Too many batteries?

I’ve recently wondered about whether it can actually be “bad” to have too much battery capacity. Say you want to be “off-grid” but want to have enough battery capacity to carry you through a few days of bad weather, and ignore the economics of that solutions.

Having that amount of capacity would probably cause you not to use more than say 20% of the bank’s capacity on a regular clear day. Wouldn’t this cause the batteries to spend too much time at high states of charge and if improperly cooled, would cause them to heat up and degrade them much quicker than if you kept them at lower SoCs most of the time (around 50%)?

I actually thought about something similar the other evening (yes, my brain decides to start thinking about these things when I should be winding down and going to bed)

In my case I thought about just the normal State of Health (SOH) of a battery seeing how it will be at ~60% after ~10 years, so do you then add more to your bank from the start to ensure you have the required capacity after ~10 years which will in turn mean using less from the bank which should then actually extend the life seeing how it will cycle less.

What I didn’t think about was your thinking of a higher SOC meaning higher temperatures. I’m actually not entirely sure if that does mean higher temperatures or if the temperatures only really rise during charge or discharge cycles? I ‘think’ that it should be OK to be at a high SOC without a negative effect on temperature, buuuuuuut I don’t know enough to stand by that statement :slight_smile:

As far as I know, keeping a lithium battery at a high SOC doesn’t cause it to heat up. Heat is only generated during charging and discharging. So I can’t see any reason (besides an economical one as you already mentioned) that too many batteries would be bad.

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@Stanley is right. No heat is generated from just sitting there at a higher SOC. It is true that the batteries do tend to degrade slightly faster if you constantly hold them at a higher voltage, but 1) this is less of a problem for LiFePO4 and more of a problem with other chemistries, eg the one used in your laptop battery, and 2) it really becomes a problem only at higher voltages, eg 3.6V per cell and up.

With a really large bank, you’d keep the bank at a lower 3.45V per cell or so, because you have all the capacity in the world and you don’t have to wring that last bit of capacity out of the battery by going higher than that. So the extra degradation is going to be rather minimal, if at all…

Ah okay, so charging them up to a high voltage generates heat, but once they are there they should cool down. However, I recall a study showing that if LiFePO4 batteries are kept at a higher temperature (above say 25 degrees), they degrade faster if they are at a high voltage. If so, then I guess my point still holds, but only if you can’t keep them cool enough in the warm summer months?