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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?

schematic

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.

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    \$\begingroup\$ You forgot state 4 : batteries fully charged, but overcharging under solar power. Useful search term : solar charge controller. \$\endgroup\$ – Brian Drummond Dec 9 '17 at 10:24
  • \$\begingroup\$ 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. \$\endgroup\$ – goodie Dec 9 '17 at 10:42
  • \$\begingroup\$ "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? \$\endgroup\$ – winny Dec 9 '17 at 11:25
  • \$\begingroup\$ My idea has been suggested in this document: cdtechno.com/pdf/ref/41_7952_0112.pdf Figure 7 - Parallel strings with diode insulation \$\endgroup\$ – goodie Dec 9 '17 at 14:13
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We have to make a few assumptions because I think you forgot to mention some things:

  1. solar panels use a change controller for the appropriate chemistry and voltage
  2. the load is actually an inverter

Sometimes both the charge controller and the inverter is integrated in the same box.

The configuration of the batteries greatly depends on what inverter and charge controler you chose.

12V inverters rarely provide enought power to be very usefull in a house. If you want to be able to use at least 2000W ocasionaly (microwave popcorn while watching TV with some lights on) you'll probably have to go for 24V or 48V for your pack.

I wouldn't mess with saving too much money because if you skimp on using a propper solution you could burn your house down. Get propper electronics and use propper fuses.

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