Can a DC to DC Charger Be Mounted in a Trailer's Battery Box to Keep the Trailer Battery Charged?

Updated 12/31/2025 | Published 11/02/2025

Products Featured in This Question

OptiMate Smart Battery Charger - DC to DC - 12V - 2 Amp

(5)

$109.95

10 Gauge Wire - Blue Primary - Per Foot

(108)

$1.61

Question:

Hi, im looking for a dc to dc charger than can be mounted in a battery box in a trailer to help charge the trailer battery. This is a dump trailer which is at times hauled by multiple types of vehicles using a 7way connector. Its being towed by a chevy 3500, ford f150 and a ram 1500. We also have an enclosed trailer with a deep cycle battery that powers the inside trailer lights and power jack... Will the option also work on this trailer if we get 2? Anything else we need?

asked by: Dan C

Expert Reply:

Hey Dan. The only DC to DC charger that you might be able to get away with mounting in a battery box is the OptiMate Smart Battery Charger item # MA99JR. It has a weatherproof case but it's still restricted to inside use only.

An easier and more reliable solution is to use the 12 Volt auxiliary terminal on your truck's 7-way connectors to keep the battery on your trailer charged. You will need to run a 10 gauge power wire like item # 10-1-1 from the 12 volt auxiliary terminal on the 7-Way trailer connector on the trailer to the positive terminal on the trailer battery and ground the negative terminal of the trailer battery to the trailer frame. This will allow you to maintain a charge on your trailer battery. Just a heads up it will not charge the battery from a discharged state.

Take a look at the picture attached below. The terminal you want is labeled 12v Battery Power.

What years are those trucks and more importantly are the 7-ways on them factory?

You can add this information and any other questions you have where it says Reply to Jerred H and I'll be happy to answer them.

expert reply by: Jerred H

7-Way Trailer Connector
(click to enlarge)

Dan C.

11/3/2025

The vehicles could vary…. It would be installed in a large metal battery box on both trailers. Would it help if I found a plastic enclosure that could be stored inside the battery box? All three vehicles have factory installed seven way plugs.

Jerred H.

11/3/2025

@DanC I was asking about the 7-way's on your trucks to make sure they were wired in the standard way so the 12v battery terminal for the trailer's 7-way connector matched up. Anything would help but it doesn't change the fact that DC to DC chargers like the OptiMate Smart Battery Charger item # MA99JR aren't made to be installed on the trailer. The long term solution is going to be charging your trailer's battery through the 7-way.

Jeff

12/31/2025

I saw this reply. But my question is, if the 7pin has a constant hot and you leave it connected awhile could it overcharge the trailer battery? I was looking to do a similar setup on our enclosed trailer but our motorhome 12v is constant with the key off. So if we are on a trip for 2-3 weeks and it continues to charge wouldn’t it over charge? I would think a maintainer or charger would need to be between the hot lead and trailer battery.