Adding to Battery Bank

[wilsoncm1]

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?

 

[peakbagger]

Probably a question best asked on the solarpaneltalk.com forum. There are several battery pros that can answer your question. Suggestion is they like detail in your post.

 

[wilsoncm1]

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

Sent from my SM-J320V using Tapatalk

 

[TonyVideo]

If you were to mix a new battery to an old string or a new string to an older bank, the new batteries would reach their state of charge sooner than the older batteries. This could lead to them boiling. In terms of longevity, the new batteries will tend to cycle more often than the older batteries. More cycles equate to a shorter life span for those batteries. Typically this will not cause the older batteries to die first, but instead reduce the lifespan of the new one you have just installed.

 

[semipro]

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.
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.

 

[wilsoncm1]

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.

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?

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.

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

 

[semipro]

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?

So what I found here seems reasonable: https://www.upsbatterycenter.com/blog/mixing-old-and-new-batteries/
The problem seems to arise when batteries wired in series are discharged in use. The newer (assuming better) batteries provide a higher fraction of power than do the older batteries but the current flowing through all the batteries is the same because they are wired in series. This forces more current through the older batteries with the higher internal resistance creating more heat and degradation.
It seems to me that under this scenario the older batteries would only get relatively worse with use rather than approaching the performance of the new batteries.