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Discussion Starter #1
I have 6 AGM batteries in series. 4 are good and 2 are bad.
The 4 that are good are 100ah.
I have two others that are 89ah that I can use to replace the 2 bad ones.

I know that it's bad to mix different capacities but I was thinking that these are so close it might not matter. They are also all used so capacities are most likely different anyway.
 

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As long as you understand the consequences, it's fine.

1 - You'll be limited by the weakest cells in a series string.

You used to have 4 @ 100 = 400ah.

Now you'll have 6 @ 89 = 534ah.

It's not 600, but it's better than 400. And more importantly, if you need the higher voltage, you have it.


2 - You'll harm the life of the weak cells faster.

The amount of energy you can get out of a battery over it's life (number of cycles and duration thereof) is non-linear with respect to the depth of discharge. That is, by discharging half as much, they will last more than twice as long.

But since some of your cells are weaker, they'll be more deeply discharged than the stronger ones, and even though the stronger ones will also degrade with use, the weaker ones will be bullied even more.

...

Beyond that, it'll work fine.
 

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1 - You'll be limited by the weakest cells in a series string.

You used to have 4 @ 100 = 400ah.

Now you'll have 6 @ 89 = 534ah.
Yes, you'll be limited by the lowest-capacity cell; however, "amp-hour" capacities don't add but energy does...

You had 100 Ah @ 4 x 12 V = 48 V, or 4.8 kWh
You could have 89 Ah @ 6 x 12 V =72 V, or 6.4 kWh
 

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Discussion Starter #6
Thanks all. This is primarily to test out the systems in a car that I have bought. If all goes well (wheels move, systems are working) I will move to leaf modules. If it moves 10ft I consider that a success.
 

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1 - You'll be limited by the weakest cells in a series string.

You used to have 4 @ 100 = 400ah.

Now you'll have 6 @ 89 = 534ah.

It's not 600, but it's better than 400. And more importantly, if you need the higher voltage, you have it.
If they are in series you only have 89Ah total.
 
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