Maintaining battery health for a trailer is very important. Dealing with a dead trailer, tow vehicle or accessory battery is frustrating and can ruin the plans you had for your trip. That why learning how to charge a trailer battery while driving is crucial. In this article, we look at some of the common problems associated with trailer batteries and their solutions, so as to help drivers out of this predicament.
Best Way: Using a DC to DC Charger
The best way to charge a secondary battery in your vehicle when driving is using a DC to DC charger. Such a charger provides perfect isolation between your secondary battery and the primary cranking battery.
If wired right, the charger will only work when the engine is running. So technically, it will draw power from the vehicle’s alternator. This ensures that you don’t drain your cranking engine and will always have the juice to start up your car no matter how low your secondary battery is. Constantly cycling your main battery not only drops the charge low but can also make it stop holding charge.
It will also protect your expensive trailer batteries from overcharging in case you have an overcharging alternator.
A DC to DC charger is also mandatory if the batteries in your trailer are lithium technology. This 40A Renogy DC to DC charger is a great choice. The charge controller goes between your trailer battery and the main battery terminals.
Since it will draw up to 40Amps, it might be wise to run dedicated cables of the correct gauge straight to the battery instead of drawing power from the trailer harness next to the trailer hitch.
ProTip: Add some Anderson Connector terminals next to your trailer hitch so that you can plug in the trailer when you need it and disconnect easily when you don’t.
Protect your wires by installing an additional 45 Amp fuse on the positive cable as close to the car battery as possible.
Using a Voltage Controlled Relay
- Start with the VCR. Organize the connections where the relay disconnects and parallels the start and auxiliary batteries instantly. The VCR will have its own wiring diagram and is essential as it disconnects the RV battery from the system when the car is switched off to avoid draining the cranking battery.
- Use good quality, high amperage cables to connect your RV battery to your vehicle’s battery. The high amperage cables minimize voltage drop, and Anderson plugs provide a great connection with minimum voltage loss and can carry large amperage.
- Allow the cable to be detachable at the towbar.
- Ensure the RV battery is securely connected to your truck’s battery.
- Do not overcharge your RV battery.
To charge an extra battery while driving;
- Disconnect the cable from the negative post of your main battery.
- Put the second battery in your vehicle and hold it in place using battery tie downs.
- Attach battery isolator to the firewall of your engine compartment.
- Trace the wire from the battery post on the alternator (marked ‘B’ or ‘B+’) to your vehicle’s fuse box.
- On the opposite end of the fuse the alternator wires to, attach and run a wire to the post marked “IN” on the battery isolator.
- Attach a wire from the “OUT” post on the battery isolator and run it to the positive post of your second battery.
- Ground the middle post of the battery isolator directly to the frame of your vehicle. Do this by attaching a wire connecting the post to a screw or bolt passing through the frame.
- Finally, run a wire from the negative post of your second battery to the floor pan of your vehicle.
Does a 7 Pin Trailer Plug Charge Battery?
Yes it does, but 7-way connectors may not provide a sufficient charge. To charge, run a wire from the hot pin on your trailer plug to your trailer battery.
How Long Will it Take to Charge the Battery?
This varies depending on a couple of factors. The most important are:
How much power is your alternator producing
Your alternator power output charges and powers everything in your vehicle. The charge getting to your auxiliary trailer battery would be the residual after:
- The main battery is fully charged
- The car takes the power it needs to run lights, electric pumps and other electronics
You will get more power to charge the extra battery if your alternator provides enough to meet basic needs and more. This will only happen if your vehicle has an aftermarket or stock high current alternator. Most manufacturers install an alternator just big enough for the vehicle and don’t factor in extra batteries.
You will also get more juice immediately after the main battery is done charging as the alternator will no longer have to push amps into the main battery.
How Far You Drive (and At What Speed)
The other factor is how far you drive at highway speed. Alternators produce more power at highway speeds so you will charge everything faster once you get up to speed. We did a piece on how fast will an alternator charge a battery. You will find more detailed information and estimates there.
The Size of the Trailer Battery
The last determinant is the size and type of the trailer battery. A bigger battery will take longer to charge simply because it stores more power. Lithium batteries will charge faster than lead-acid batteries of the same capacity since Lithium chemistry accepts charge to 100% faster than lead-acid.
There you have it, charging your trailer battery while driving is not a daunting process after all. Hopefully, this guide has made it simpler to do on your own.