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How to Charge a Trailer Battery While Driving: 3 Common Problems (And How to Fix Them)

A dead battery can be more than an inconvenience—when it comes to your trailer, letting your battery charge drop too low can permanently affect its ability to hold a charge. Not to mention, dealing with a dead trailer, tow vehicle, or accessory battery is a huge pain that can put a damper on your trip.The truth is, any time you're dealing with a trailer, maintaining battery health is crucial. Below are three common problems our customers run into when it comes to their batteries, as well as our recommended solutions.
Discover what to do if:
  • Your Trailer Battery Doesn't Fully Charge
  • Your Trailer Battery Drains Your Tow Vehicle Battery
  • You Need to Charge Your Winch or Dump Trailer While Driving
Vehicle Pulling Trailer
Battery HealthA note on battery health before we begin—always monitor your trailer battery's charge to ensure it doesn't drop below 50%. Dropping below a 50% charge will shorten the lifespan of the battery. If you don't have a built-in indicator, a battery monitoring system can provide the status of your charge. A half-charged 12V battery will put out somewhere between 12.06 - 12.1 volts (NOT 6 volts). A fully-charged 12V battery will put out about 12.6 volts or more.
Depleted Battery Symbol

My Trailer Battery Doesn't Fully Charge

If you've ever tried to charge a dead trailer battery with just your vehicle and a 7-way connector, you know that this plan is doomed to fail. The distance between the vehicle battery/alternator and the camper battery is simply too far to fully charge a trailer battery from a depleted state. The most you can do is provide a maintenance charge to prevent your trailer battery from being DOA at the campsite, and even this will only charge your battery to about 80%.So can you actually charge your trailer battery with your vehicle's alternator? Yes—but you'll need more than a 7-way.To charge your trailer battery, you'll need a DC to DC battery charger. These chargers typically connect directly to your batteries (rather than your 7-way) to provide the power needed to charge your trailer battery from a depleted state.In addition, many models are designed to draw power from both your vehicle alternator and solar panels, if they're available. So these chargers are excellent choices for keeping your batteries charged while driving or boondocking.
etrailer Recommended:
Redarc BCDC Battery Charger
  • Brings battery to 100% without overcharging (compared to about 50% through 7-way)
  • Won't drain tow vehicle battery
  • Green Power Priority ensures power is pulled from available unregulated solar panels first, only tapping into vehicle battery when necessary
Required to Install:
Redarc BCDC
Watch: Video Install
Isolation Solenoid

My Trailer Battery Drains My Tow Vehicle Battery

Those of us who have ever left our vehicle lights on overnight can attest that dead batteries are terribly inconvenient.Even worse than a dead battery in the driveway, however, is a dead battery in the middle of nowhere, two days into your boondocking adventure. Unfortunately, if you use your 7-way connector to maintain your trailer battery and don't disconnect (or use a battery isolation solenoid) once you shut off your vehicle, then this is exactly what you can expect.You have a couple of options here. The first is to simply disconnect your trailer from your vehicle when you reach the campsite. Another option is to use a battery isolation solenoid so that power is only drawn from the vehicle battery when the vehicle is running. Isolation solenoids are a simple, economical way to preserve your vehicle battery, so you never have to worry about forgetting to unplug. You can also use a battery charger with a built-in isolation function. A battery charger has the added benefit of fully charging your trailer battery while you drive.
Electric Winch

I Need to Charge My Winch or Dump Trailer While Driving

Unfortunately, 7-way connectors don't provide a sufficient charge to power a winch or dump trailer, but you can keep a winch or dump trailer charged by drawing power from either the tow vehicle battery or trailer battery.

Option 1: Draw From Vehicle Battery

The first option is to install a winch wiring kit to connect the winch or dump trailer directly to your tow vehicle's battery (without splicing into the 7-way).You'll also want to use a battery isolator to prevent your vehicle battery from draining when the ignition is off.

Option 2: Draw From Trailer Battery

Although it's not a good idea to power your winch or dump trailer directly from your 7-way connector, you can use the 7-way to keep your trailer battery topped off. Then you can use your trailer battery to power the winch or dump trailer.Or, for maximum efficiency, charge your trailer battery with a charger like the Redarc BCDC. Then use the trailer battery to power the winch or dump trailer.
Warning: We recommend unplugging the 7-way connector before using the dump feature on a dump trailer. Leaving the 7-way plugged in while dumping the trailer can potentially short out the 12-volt accessory circuit.
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Related ProductsWritten by: Amber S Updated by: Jacob J Updated on: 12/13/2023

John H.

2/26/2024

Can I mount an isolation solenoid to the firewall?

Etrailer Expert

Andrew N.

2/26/2024

@JohnH Hey John, some battery isolators/solenoids do ground through the metal mounting tabs, so you could mount that type of isolator to the firewall or any metal surface of your vehicle (assuming it's a negative-grounded chassis, which is pretty standard). That said, something like the Redarc Smart Start Battery Isolator - 12 Volt - 100 Amp # 331-SBI12 is actually supposed to ground by connecting its negative black wire to the chassis/metal surface of the vehicle. If you were to mount this isolator to a bare metal spot on the firewall, you could run the black wire to the mounting bolt to get a good ground connection. The instructions do state to keep it away from excessive engine heat, so keep that in mind.

Leah S.

2/27/2024

@JohnH You can with most, it just has to ground through the mounting tab.

John H.

2/27/2024

@LeahS Thanks! The isolator I'm looking at says it grounds through the hardware, so I should be good to go.
See All (4) Replies to John H. ∨

Greg L.

2/15/2022

There is too much fluff in these articles. They are good but 50% or more of text is just distracting fluff

Dave

1/2/2022

An option: Already having 300Ah of LiFePOs, 800W of solar on trailer roof, solar charge controller and 2Kw inverter/Charger in trailer, I’d like to “add” (place) 200W flexible/light solar panels on my truck’s tonneau cover (MPPT built in) and run an SAE cable to my pre-wired solar on side port (conveniently located at front of trailer). I could thereby charge while driving, but more than that, I can add 25% more power and also have panel adjustment while camping. Thoughts?

Les D.

1/3/2022

Are the charge ports on your trailer connected directly to the batteries or to the Charge Controller? Would need to have buss bar or similar connection and controller would have to be big enough to handle the 1000 watts of solar power input.

Dave

1/3/2022

@LesD 2 controllers, each connected to batteries via bus bar.

Les D.

1/3/2022

@Dave yes, that can work.

Gerry

12/14/2021

I just set up my 2019 equinox for flat towing. I have the Readybrute elite ii which is a surge brake tow bar. I have installed a blu ox tail light kit (bulb and socket) BX8869 so my car now acts like a trailer for running and signal/brake lights connected directly to my 7 pin outlet on my motor home. When I brake hard enough for the surge brake to engage is the only time my equinox brake lights will come on using the car battery power. Do I need something to keep the equinox battery charged and if so can I just run a wire from the center power pin to my car battery or do I require something more?

Les D.

12/15/2021

@Gerry, yes, to apply a maintenance charge to your towed car you basically wire the 12-volt accessory line from the towing vehicle to the battery on the towed vehicle. If you purchase the Roadmaster # RM-156-25 kit it will include everything you need for this chore. I have also attached an article on the pin outs for various connectors.

Gerry

12/15/2021

@LesD thank you for such a prompt reply. I will get that done before spring.

Les D.

12/15/2021

@Gerry, glad we could help. My own early spring RV maintenance list is growing as well!

Rian S.

8/30/2021

Can you add the DC-DC charger in addition to the 7 pin connector or do you need to eliminate the one from the plug?

Victoria B.

9/8/2021

You would not need to choose between the two, you can have a DC to DC charger with it's own connection as well as a 12v hot lead on your trailer connector.


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