Adding 12v Deep Cycle Marine battery for CPAP and camping

donzoh1 · Oct 23, 2021

I'm wanting to add a 12v deep cycle battery to my minivan to power some items. These are a CPAP machine and some small vent fans (like USB powered computer fans.) At the rear of the van is a 12v cigarette lighter socket I'd like to use. Can this be connected directly to the battery (positive to positive, negative to negative) to charge the battery as the van drives? Anything I should do to prevent draining of the primary battery in the engine compartment. I also have a 110v charger and an inverter (400w) that I could use here but those would have an efficiency penalty.

SparkySy · Oct 23, 2021

If you connect the batteries directly together it will deplete the charge in both of them. You ideally need a split charge controller for the second battery so that it can be charged and supply independently from your main cranking battery and will not be directly connected to the primary one. Also modern alternators don't like charging more than they have to for the primary cell unless told by the ecu.
something like this should work.

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Be careful on cable lengths it's best to keep the batteries as close together as possible or you will need to use larger cable! Also make sure you fuse the connections appropriately.

Before modern ecu controlled alternators you would get away with this system

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