I have several Sealed Lead Acid batteries (12v 100Ah). The batteries are from a commercial UPS and they are not brand new. I wish to connect them to my home DC solar system to store solar energy for night time use. I would like the batteries to last for as many years as possible.
If I connect them in parallel, I am concerned that when no load is connected, the strongest battery will continuously "charge" the weakest one as soon as the sun goes down and the lifetime of the batteries will be reduced, as well as wasting energy.
The benefit of having them in parallel is that the load on each battery will be distributed and the depth of discharge will be less, leading to longer service life.
So there would be three states:
- Charging from the solar system (current going into each battery),
- DC load at night (all batteries contributing to the load), and
- no load at night (batteries discharging into each other).
I have come up with a scheme using diodes to reduce battery damage from State 3.
Is this a good idea or is there a better way to deal with State 3?
Another idea is to have 2 relays per battery (where the diodes are) and a current sensing system which would connect/disconnect each relay optimally.
And a third idea would be to have a low-current, 12.8v, float-charger connecting the batteries to mains power in State 3 in an effort to prevent batteries discharging.