Adding to Battery Bank

Howdy gang.

I keep hearing that enlarging a battery bank at a later date is a 'bad idea'™. Battery capacity degrades over time and when you add to a bank the new batteries are forced to perform at the state of the old ones.

My question is why does it matter? Let's say I buy 8 batteries now (48v bank) and in a year they degraded 10% and they're at 90% performance. Then I add 8 more batteries and so the entire bank is at 90% performance since the new will match the old. Versus buying all 16 now and they degrade 10% over the next year and the whole bank is at 90%. Aren't I still at the same place a year from now?

What am I missing?

Ya, I thought about posting over there. Not necessarily the nicest place sometimes. Thought I'd pick the brains here before I try there.

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semipro said:

No expert, just tossing this out there.
I think the primary issue is that a battery's internal resistance increases with age.
If the old and new cells are connected in series this would mean that older cells would end up being charged to a higher voltage than newer cells since current is constant through all cells. Thus, newer cells wouldn't be charged to the optimum level for best storage performance.

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Yup, I understand that. What I'm asking about is whether is makes a difference over time: 1 large bank degradation over time vs an added to bank over time. Will the output be the same after 3 years?

semipro said:

I would think that a good battery management system that operates individually on each cell would compensate for this issue though by adjusting the voltage of each cell to the optimum level of performance.

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A BMS is an interesting idea but that's another additional cost plus another failure point.