it that require personal monitoring. I'll depend on you smart guys to find a way to put this all on a board, with programming to do everything including washing the dishes.

When you get down to it, I'll have a 50 cell (50, 3 cell buddy packs in my case) protective charging system with high and low cell monitoring and LVC or LVM.

You will also have very high charge rate, it could be a 48 volt dump pack with a tickle charger at home and you could carry a good variable input voltage 48 volt 20 amp charger or power supply with the car. You don't need fancy/fussy $1500.00 battery chargers. and +$1000.00 BMS.

Again using off the shelf components, I've got everything so far for $1000.00 +-10%. Want to bet what some smart EE could come up with?

Sorry, I do run on.:eek::eek:

Jim
 
#35 ·
What we plan is using, one, reletivly inexpensive, isolated DC to DC converter attached to each battery (large format prismatics) or battery group (small formats other then prismatics) to charge it.

The DC to DC is set to be able to only charge to a specific set voltage. The DC to DC then CAN NOT/WILL NOT over charge the cell. Since many of the DC to DC units can have a trim pot each unit can be adjusted to bring your cells in to perfect top balance.
I want to point out that you need to use very reliable cell level DC to DC converters. If you are using units with a 100,000 hour MTBF (mean time between failure) with a 50 cell pack then you can expect to experience a cell not charged issue about once a year. It is important that your system can detect that failure ASAP, hopefully before you are 5 miles from home.
 
#40 ·
I want to point out that you need to use very reliable cell level DC to DC converters. If you are using units with a 100,000 hour MTBF (mean time between failure) with a 50 cell pack then you can expect to experience a cell not charged issue about once a year. It is important that your system can detect that failure ASAP, hopefully before you are 5 miles from home.
As per the dc/dc converter data sheet:
RELIABILITY CHARACTERISTICS

Calculated MTBF 2.8 10
6 Hrs. Telcordia TR-NWT-000332; 80% load,300LFM, 40oC Ta

Calculated MTBF 1.8 10


6 Hrs. MIL-HDBK-217F; 80% load, 300LFM, 40oC Ta
Demonstrated MTBF >25 106 Hrs. Field demonstrated MTBF

Formatting didn't come through 10
6 Hrs. is 10 to the power 6 or multiples or one million hours.

Demonstrated MTBF greater than 25million hours seems pretty good to me.
The power supplies I am using have a 5 million hour MTBF.
 
#34 ·
Jim, Thats a great idea.... It's referred to as the power conversion type of BMS I stated above. I used pretty much the same system on my lead pack for my motorcycle several years ago. individual DC-DC converters that charged each battery. The only diff was, I didn't have Cell Logs, I used microcontrollers on each DC-DC that looked at volts and controlled the ON/OFF of the DC-DC so they would use less power in standby. They'd comm to a mother unit that could cut off the contactor if needed.

Functionally, your cell-log and dc-dc converter system is the same. I actually have 36 Vicor 3.7V DC-DC 20A converters with a 48V input sitting in my garage. I also have a few 48V DC-DC power supplies that can output 800W on 110V and 1500W at 220 and can be hooked together. Only thing missing is an HVC/LVC. The cell-log would work great.

Another thing...... your system balances the pack EVERY charge. Resistive balancers can often take several charges to balance an unbalanced pack. With yours, you never have to worry. Charge: Protected. Discharge: Protected (depending on how you "react" when you hit LVC).
 
#36 · (Edited)
Frodus,

Thanks for the praise, but I can't take all of the credit.

RWaudio is also on this path (a bit ahead of me in fact in actual hardware and research) and I've seen many others I think that are getting there or have been most of the way there (like you).

IMHO the OEM battery makers, at least for the prismatics should be the ones picking up on this. I.E. each battery comes with the charger built in or snapped on and a set of 2 wire connector cables to go between DC to DC units for the charger power loop. They can have voltage sensing to turn them selves off when the battery reaches charge, warning triggers, etc. etc. built in. The DC to DC I've bought cost about $5.00 each in the volumn the battery makers would need I bet that could go waaaaaay down. each board could be given a unique ID so all of the sensing could be a CAN bus loop and a generic CPU could sort out what battery is doing what for diagnostics and servicing as well as LVC or LVM, no matter what the battery count.

Enough preaching,:eek:
Jim
 
#45 ·
I think Jacks idea of "bad thing" refered to anything connected to the batteries that could be considered BMS related. As well as the fact that I will be top balancing every time I charge and Jack is against top balancing almost as much as he's against BMS systems.

Personally I like the idea of top balancing way better than bottom balancing because my pack will spend 100-200 cycles at the top for every 1 cycle that I may even come close to the bottom.

And from a safety perspective, sure I can damage/destroy cells with overdischarge, but they don't seem explode like an overcharged cell. If something bad is going to happen I would prefer a LiFePO4 paperweight to LiFePO4 shrapnel. So for me it's better to be safe that 100-200 charge cycles instead of being safe that 1 discharge cycle that I push the car too far.
 
#44 ·
If you're going completely BMS-less... What happens if a cell/multiple cells go(es) bad? I mean, manufacturer defect-type of thing.
If I understand your question.

First, we are not going completely Battery Monitoring-Less, there are the Cell Logs with their low voltage warnings.

In a single series system the warning would go off when the battery went below the set voltage and you would most likly come to a complete stop if the battery failed open. If the battery faled shorted you might continue at greatly reduced power, again the warning would go off when the battery went below the set voltage.

Refering to the Cell Log readouts would tell you which battery was failed. you could then jump out (to get you home) or replace the defective.

In a parallel system depending on how you wire your parallel packs you might not immediatly notice if one battery of a buddy set went down, but a look through of the cell log readings would/should show you a pack with an out of sync reading. E.G. If all other cells/cell sets showed 3.65 volts and one shows 3.50 volts you would probably want to look into it.

Even if you were to have a cell in a buddie pack go down open the current should still flow around that battery and what I beleive would happen is that cell group would discharge faster do to the lost capacity and would reach Low Voltage and trip the warning sooner then all others. A quick look at the Cell Logs should tell you quickly what is wrong.

No system is perfect, especially one built up from off the shelf generaic parts, you just do the best you can.

Jim
 
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