# Bottom Balance BMS Idea



## Tony Bogs (Apr 12, 2014)

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?


----------



## Sunking (Aug 10, 2009)

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.


----------



## Ivansgarage (Sep 3, 2011)

Here is Pete doing a bottom balance.








-


----------



## EVfun (Mar 14, 2010)

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 (Apr 12, 2014)

@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 (Aug 10, 2009)

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 (Jul 22, 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 (Apr 12, 2014)

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 (Jul 22, 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


----------



## onegreenev (May 18, 2012)

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
> ...



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


----------



## dcb (Dec 5, 2009)

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


----------



## Tony Bogs (Apr 12, 2014)

@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 (Jul 22, 2017)

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 (Jul 22, 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 (Aug 10, 2009)

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 (May 18, 2012)

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 (Apr 12, 2014)

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?


----------



## WolfTronix (Feb 8, 2016)

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 (Apr 12, 2014)

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.


----------



## EVfun (Mar 14, 2010)

Sunking said:


> 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


With a relay you can make the board bypass what you want and completely shut off when the low target voltage is reached (no slight parasitic drain from an watching BMS module.) The load on the BMS board can be replaced with a 3 volt relay and Transzorb. With that you can set a shunt load of up to 2 amps and also power the reg itself with the relay contacts. One N.O. contact is connected to the cell +. The other N.O. contact connects to one side of your load resistor and the reg board +. The other side of the load resistor, the cell -, and the reg - terminals are connected together. The reg set point to switch off the load is reduced to 2.6 volts, or whatever you want by changing 1 or 2 resistors on the board. A push button N.O. switch is placed across the contacts you can turn turn the reg on with the push of a button. It then stays on until the voltage set point is reached and then switches off completely. 

Mouser part numbers -- just a first pass choice on my part based on price and description you see in the page. With a load resistor perhaps $4 in parts per board. If you build more than one board you want to insure the voltage set points and discharge loads are very accurately the same. 
Relay Transzorb


----------



## Tony Bogs (Apr 12, 2014)

Evfun said:


> (no slight parasitic drain from an watching BMS module.


Now that is a important potential source of imbalance in FET based balance boards. 
A low Rdson FET can have a leakage current of 50 microamps and more at low temperatures (<= 50 deg C). And more important: they're not all equal. There is a stochastic (statistical) distribution profile.

But: for safety reasons (and regulations) it is not a good idea to do without a minimal monitoring circuit. 

Minimal means: minitor temperature during operation and low and high voltage thresholds continuously. No automatic corrective actions.

Relay: I don't have data on the subject, but how constant are the switching voltage thresholds of a relays?. I think they may show quite a bit of variation.


----------



## EVfun (Mar 14, 2010)

Relays vary greatly in switch off voltage and the one I listed doesn't switch off until down to around 0.3 volts. The function of the shunt regulator is to turn the relay coil off, the opening relay contacts then disconnect both the shunt load and the reg board itself. That eliminates the issue of the parasitic load of the regulator board when it is off. 

Usually the voltage set point of a cell regulator board is set by 2 resistors. Changing the value of one or both can be used to adjust the switch off point of the board down to where needed to use as a bottom balance board. Personally, I top balance and run without regulators, but that debate is for another place. The original poster wanted a bottom balance assist device. Several of us created a similar device for measuring the capacity of our AGM lead acid batteries. It was coupled with a timer across the contacts to allow us to compare the capacity across our packs as they aged.


----------



## Tony Bogs (Apr 12, 2014)

Ah, I missed the switch-off by a regulator without a schematic in front of me. 

For a larger pack (96S) a BB assist device for a single cell is unpractical. 
I'm thinking more like one board does twelve cells simultaneously. 

When balancing is performed close to an ambient temperature of 20 deg C 
temperature drift is not an issue and aging will not pose a problem when a suitable ( a bit more expensive) reference device is selected.


----------

