question

DanieRichter avatar image

4WD Battery Box

Hi , I'm building a battery box which will be removed from the vehicle, and charged off solar, when camping. I'm fitting a MPPT 75/15 solar controller to the battery box to control solar charging and loads while camping, and will be hard-wiring the controller to the battery.

When in the vehicle, the battery will charge off the alternator via a uni-directional voltage sensing relay.

Question: Do I need to isolate the solar controller from the battery while it is being charged off the alternator, or is it OK to just leave it connected to the battery in parallel? (No loads will be connected to the controller while it is in the vehicle).

If I have to disconnect it, would normally-closed 30A horn relay in the + line between the battery and the regulator do the trick? The trigger current for switching the relay (to break the battery-controller + connection) would come form the battery box connection point to the alternator circuit. I could also do this with a manual switch, but would prefer to automate it. One less thing to forget.

Thanks a lot!

Danie

solarBattery Protectsmart solar set-up helpcharging
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"When in the vehicle, the battery will charge off the alternator via a uni-directional voltage sensing relay. " - no this will not charge it properly, you need a DC to DC charger...

@ Daniel1957 Thank you for the opinion, Daniel. I am aware of the debate.

2 Answers
DanieRichter avatar image

@Mark Thanks a lot, Mark.

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No worries - all the best with it & happy camping!

Mark avatar image

You can leave the MPPT connected - no problem.

However there will be a very small current draw. If you use the car regularly it would not be worth worrying about.

PS. The current draw is that small that it doesn't even register on my battery monitor...

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Thanks Mark. That keeps it nice and simple. d