I am installing a Deep cycle AGM battery in my car with a DC to DC charger via a battery box. The battery box has some peripherals such as usb/cigarette lighter outputs inline with the normal current flow.

I was wondering however if the DC charger starts to charge the 2nd battery, while a usb device or peripheral is plugged in will this cause problems? As from what I have heard batteries are generally charged by pushing electricity through them in the opposite direction? Would the opposite polarity have an affect on the peripherals? Or if the charger has a higher voltage then the battery to charge it would that voltage transfer to the peripherals, making them more then 12v outputs? and potentially causing damage?

The 2nd photo is the lid of the battery box and the highlighted green wires are the peripheral's circuit from the positive terminal of the 2nd battery to the ground of the box. Everything connects to the box's ground (usb ports, cigarette accessory, DC Chargers negative, 2nd Battery's negative) and the box's ground connects to the Main car battery ground. The car battery Positive comes into the box and through the DC to DC charger (25amp projecta), which comes out the DC Charger (Green wire from the bottom right gland) and connects to the positive of the 2nd battery. There is a switch between the positive 2nd battery terminal wire and the peripherals circuit (the 2 gold nuts, with thick wire to thin wire) and a 10amp circuit breaker before the actual peripheral components (black box with the insulated spade connectors).

My circuit diagram is roughly how I have wired it all together, with the joins being anderson plugs. I am adding solar soon. From my understanding I do not think there would be any major problems with the setup in terms of current, as I believe the peripherals would only draw what they are needing. But I am not sure if the extra voltage from a charger would change things or if there would be a backwards current present.

Any guidance would be appreciated.

Hand Drawn Wiring diagram

Actual Wiring


1 Answer 1


In short: no, there should be no problem.

There's no reverse polarity, so nothing to worry about there.

Yes, the voltage gets higher when the battery is charging, but that's exactly what happens in a car anyway when the alternator is charging the battery. Electronics built for a car's "12V" already have those voltage variations in mind. Usually they will operate correctly in a range of something like 10 to 16 volts, and tolerate brief spikes of 20V without damage.

These are just rough numbers, and you should look at datasheets or manuals (if available) for anything you want to plug in, but my point is that it looks like what you're creating is no worse than a normal car environment, so anything that works fine in a car normally should keep working fine.


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