We have a 24 volt off grid solar system. At the moment if the cloud cover is heavy and no solar energy is generated we have 3 days max of power in the batteries before they drop to 70% charge. Our batteries are BAE valve regulated lead acid batteries at 24 volts and 660 amp hour.
We are thinking of getting another bank and using a switching system to change over banks when we have cloudy weather. ( in winter we are in the clouds a bit).
It is rare that we have no power input at all but it does happen, recently I had to resort to a small petrol motor and car alternator to top the batteries up.
The spec sheets for the batteries say that if we only use the top 30% of the capacity they should last around 15 years. That would be nice.

So I have ordered two 500 amp kilovacs to do the switching, but I need both banks to be kept charged when the solar input is enough.
I was thinking of using a voltage controlled relay to switch the kilovacs, so that when one bank is fully charged then the other would be switched on to supply power and also be charged up.

I am just wondering if anyone here has ideas on the best way to run the system.
Our original bank of batteries are now about 8 years old and still in good condition, obviously I don't just want to parallel a new bank to them.

Thanks
Pete

I have read in a few place's that some run a LFP battery along side the Old Lead acid to extend the life of the Lead Acid by quite a margin, the LFP battery will do most of the work, when they get down the Lead acid takes over and carries on making them an ideal when the weather isn't great.

Actually I have this setup in the shed but only for welding and other high surge stuff so have never run it flat to see the FLA batteries take over.

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I was thinking of using a voltage controlled relay to switch the kilovacs, so that when one bank is fully charged then the other would be switched on to supply power and also be charged up.

Probably something to measure the SOC on either pack and programmed to switch the relay.

One of our systems I built as an interim setup until the big system goes online, was a 24v sealed Lead Carbon 300 AH bank, it had proven to be a tad small due to users piling on more and more loads, then leaving things running over night when not in use.

As the bulk charge voltage was almost 28v I decided to augment its capacity by paralleling it with a 24v 200AH bank of Lifepo4 cells re-purposed from dismantled 48v 100AH battery packs I had rescued. This has proven to be hugely successful, the Lifepo4 cells only get to 3.5V each, which is ideal, they do all the work, supplying power to the inverter until their voltage drops where the L.Carbon sealed cells take over.

I use one of those Kilovacs to isolate the Lifepo4 bank from the L.Carbon bank when their voltages start dropping near 22 volts, the BMS senses individual cell voltages and switches the bank out via the contactor, it re-enables it again when the Lead cell voltage > 24 volts.

So that is maybe another option, a new bank of 24v Lifepo4 cells will do all the heavy current grunt work, only drawing down on your older Lead as they start getting discharged. Here in NZ, Lifepo4 banks are less expensive than any form of lead.

Cheers
Mike
Edited 2026-06-20 20:13 by Solar Mike