Pack capacity balancing

Hey guys
As I mentioned in previous threads Im building a 48v 10p14s set of packs for my powerwall.

I wanted to check about balancing cells with similar capacity. I am using a basic 14sBMS that will do 40a discharge continuous & 15a charge.

I have been testing all my cells which are mostly in the 2250-2350mah range.
I put together a spreadsheet of each pack with each cells capacity and have balanced each set of 10 parallel cells with the 13 others...

Im hitting about 23000 to 23100 mah per bank of 10 cells, with a cell avaerage per 10 of 2305.5 across all 14 series sets.

Is this the correct way to balance them? I read somewhere they had to be within 50mah or they would quickly go out of balance?

Obviously this is why I have the BMS, I have 1 per pack, as well as a digital display showing pack voltage and current (via 100a shunt)in real-time.

I plan to start with 4 of these packs initially mounted in a metal electrical box hanging on 12mm stainless steel rails which will be mounted inside the box.

So for 560 cells I will have 4 BMSs and 4 digital readouts.

Am I being overly OCD with balancing or is this normal?

Cheers!
Oz

What type of BMS do you plan on using? It appears most small BMS boards barely make a dent in usefulness when it comes to balancing to be honest I have started to consider them to be more protection devices than items that can provide a useful balance.

I am building a larger system right now where each pack is actually at system voltage due to the cells being arranged into pre-manufactured modules, I have decided to connect each series string in parallel across each pack.

This is being done for no other reason than BMS economics for the level of performance and quality, that allows me to use Batrium without going overkill on longmon count.

I have yet to find a per pack BMS that is going to give me the performance I want if I went with a dedicated BMS per pack at system voltage.

This is a good bms
https://github.com/stuartpittaway/diyBMS

JasonMorris said:

This is a good bms
https://github.com/stuartpittaway/diyBMS

Click to expand...

Looks interesting and similar to my situation except where I am in the UK we
Get 8-12 power cuts a year and myBaseline draw is nearer 750w with regular spikes over 2000w and occasional spikes to 3900w...

I purchased this from eBay to try as a per pack balancer:

https://www.ebay.co.uk/itm/323028794905

As Im being rather ocd about the balancing of my packs & the fact Im using the same brand and age (almost new) cells Im hoping this will be enough to keep the cells balanced.

Its only going to top balance at 50mA, but if all the cells are newer and closely matched, you should be fine.

When you build your pack, make sure there is plenty of series connections between the cells. That is what keeps your battery in balance, it will not cause any bottlenecks that will cause one cell from getting less charging/discharging current while in use.

I build my battery pack with recycle batteries from 1100 mah up to 2600 mah in each P row, but all the S rows equal about the same as best as I can get them. Even with old batteries my 168 cells 3s56p 94ah, pack stays in perfect balance.

If you do have balancing issues, the best device to fix that is active balancers, they might get expensive on 48 volt pack, but they balance at up to 6 amps. I use them on my 4s lifepo4 110ah that has balancing issues. There performance is outstanding. "electriccarpartscompany" sells them. They balance during charging/discharging. I recommend these for severe balancing problems that you can't fix otherwise.

First of all its 14s10p and not the other way around

A pack doesnt get more unbalanced because they have uneven capacity. Thats a myth if all other factors are the same.

What will happen is that if you top balance with lets say 10Ah, 10 Ah and one is 5Ah pack you will not get more than 5Ah out in total... And when charged up again they are all in balance again.

Balancing is one thing the BMS generally do. The even more important thing is to protect! IF and WHEN it goes south you want a BMS that can disconnect. Thats more important If you are worried get your self a BMS that actively can report back to you with info like Batrium or other high end systems. The cheap 20 dollar china BMS systems should not go near a powerwall if you ask me. they are for ebikes and others where you dont care much.

daromer said:

First of all its 14s10p and not the other way around

A pack doesnt get more unbalanced because they have uneven capacity. Thats a myth if all other factors are the same.

What will happen is that if you top balance with lets say 10Ah, 10 Ah and one is 5Ah pack you will not get more than 5Ah out in total... And when charged up again they are all in balance again.

Balancing is one thing the BMS generally do. The even more important thing is to protect! IF and WHEN it goes south you want a BMS that can disconnect. Thats more important If you are worried get your self a BMS that actively can report back to you with info like Batrium or other high end systems. The cheap 20 dollar china BMS systems should not go near a powerwall if you ask me. they are for ebikes and others where you dont care much.

Click to expand...

Thanks, I only have about a 100-1200 budget for the initial build so I cant spend 80% of that on something like a Batrium system. Any other options?

I also did review a cheaper option that do report some data. With that said you can buy a cheaper BMS as a start and go with but you should test it.
Its not easy to get a decent bms under 100GBP i would say.

daromer said:

I also did review a cheaper option that do report some data. With that said you can buy a cheaper BMS as a start and go with but you should test it.
Its not easy to get a decent bms under 100GBP i would say.

Click to expand...

Sorry that was a typo... 1000-1200gbp

Expecting 700 on hybrid 5000vainverter so need a BMS less than 250gbp.