How do you balance different battery PACKS with each other?

Alfalfameister

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If I have a 16S 280Ah LiFePO4 pack where I have already top-balanced the cells, and it's now in use, then later on I want to add another 16S 280Ah pack (cells also top balanced), how do I make sure the two packs are balanced? It says that two of them in parallel will eventually balance, but I was thinking of "getting them close" by fully charging both packs individually then connect them in parallel.

Assuming cells within each pack is top-balanced:

1. Day one, fully charge Pack 1. Then disconnect (yes, it will be "stored" but maybe for a day or two at most, at full charge);
2. Day two, fully charge Pack 2. When it's fully charged, I can parallel connect pack 1 to the busbars.

In theory, both packs should be closed to balanced?

Finally, what if I wanted to add another pack, but different Ah (say, a 16S 200Ah battery pack whose cells are already top balanced). Can I just fully charge all three packs individually then connect them in parallel to the busbar?
 

Craig

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Yes as long as they are relatively close when you finally connect them together there should not be an issue. You could in theory connect packs forever. The only thing to consider is an old pack that has weakened could possibly drag down the newer packs and wear them a bit faster. Personally I wouldnt worry too much about this but it is a possibility.
 

snoobler

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(1) In theory, both packs should be closed to balanced?

Finally, what if I wanted to add another pack, but different Ah (say, a 16S 200Ah battery pack whose cells are already top balanced). (2) Can I just fully charge all three packs individually then connect them in parallel to the busbar?

(1) Yes.
(2) Yes, but you could just charge the two that are already together as one.

There will be some voltage drift between them, i.e., two day old charged pack will be lower voltage than the fresh charged pack. I'd put yesterday's pack back on the charger to bring it back up to today's pack. At the moment of connection, I'd want them to be within about 0.2V of each other. They'll figure it all out after that.
 

100kwh-hunter

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If you manage for example 10 packs in 10 years, than the oldest (or lowest capacity)pack could drag down the newest.
Assuming you are not charging a ev or have a heavy water boiler/heater and such connected to it...

Connect 1 old and one new in parallel, so they will "act" like one cell.
So basically you will get 16 in series two in parallel
It is possible to have 9 packs/strings of 280a and one of 200, yes there is always a but:
The 200a will wear out faster, up to 4 times, how? when you drain the packs the other packs are "charging" the 200a pack, but at the same time the 200a pack is also discharging.
In this case it would be very clever to connect within a year of use 80a in parallel.

For the best case scenario:
Option 1: test your packs every 1 to three years, if the oldest pack(assuming they are all 280a) is 25% less (~200-210a total remaining capacity)then the newest cells, its time to mix and match or add 50%!(total of ~350a) extra capacity to the old cells.
Option two: buy every three year a slight smaller pack.
Option three and the most likely:
The cells are growing bigger every year, if somebody would tell me that there are 280ah cells coming.... 10 years ago....
When buying a new string (500ah in 5 years?), buy also extra to keep your oldest packs at the same capacity as the new ones.

Even with a dod of 50% it is very important for the cycle life of your cells to have every string in the same capacity.

If you do it like this you can expand to even 1Mw without worry and your (great)grand children will have to replace them....

As stated before yes and yes, dont worry if you are getting the packs at the same voltage before connecting, if not at the same voltige...you will quickly find out what 1c is capable of.


Best Igor
 
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