DIY Electric Car Forums banner

1 - 20 of 21 Posts

·
Registered
Joined
·
362 Posts
Discussion Starter #1 (Edited)
Hello,

Been a while since posted. My latest effort is to build a series of 12VDC packs that are 'drop-in' replacements for a standard 12 deep-cycle battery for my camper.

Battle Born batteries is already doing this for a fee of nearly $1000 per batter for a 100 Ah battery.

I happen to have 45 130 Ah CALB cells that I MIGHT want to pull from my EV project and convert for use in my camper. The idea being to extend my off-grid stays through a much larger bigger battery bank. Yes, I know, it might be a bit of over-kill but this seems to be the best way to use those cells if I can't put the EV back on the road.

So, anyone know of a BMS that might be suitable for this sort of application?

Been thinking about trying to design my own using a Arduino as the brains of the system but before I follow that path, I thought it would be good to ask.

Thanks,
Pete
 

·
Registered
Joined
·
1,469 Posts
take a look on evay for 4S BMS boards with the current rating which you desire. They usually do under voltage, over voltage, over current monitoring and they us some FETs to handle the current load. The FETs get turned off if the limits of any one cell are exceeded. Read carefully to get the right limits for your chemistry, the little monitoring chips are made with a variety of presets with various limits, although some allow for custom setting. Some have balancing features but i'm not sure how much real use or value that would be in your situation.
 

·
Registered
Joined
·
739 Posts
Hi
12v is only 4S
I would question the need for a BMS at 4S
Yes especially for a low current use case.

All you need is

low temp protection in freezing locations,

LVD at 12V,

HVD/stop charging at 13.8V,

keep under .3C,

manually check on cells once in a while, should rarely if ever need re-balancing.
 

·
Registered
Joined
·
389 Posts
You cant just drop some cells in and hope it will work with the stock charger ect. you need A BMS and you need a dedicated CC/CV charger with load sharing, its an absolute must. A BMS wont protect you in all instances.

Correct me if i am wrong but If you dont have the ability to bypass the battery with load sharing you could run into a dangerous overcharge situation that the BMS wont rectify, for example your solar array is out putting an amp and you have a load of an amp and you battery is nearly full, that would mean it would be kept at close to its maximum voltage indefinitely untill the situation changes.

With load sharing all power would be taken from the solar array and the battery would be off charge

If you need a BMS you could look on ebay or aliexpress, there are some new models that are very inexpensive that have an android or PC app to measure battery voltage, temperature and current draw. make sure to match the battery chemistry and get one higher current than you need, they are not rated conservatively at all.

The other option as John61ct has said is running them at their float voltage, which is a great way to avoid the above hassle but capacity will be lower, You will still need a BMS in this case one with adjustable balance voltage, the ones that connect to a PC like i said above, have this function like this one:
https://www.aliexpress.com/item/4S-80A-Lifepo4-smart-bms-pcm-with-android-Bluetooth-app-UART-correspondence-bms-wi-software-APP/32864142100.html
 

·
Registered
Joined
·
739 Posts
The actual AH capacity difference between

14.6V until zero amps accepted, and

no Absorb/CV at all, "just stop" when 13.8V is reached

is at most 4%

Lifespan is so much longer avoiding the shoulders as spec'd above, totally worth it.

If Float cannot be avoided, I use 13.1V.
 

·
Registered
Joined
·
1,627 Posts
I use a Zeva unit for an RV 12V application: http://www.zeva.com.au/index.php?product=125
You manually balance the cells and the BMM8 is there to save a cell in some rare cases (solar panel source and park few days under a tree, by example).
 

·
Registered
Joined
·
389 Posts
The actual AH capacity difference between

14.6V until zero amps accepted, and

no Absorb/CV at all, "just stop" when 13.8V is reached

is at most 4%

Lifespan is so much longer avoiding the shoulders as spec'd above, totally worth it.

If Float cannot be avoided, I use 13.1V.
Agreed this is the way i use LifePo4 on my car, with 3.45 float voltage.
I was under the impression that it was a 40% - 60% capacity loss compared to cc to 3.6v cv with C/20 cut off? or maybe i haven't understood what you are saying?
 

·
Registered
Joined
·
739 Posts
You understood, but your prior understanding was incorrect.

Also, cutting off discharge at 12V (2.99Vpc) is also a **very** minor sacrifice of capacity at the low end, certainly nowhere near the 5% many believe, especially at low C rates.

Very common for people to think / say they're cycling between "10% and 80%", but not at all true.

Going all the way down to 10.5V is very dangerously close to rendering your very expensive bank instant scrap, I would not do so even occasionally for capacity / load testing.
 

·
Registered
Joined
·
389 Posts
Yes my previous statement only holds true if you want to use all available capacity in the cells safely, which I assumed you would want to do.

Can you show me a SOC graph that supports this, I cannot believe that what you said is accurate based on the graphs for various LiFePO4 cells i have seen, and the behaviour of the cells i have on a charger.
 

·
Registered
Joined
·
739 Posts
No, I am going off my real-world measurements, mostly CALB and Winston, both second-hand and new.

Note actual AH capacity has almost always been higher than rated.

Keep in mind you need to use a full load / discharge test to determine actual AH.

Say you start with a 480AH bank, charge at 20A to 13.8V and stop, no Absorb/CV, and that turns out to be 467AH when drawing down and stopping at 12V.

From that definition of Full, you may be able to pump in another 40+AH to get from there to 14.6V absorbing until current stops, or say change less than .1A over an hour.

However many if not most of those "charging AH" are actually just dissipated as heat, maybe only 20AH are actually held by the bank.

And if longevity is your goal, stupidly pushing up the charging curve like that just to get that last 20A in there is going to result in your only getting the mfg rated cycles lifetime, even if the bank is otherwise coddled.

Say 2-3000 cycles rather than many times that if the shoulders are avoided, I believe 8000+ but we just don't know yet, most pioneer owners for House bank use are only going into years 6-8 or so.

Those keeping to the "avoid the shoulders" regime, who've avoided other "events" have yet to see any drop in capacity, and most stop worrying about cell-level balancing, even annually.

Of course most just follow the cell maker specs and don't want to bother paying much attention, and are happy to get less than a decades's lifespan. In which case they can use cheaper lead chargers with fixed charging profiles.
 

·
Registered
Joined
·
389 Posts
I have charged LiFePO4 cells to 3.45v and then used another charger to top them up to 3.6v CC CV to C/20, they have taken above half the rated capacity to get them there, I cannot see how you are only getting a 4-5% capacity loss with charging to 3.45v compared to ending at 3.6v, all the SOC graphs i have seen support this as does the calorie counter on my charger. I have never experienced excessive heating charging like this either, tho the cells are significantly smaller.

Float charging is safe for any amount of time to 3.45v on LiFePO4 in my experience but will still need a BMS as any deviation by one cell even by a few mv will significantly change the energy stored in the cell, finding a BMS for balancing to float is possible but you always pay more.
 

·
Registered
Joined
·
739 Posts
I am talking about comparing the AH **actually stored** in the bank, via load discharge tests.

Again, that has nothing to do with measuring the AH **output from a charger**, and easily verified without specialized gear.

I also did not say anything about the heat conversion being human perceptible, just explaining "where it goes" when you're in effect just adding a little superficial surface charge.

For those motivated by longevity, LFP should not be allowed to sit Full for any length of time.

Floating the bank should really be avoided if possible, and certainly never to be used when the bank is at a high SoC.

If the charge source is sufficient to carry loads, just take the LFP bank offline and let the starter batt(s) be the buffer / load dump.

If your design or variable / heavy load "requires you" to Float with a near-Full LFP bank online, then I would keep V below 13.2

Again, I am not talking about EV usage nor any live / active balancing, IMO more risk with no benefit for the use patterns spec'd above, the only "BMS" functionality needed is pack- or bank- level LVD / OVD and low temp protection in freezing environments.
 

·
Registered
Joined
·
389 Posts
Seems you are right about the capacity vs charge voltage i found this:

https://www.powerstream.com/lithium-phosphate-charge-voltage.htm

It seems odd that anyone would charge to 3.6v as it doesn't seem worth it at all for the tiny amount of capacity it seemed to add in the test, its a shame that nearly all cheap BMS boards seem to balance at this voltage.

Thanks for the info im sure you were right about them lasting alot longer, if its not doing useful work in the battery it will definitely be ageing them.
I have just started using an old LifePO4 battery to power a sound system and i wanted to charge it with solar and was planning to implement load sharing that was a real hassle, do you think it would be ok to set the charger to 3.45v and use the battery at the same time in your opinion? I was under the impression from what i have read that they are okay with float all the way to 3.45v.
Thanks
 

·
Registered
Joined
·
739 Posts
It seems odd that anyone would charge to 3.6v as it doesn't seem worth it at all for the tiny amount of capacity it seemed to add in the test, its a shame that nearly all cheap BMS boards seem to balance at this voltage.
The whole industry seems to just blindly accept the mfg spec'd profiles.
 

·
Registered
Joined
·
739 Posts
I have just started using an old LifePO4 battery to power a sound system and i wanted to charge it with solar and was planning to implement load sharing that was a real hassle, do you think it would be ok to set the charger to 3.45v and use the battery at the same time in your opinion?
The level of care I'm talking about is for banks costing thousands.

Old / small stuff, no sweat.

Plus, if the load requires more than the source puts out, batt won't be staying at Full anyway.
 

·
Registered
Joined
·
739 Posts
Here's my "boilerplate LFP House banks" summary, mostly from marine electrics discussion forums involving long-term users and professionals, with special thanks to Maine Sail (see below).

Any and all feedback is welcome, especially if more "canonical" information from the links cited conflict with my summary.

______
Systems: OceanPlanet (Lithionics), Victron, MasterVolt, Redarc (Oz specific?)

Bare cells: ​Winston/Thundersky/Voltronix, CALB, GBS, A123 & Sinopoly

Best to size your cells for two parallel strings for redundancy, unless you have a separate reserve/backup bank. Don't go past three, or you may see balancing issues that affect long-term longevity, maybe four in a pinch.

Note nearly **every** vendor, also those of ancillary hardware touted as "LFP ready", gives charging voltages **way too high** for longevity.

Optimizing for longevity is just not high on their list of priorities. All their focus is on their main customers, the military and EV propulsion, where range/per-use capacity is everything and expected lifespan is short anyway. EV's high C-rate usage is **very** different from our much gentler House bank cycling. Note also that most EV people talking "lithium-ion", mean other chemistries not as safe as LFP, much shorter lifetimes, and with completely different setpoints and behaviors.

Gentler House bank*storage*use cases are .0001% of the cell manufacturer's market, so therefore no testing / different specs are developed for that use case; that has been left to the end users, and the more objective/technical vendors like*Maine*Sail.

Consider also, that the auto industry could easily have developed utilitarian cars and*parts*supply chains, so that consumers could routinely drive them for 50-100 years. For purely rational free-market economic reasons, that has never happened.

So, following those vendor charge specs is fine if you only want the cycle lifetimes they advertise. You can however get **much** longer life by "avoiding the shoulders", look at the SoC vs Voltage chart, and avoid the "shoulders" at both ends, stay in the smooth parts of the curve.

________
My end-charge voltage setting for charging LFP is 3.45Vpc, which for 4S "12V" = 13.8V max. Note that is usually at an amps rate of around .3C, or 30A per 100AH. At higher rates, to shorten ICE run-times it is safe to go to 3.5Vpc / 14.0V. Also at **very** low charge rates, I back off to 3.40Vpc / 13.6V.

For daily use cycling, best and simplest is to "just stop" charging when your end-point voltage is reached. A long Absorb stage is holdover "lead thinking".

For precise benchmarking of 100% SoC, an endAmps spec of .03C (3A per 100AH) or even .05C is fine, but otherwise Absorb time only gets you another percent or two actual SoC capacity, mostly surface charge or dissipated as heat, and at charge voltages over 3.5Vpc will reduce longevity.

Note even at the "low" max charge voltage, letting the charge source continue to "push" even low currents long **past** the endAmps point is over-charging, and will reduce lifecycles.

Note that "stop charging" may simply mean isolating the LFP bank, if you want your charge source to carry ongoing loads rather than discharging your LFP bank.

But if you can't do that or just prefer to Float, then at least set the Float voltage well below your bank's resting Full voltage point, at say 13.1-13.2V. But that is a compromise, and *may* somewhat shorten life cycles.

With LFP, you don't need to ever fill up all the way, as far as the cells are concerned. In fact, it is bad for them to sit at Full for more than a few minutes. Therefore only "fill up" if consumer loads are present, ready to start discharging, ideally right away.

Charging at .5C or even higher is no problem if you want to minimize ICE runtime, as long as your wiring is that robust; vendors may spec lower rates out of legal caution. .3C or lower charge rates are better for longevity.

And of course, we're talking about gentle "partial C" House bank discharge rates, size appropriately and be careful feeding heavy loads like a winch or windlass.

Many sources claim there is a "memory effect" from keeping charge voltage and ending point exactly the same every time lower than manufacturer specs, that can apparently over time lead to apparent lower capacity. Their recommended fix is to "go higher, into the shoulder" every so often, similar to "conditioning" a FLA bank monthly. IMO you can also prevent the issue, by varying your setpoints a bit, sometimes go a point or two higher or lower, vary Absorb time a bit etc. There is no consensus just how serious this problem is.

Store at a low SoC. "Not over 50-60%" say most, to compensate for self-discharge, if not getting topped up regularly (I would at least monthly). "Lower the better" say many more recently, as long as you take **zero chances** letting self-discharge drop below 10-20% SoC, certainly not below 3Vpc. Isolated from everything including your BMS and cooler the better.

Letting the batts go "dead flat" = instant **permanent unrecoverable** damage.

Same with charging in below 32°F / 0°C freezing temps.

Persistent high temps also drastically shortens life.

Again, going above 3.5Vpc won't add much if any AH capacity, but will shorten life cycles dramatically.

Following these tips, letting the BMS do active balancing is unnecessary and potentially harmful, IMO just look for LVD / OVD and temp protection. Multiple layers of protection are advised if it is a very expensive bank, so you don't rely on any one device to keep working.

Check cell-level voltage balance say monthly to start, then quarterly, finally every six months if there are no imbalance issues, but only if that seems safe to you.

This thread is long but informative
http://www.cruisersforum.com/forums/f14/lifepo4-batteries-discussion-thread-for-those-using-them-as-house-banks-65069.html

, make sure to give both Maine Sail and Ocean Planet your close attention.

Also MS' summary notes here
https://marinehowto.com/lifepo4-batteries-on-boats/

**Everything** at that site is worth reading, very valuable. https://marinehowto.com/support, feel free to make a donation to help with those expenses. He also has great articles in Practical Sailor.

Anyone actually setting up House banks from LFP, do please report back here!
 
1 - 20 of 21 Posts
Top