Best Way To Charge A Lithium Battery With A 7-Pin Trailer Plug On A 2021 Ram 2500

Updated 12/08/2025 | Published 11/19/2025

Question:

Good Morning, I am wiring up my enclosed trailer with 50amp RV service using a progressive dynamics 4590 and using a 100ah LifePo4 lithium battery. My question is, how should I wire in the charging wire from the vehicle 7 pin trailer plug? Is it as simple as the power wire from 7 pin to positive of battery? And positive from battery to pd4590? Obviously Ill install a couple inline fuses as well. Been reading too much on the vehicle not being able to charge lifepo4 battery correctly or the lifepo4 over running the vehicle battery and kills vehicle batteries. Vehicle is a 2021 ram 2500 6.7 diesel with tow package and factory trailer brakes.

asked by: Joe B

Expert Reply:

Hey Joe, great question. You’re thinking about it the right way. You can technically run the charge line from the 7-pin’s 12V power wire straight to the positive post on your battery, but with lithium you’ll want to do it safely. Your truck’s alternator and factory charge line aren’t designed for lithium charging, and without control, the battery can either overdraw or back-feed to the truck.

To fix that, install a DC-to-DC charger like part # 331-BCDC1225D between the 7-pin charge wire and the battery. That keeps current flow safe in both directions and ensures proper voltage for the lithium battery. You’ll still run your positive from the DC-DC charger to the PD4590 and your negative back to the frame or ground bus, with inline fuses on both ends. That setup will keep your lithium battery and truck system protected while charging efficiently.

expert reply by: Jesse M

331-BCDC1225D
(click to enlarge)

Jesse M.

12/8/2025

Hey Joe, how did things go with adding the DC-to-DC charger between your truck’s 7-pin and the lithium battery? It’d be great to hear if that setup kept your charging system stable. Your results could really help others wiring similar trailer setups.