Why is Truck Battery Draining With Trailer Plug Disconnected

Question:

I dont understand why when my truck is connected to my horse trailer via the hitch but the electrical plug is NOT plugged in it still drains my battery in a matter of a couple days?

asked by: Amy

Helpful Expert Reply:

There could be many reasons why your truck's battery is draining when the trailer's electrical plug is disconnected. However, only one has to do with the trailer causing the issue.

If your vehicle battery maintains its charge when the trailer is NOT plugged in, this would point to a rather unusual situation where your trailer battery is actually grounding through the gooseneck coupler and passing the current through the ball and eventually to the frame of your tow vehicle.

Check both wires going to the trailer battery and make sure they are not pinched, frayed or damaged anywhere along the length of the wires. Your best bet is to replace the wires with # 12-2-1 (per foot) and secure the connections with heat shrink butt connectors like # DW05745-5.

On the other hand, if your vehicle battery is draining whether it's coupled with your trailer or not, the most common reason for what's referred to as "parasitic battery drain" is usually lights that remain on even when the door is closed. For example, glove box lights or trunk lights (in your case maybe a camper shell light), which are easy to miss.

As with anything electrical, there is also the possibility for a short that would explain the excessive current draw. Perform a visual inspection and check for any loose or damaged wires.

If the diodes in your alternator are going bad or you have relays that are stuck in the "on" position then that would be another possible cause.

The # 329-GEN1 charger provides a simple and reliable way to charge your battery when shore power is available, so it's always a good idea to have one of these handy.

Check out the FAQ I've attached at the bottom of the page.