Electronics > Power/Renewable Energy/EV's

Dual battery bank, Split charging, Voltage sensitive relays

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StokesPen:
I've got a question about dual battery setups. Let's say it's in a boat with a 12v battery to start the engine and then a larger 12v bank for use when the engine is off.

You can use a voltage sensitive relay or manual isolator to ensure the battery for the engine isn't drained.

Overnight the "leisure" battery is depleted. There is now a large differential in voltage, the engine battery is at 13.2v and the leisure battery 12.2v.

When the engine is started and the connection is made joining the batteries together again, my understanding is the batteries will basically be directly connected in parallel and desperately trying to equalise voltage, this could draw hundreds of amps from the engine battery and damage it along with pushing to high current into the leisure battery? Of course the alternator will be chugging along pushing 100 amps or whatever so both batteries will end up charged but my question is this:

Is there anything to stop this high initial current draw, am I missing something? is it as bad as I think it is? is there a current regulator device for such setups?

Thanks

 

SeanB:
Typically you use a split charge controller, which is basically just 2 very beefy diodes, so the alternator charges both batteries in parallel, though the charge is going in the majority of time to the battery with lowest voltage. Diodes keep the one battery from charging the other, but the alternator has to provide the current. Yes if the one battery is flat the alternator will be running at maximum current, which in many cases will be bad for a modern cost cutted alternator, or one that is controlled by an engine management unit that is only using the engine battery as reference, and in many cases you will probably want an aftermarket alternator or a second one for the auxiliary battery.

StokesPen:
ahhh, I'm starting to see, the diodes make a lot of sense and solves the big problem I was thinking of. Just the mention of it has enabled me to search and find more relevant information so thanks for that.

Codebird:
There is another issue to be aware of: Diode drop. Even the best schottky is going to be at least 0.2V, and if you are floating your engine battery at 13.6V and your leisure battery at 13.4V... well, one of those isn't going to be spot-on ideal. It'll work, it'll just shorten your battery life a little bit. Maybe you'll have to replace it in four years rather than five.

There is a simple solution to this: Use an ideal diode circuit.

Nauris:

--- Quote from: StokesPen on June 21, 2017, 04:52:59 pm ---
Is there anything to stop this high initial current draw, am I missing something? is it as bad as I think it is? is there a current regulator device for such setups?

Thanks
 

--- End quote ---
No, it is not that bad. There is resistances in wiring and in the batteries so 1 volt difference is not going to make too big currents.

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