# Method for rechargeable batteries in Parallel

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:

1. Charging from the solar system (current going into each battery),
2. DC load at night (all batteries contributing to the load), and
3. 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?

simulate this circuit – Schematic created using CircuitLab

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.

• You forgot state 4 : batteries fully charged, but overcharging under solar power. Useful search term : solar charge controller.
– user16324
Commented Dec 9, 2017 at 10:24
• I'm assuming that the solar panel system includes a charge controller and battery charge management system. You could assume that the power source is just a battery charger. However, I notice now that the diodes would prevent the charging system from sensing the battery state. Commented Dec 9, 2017 at 10:42
• "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" XY problem! What evidence do you have to support your hypothesis and that it will determinant of the lifespan of your battery bank? Commented Dec 9, 2017 at 11:25
• My idea has been suggested in this document: cdtechno.com/pdf/ref/41_7952_0112.pdf Figure 7 - Parallel strings with diode insulation Commented Dec 9, 2017 at 14:13