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Installing Wiring for Vehicle 12V Battery and Trailer Connection to Charge Trailer Battery  


My black trailer wire is connected to the trailer battery. Do I connect this to the positive symbol on my 7 prong truck trailer adapter outlet? I show trailer battery voltage on this wire with my multimeter. Does this wire allow my truck alternator to charge the trailer battery or will it blow my tuck fuses when connected? Thank you!


Expert Reply:

The wire connected to your 12V (1 o'clock) position on your 7-way should be connected to a 40 circuit breaker like # 9510 and then to your vehicle's positive battery terminal using 10 gauge wire like # 10-1-1. The 12V power at your trailer connector is often connected directly to your trailer battery to provide a maintenance charge while you drive. This is a slow trickle charge based on your vehicle battery charge and your alternator. It is normally around 5-10 amps but this connection will not blow fuses. If you vehicle already has the 12V connection at your vehicle side 7-way from the factory, then it already has the proper protection for this application. Just make sure the trailer side has the battery negative terminal grounded bare metal to metal on the trailer frame.

expert reply by:
Jason S
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Rob G.


If I use the vehicle side 12v power from my 7-pin, can it be used to charge a lithium-Ion in the trailer? I was thinking of upgrading to a lithium ion for the trailer battery.

Etrailer Expert

Chris R.


The 12V circuit on the 7-Way likely wouldn't be enough to actually charge the battery up a lithium-ion battery, which requires a higher input voltage than standard lead acid units. You'll need to add add an on-board system like the Redarc # 331-BCDC1225D to boost up this circuit.

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