adding batteries to string?

[krementz]

I know adding batteries to a string is not recommended. However....

I have  4 Deka gel batteries installed a year ago. Most of the time they have been continuously charged from my large PV system, as if they were connected to the grid. They have only been on the small  PV system for 20-30 days, and never discharged to 50%.

I want to add 4 more identical batteries. I know I have to be careful about cabling lengths.

Would doing this be stupid, or just a minor degradation in capacity/life?

[ClassicCrazy]

I think the issue if more with the age of the batteries - their life cycle begins with when they are manufactured.
So over time as the first batteries age and might need tweaking to their voltage or ending amps setpoint, it would be more difficult to do with some one age and the other different.

So not recommended but that doesn't mean I wouldn't do it depending on if you are talking about series or parallel , the expectations for lifespan, the use, and monetary costs compared to replacing the whole pack.  There are a few considerations to think about. 

Ideal would be to try to find someone who would want to buy your 4 batteries for a halfway decent price and replace them all with new ones.

Larry