To take this one step further I'm considering adding 2 more 6V batteries as my motor controller will support 36V & 48V. I have 6 GC2 batteries from another 36V application that are maybe 10 years old, but had very light use. How would I pick the best 2 to add to the existing 6 batteries to give me a 48V system? I know it isn't ideal to mix battery ages in a bank, but would there be any significant problems in doing this?
I did order the hydrometer and I have a multi-meter.