Bottom Balance BMS Idea

3562 Views 23 Replies 8 Participants Last post by Tony Bogs, Oct 31, 2017

Sunking

Discussion Starter · Oct 24, 2017 (Edited)

Ok for those of us who Bottom Balance our batteries have slim pickens out there for equipment options leaving us to be creative. The Initial or Bulk Bottom Balance is simple enough. We recieve our new batteries, wire them in parallel, discharge to 2.5 volts, and we are set to go. All we have to do is install them in series, and charge up. Ready to go fast down the road.

However over time due to self discharge rates and parasitic losses (natural aging) we incur an imbalance and thus need to rebalance the pack. That is a PIA to do if you have to remove them and wire in parallel again like we did to start with. Well I guess that is fine for a informed DIY who knows what they are doing, but eliminates other DIY and any commercial consumer application. Then it dawned on me how to get around the problem. Not that I think it would have any real commercial market potential because the commercial world is stuck in Top Balance for various reasons.

But what hit me is, why not do it the exact same way a commercial Top Balance BMS works except in Reverse using Vampire aka Bleeder Boards. Instead of Turning On at 3.6 volts and Turn Off at 3.5 volts, why not Turn On equal to or greater than say 2.6 volts, and disconnect at 2.4 volts?

Does not need to be a lot of current. All one needs is about 1 amp load to do a corrective aka Maintenace Bottom Balance. I built a prototype for a 3S LiPo pack I use on RC planes. So if I burned it up, no big deal. At .25 amps worked perfectly on 3 AH 11.1 volt LiPo. I just used the JST Balance plug already on the battery to BB. At 1-Amp drain is less than 4 watts of power per cell board. Real doable using off the shelf components. Example 5-Watt 2-Ohm Power Resistor and a FET switch.

I do not see any reason this could not be scaled up, especially for folks using LFP on Off-Grid Solar using 8S and 16S. Just make a Balance Wire Harness hard wired to the batteries to a Balance Plug. On an EV just as simple, just more cells in series.

So when the time comes, drive the EV until Turtle or Limp Home mode activates to do the bulk discharge, plug the Balance Port up to Bleeder board to finnish the complete Balanced Discharge aka Bottom Balance.

Anyone else try this, or have comments good or bad? Just keep it friendly and constructive. This is NOT a Top Balance vs Bottom Balance debate.

THX

Dereck

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Tony Bogs

· #2 · Oct 24, 2017

Oh yeah, I like this approach. It is definitely what I'm going to try in my build.

First thoughts:

A bleeder unit for a 12 cell bank might be a good idea. Nice "safe" differential voltage.

How about environmental conditions? Temperature?. Probably has to stay constant during the process.

Discharge amp rating as high as possible, but then again not too high, since
heating up an adjacent bank before it starts its balancing act might have an adverse effect.

Perhaps a little bit of cooling down in between discharging sessions or maybe a scheme selecting banks with enough distance between them?

A temperature sensor should already be in the bank for safety. Use it to adjust the turn-off voltage?

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Sunking

Discussion Starter · #3 · Oct 24, 2017

Tony thanks for the feed back. I am taking a KISS approach to this (Keep It Simple Stupid). Environmental, temp, and so on adds cost and complexity. Certainly something you might want in a Commercial production, but in all reality Bottom Balance systems is pretty much DIY where the user has above average knowledge of LFP batteries. BB will never be mainstream IMHO. Commercial is Top Balance in Power Tools and Middle Balance in the EV Market. I just do not see a Market for it.

Not sure bleeding cells at as high of current as possible is a practical or needed. Here is why. If you were to say discharge at 3 amps does not give you precision control. If you discharge at 3 amps, terminate at 2.4, I think you will find the voltage will rebound back up above 2.6. In my experience the fastest and most precise way is as capacity fades, slow down the discharge rate. So when you reach 2.4, the rebound will be less significant and acceptable. Anything that ends 2.5 volts or less, and greater than 2.4 is fine. Danger is 2.0 volts and less.

So my thinking and KISS is use a lower current like 1 amp. My thinking is do the BULK discharge by driving until she quits Captain. Then finish the cells off with the Bleeders. Granted it might take a few hours, it beat a day or two taking everything apart.

To keep expense down, at the expense of time, one does not need to Balance all cells at once. Say somethin glike 15S chunks? You just have to do it 3 times in a 45S EV battery.

So far, and I have thought about this for a while, I cannot see any reason it would not work. The circuitry is simple and cheap, and there is more than 1-way to do this. Just as easily be mechanical Relays with voltage activation.

One thought I had was purely mechanical using a custom small relay. Make a 3-Volt relay with SpSt contacts. When the relay is energized with 3 volts, will draw about 1 amp, and release at 2 to 2.5 volts. Completely passive.

Lots of ways to skin this cat.

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Ivansgarage

· #4 · Oct 24, 2017

Here is Pete doing a bottom balance.

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EVfun

· #5 · Oct 25, 2017

You can set this up with a regular top balance regulator by changing the voltage set point resistor values so the turn on point is 2.6 volts, or whatever your target bottom point is. We used to do this to measure the capacity in 12 volt AGM. The load would additionally power a relay that would run a timer to measure how long the cell could supply a given load. You might want to add a similar relay circuit and use the contacts to switch the regulator off of the cell when the discharge is complete. Regs draw a little current even when the load is off.

Tony Bogs

· #6 · Oct 25, 2017

@Sunking:

I'm thinking more like 96S for an EV. So the time it takes to do the BB is important.
A week without wheels? Hmm.

Still like the approach. And I am going to try it as soon as I have all the time in the world to start my build. Unfortunately, not going to happen very soon.

Sunking

Discussion Starter · #7 · Oct 26, 2017

EVfun said:
You can set this up with a regular top balance regulator by changing the voltage set point resistor values so the turn on point is 2.6 volts, or whatever your target bottom point is.

Glad I asked. Would save a lot of fabrication and design time and most likely would not end up costing a lot more.

Only one issue I can see but fairly easy to resolve I think. Most balance boards I have seen only bypass small amount of current of less than 1-amp. I would think you could find some out there in the 1-amp range.

THX

Solarsail

· #8 · Oct 26, 2017

So will you be manually watching the voltmeter to see when it hits 2.4V to disconnect? And do that for 15 or 45 cells? And never make a mistake over draining the cell?

The suggestion to use a N.O. 3V relay and depend on it opening at 2.4V is not practical if you are doing that for multiple cells simultaneously. The disengage voltage can vary dramatically (if you are using 15 relays) from one relay to the next - is not precise enough. The FET is the better solution.

Just use a 15s off-the-shelf top balancer for $10. No need to schedule for a BB.

Tony Bogs

· #9 · Oct 26, 2017

Just what I needed to read. An off-the-shelf top balancer for 15 cells and only $10.
Link please! I've got to see it to believe it!

Solarsail

· #10 · Oct 26, 2017

16s balancer-protector $13. Note that you can make this balance less cells, such as 15s.
https://www.ebay.com/itm/16S-60V-45...807967?hash=item3f823a71df:g:D50AAOSwXtRZbsWr

4s balancer, use 4. total $5
https://www.ebay.com/itm/3S-4S-4-2V...hash=item2cc12544eb:m:mrHaEX7xEP7leBnb-qtLO7A

20s balancer-protection $23.
https://www.ebay.com/itm/72V-20S-45...990041?hash=item1a292d6319:g:ZxEAAOSwXXxZXNut

14s
https://www.ebay.com/itm/14S-CELLS-...hash=item237520a2d6:m:mXJcv6WWEGQ-qOeNUcxNp9w

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onegreenev

· #11 · Oct 26, 2017

Solarsail said:
16s balancer-protector $13. Note that you can make this balance less cells, such as 15s.
https://www.ebay.com/itm/16S-60V-45...807967?hash=item3f823a71df:g:D50AAOSwXtRZbsWr

4s balancer, use 4. total $5
https://www.ebay.com/itm/3S-4S-4-2V...hash=item2cc12544eb:m:mrHaEX7xEP7leBnb-qtLO7A

20s balancer-protection $23.
https://www.ebay.com/itm/72V-20S-45...990041?hash=item1a292d6319:g:ZxEAAOSwXXxZXNut

14s
https://www.ebay.com/itm/14S-CELLS-...hash=item237520a2d6:m:mXJcv6WWEGQ-qOeNUcxNp9w

Will there be protection from the balancer if it happens to fail so you don't loose any cells?

dcb

· #12 · Oct 26, 2017

yah that is 4.2 volts too, don't do 4.2 volts unless you like replacing batteries.

Tony Bogs

· #13 · Oct 26, 2017

@solarsail: Thanks for the links.
Those boards are indeed really cheap. Price and appearance.

@Sunking: does it really take that much time to design one of these boards in a better quality and with improved functionality?
Don't think it does. Price is going to be at least tenfold. But still only $200. I'm OK with that.

@dcb: Discharging is the idea of the TS as a form of BB.
So 4.2V never happens.

Solarsail

· #14 · Oct 26, 2017 (Edited)

onegreenev said:
Will there be protection from the balancer if it happens to fail so you don't loose any cells?

Balancers are just simple FETs and resistors and maybe a diode. They will last far longer than any cell. There is no need to protect against balancer failure. Resistors don't fail and neither do FETs with 4 volt across.

On the other hand if you are referring to a "smart" BMS, there is a chance that the BMS will fail, and keep the shunt in place until battery is depleted. Tesla and Leaf don't seem to be too concerned.

However for a small 15s 50Ah 1C project you don't need a smart BMS. Just use off-the-shelf balancer.

Solarsail

· #15 · Oct 26, 2017

Tony Bogs said:
does it really take that much time to design one of these boards in a better quality and with improved functionality?
Don't think it does. Price is going to be at least tenfold. But still only $200. I'm OK with that.

Depends what you are balancing. If you are simply balancing a 15s below let's say 100Ah and a low C rate, then just go off-the-shelf balancer and protection, as linked above. These boards are used in millions of e-bikes and they are truly commercially used (in China, and exports). If you don't want to take the risk of eBay, try Alibaba or AliExpress. eBay often supplies seconds from the factory.

But if you are balancing high-cycle 96s, then you want a "smart" BMS to monitor the pack. This could be a used Leaf or Tesla BMS. Or you can build one. The following thread has a discussion of a smart BMS project. Many more threads on smart BMS.

http://www.diyelectriccar.com/forums/showthread.php/18650-13s10p-project-48v-x-34ah-188618p9.html

Sunking

Discussion Starter · #16 · Oct 27, 2017

onegreenev said:
Will there be protection from the balancer if it happens to fail so you don't loose any cells?

Pete glad you chimed in. Not sure that question is directed at me or not. In any event, no secondary protection. Basically a Vampire Board (Balance Board) with different voltage trigger set point. Example turns on greater than say 2.6 to 2.9 volts, and turn off at 2.4 volts. Pretty much autonomous.

onegreenev

· #17 · Oct 27, 2017

Sunking said:
Pete glad you chimed in. Not sure that question is directed at me or not. In any event, no secondary protection. Basically a Vampire Board (Balance Board) with different voltage trigger set point. Example turns on greater than say 2.6 to 2.9 volts, and turn off at 2.4 volts. Pretty much autonomous.

No. Just a question to the guy who posted those links to the cheap boards. Im all for bottom balance but not sure if Im on board for an auto bottom balance unless it is only done when needed. Ive not yet seen a real reason for re-bottom balancing unless you need to change your voltage by increasing your current voltage or if you for what ever reason need to replace a cell. I don't like auto function things unless they are either bullet proof or have a safety system in place that can monitor itself for problems or eminent failure so not to ruin a battery or pack.

Tony Bogs

· #18 · Oct 27, 2017

Sure, there are a lot of factors that can play a role when chosing a balancing system.

Statistical failure rates, cost of the entire product (cheap $500 Chinese e-bike, $100.000 Tesla model S, solar pack made from recycled EV cells),
reliability and there are certainly more for every individual case.

But for my application (96S EV pack) I really like the approach of a passive balancing solution that is only used when the pack reaches a threshold in imbalance.
The BMS can be reduced to a minimal, high quality, fully tested, always on monitoring system to detect crossing of safety thresholds.
During balancing it will be a two factor system: a high quality, fully inspected and tested, autonomous 12S balancer and human supervision.
The human factor should not be a problem when balancing a 12S bank takes less time than the average attention span.

High amps at the start will make sure that the risky part of the discharge process stays within the human attention span.
After that there will be a short recovery period, voltage bounces back up and then slow bleeding starts.
Depending on the level of paranoia and anxiety of the individual it is quite easy to add additional warning systems.
For instance a simple off the shelf cell phone dialer that sends a text message when an alarm situation occurs.
Could be as simple as an elapsed period of time since activation of the bleeders.

onegreenev said:
I don't like auto function things unless they are either bullet proof or have a safety system in place that can monitor itself for problems or eminent failure so not to ruin a battery or pack.

Is there a chance that the above mentioned can meet your standard for an auto function system?

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WolfTronix

· #19 · Oct 27, 2017

If you are worried about the FET getting struck on, and it draining your cell...

Chose a resistor value that results in 4 times your balance current.

Place a small fuse rated at 2 times your balance current.

Then PWM the FET at 25% duty cycle (this should be your balance current).

If the FET shorts out, it will just pop the fuse.

Tony Bogs

· #20 · Oct 27, 2017

Voila, an improvement to the cheapest solution.
PWM (microcontroller?) and fuses. Sounds good if I would be worried about FETs shorting out.

But I'm not. Just want to put the right mix of hardware, time, effort, money and tedious bit banging (microcontroller programming) in the pack.
And then the S factor kicks in. 96S is already starting to be a small pack for performance EVs.
Porsche tends to go for 700V if one can believe the news sites.

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