Want to charge 24 VDC 600 Ah Li-ion house battery with overflow of 12 VDC 600 Ah AGM battery

Question

I recently installed a 24 V lithium-ion battery and new inverter charger for house power on catamaran but left 12 V AGM batteries connected to separate DC system which also is charged by solar, wind and 50 A alternator. What is best way to charge the 24 V lithium-ion battery from the 12 V battery?

• 24 V MPPT charger/ inverter 3000 W 230 VAC.
• 12 V, 30 A wind, 20 A solar, and 50 A alternator

Thanks.

Answer 1

If the problem is that when everething is producing at full rate simultanously (30A wind + 20A solar + 50A alternator) it is too much for your 12V charging system (charger or batteries), then I would suggest to have a relay (commanded by the power of the generator) that switches solar+wind to the 24V batteries. Or even simpler, just make sure your don't turn on the generator when the renewable energies are producing a lot (you probably already harvest more than enough energy).

If your question is redirecting the power to the 24V system when the 12V batteries are full, then the first thing you need is a way to know when your batteries are full. This can be determined in several ways, depending your exact instalation:

• if you have some "smart" chargers or monitors involved, you might already have this signal somewhere
• or you can compare the battery voltage with a (nearly) "full" voltage: if the 12V battery is above this nearly full voltage, you can switch power to the 24V system

Then, for doing the actuall switching, you have 2 solutions:

• switching each power source directly to a 24V charger
• or just connecting a charger that drains the 12V batteries to charge the 24V batteries. If the charger draws more 12V current than what is recharged, the voltage of the 12V batteries will drop until you are bellow the threshold voltage. NB: this solution is probably a little bit less efficient from an energetic point of view, but probably less expensive than buying 3 different chargers.