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

Battery Health

A 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.

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.

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:

Fuse Kit: # 331-FK40 (for # 331-BCDC1225D) or # 331-FK60 (for # 331-BCDC1240D)
Wiring Kit: # SW1520

Watch: Video Install

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.

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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Written by: Amber S.

Updated on: 3/1/18

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.

Helpful Links

faq-wiring.aspx

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.

Tom

7/31/2021

I’m rebuilding an old truck camper. The electrics are completely ripped out. I ideal want to be able to charge the battery from the truck whilst driving. From shore power and from solar. However this camper cost only $500 and I don’t want to spend alot (max $1000 ) on it. What is the cheapest way to do this?

Lois

2/12/2021

We have a Class A, 33ft and tow a Ford Taurus. We have a 7pin connector. When we travel more than two hours, our battery is dead on the car. We have a booster charger and it starts right up. Why does it drain while we are driving? It is a new battery, too.

Jon G.

2/15/2021

I'm not sure what year Taurus you have but if the battery is dying then it sounds like you probably have to keep your vehicle in the ACC mode while flat towing. This means that something is needing to pull power from your battery while flat towing which is why the battery keeps dying. A charge line kit like part # DM66VR should remedy this issue but it's best to make sure that you are following all of the steps outlined in your owner's manual for flat (recreational) towing. If you let me know what the year is of your Taurus I can confirm whether you should really need the charge line kit or not.

Casey B.

12/20/2020

I have a Buick Enclave towing a teardrop with a 100 amp hr lithium house battery. Whats the best solution for charging while driving. The full voltage for house battery is 14.6. I have a200 watt solar panel and mppt controller. My fridge is just a two way and I would like to keep that running. Thanks in advance.

Jon G.

12/24/2020

The best solution for you is to install the Redarc BCDC # 331-BCDC1225D (with fuse kit # 331-FK40), and then tie your solar panel directly to the BCDC - bypassing your current solar panel controller. The BCDC optimizes power that is coming from your Enclave through the 12V power pin on the 7-Way connector so that your battery will get to a 100% charge. Tying the solar panel in means that the BCDC will pull as much power as it can from the panel before it starts pulling from the vehicle. Attached is an installation video that you can check out.

Helpful Links

tv-install-redarc-in-vehicle-battery-charger-331-bcdc1225d.aspx

Casey B.

12/24/2020

@JonG thanks for the informative reply.

Jon G.

12/24/2020

@CaseyB Happy to help out!

Brett C.

10/1/2020

I was explaining to someone hooking up a 50 AMP DC to DC charger on my F350 to charge my trailer batteries running an inverter to power the refrigerator while driving down the road. I don't want the propane on while driving. Using a KW we saw that the refrigerator should pull about 40amps DC. This person who has a background in electronics asked why use the DC to DC charger. If both battery chemistries are the same why not eliminate the converter and wire (through breakers and a relay) from truck battery to trailer battery. Use the same 2 gauge wire that we were going to use for the DC charger from the truck through an Anderson plug then to the trailer. Would the truck alternator keep all battery voltages up in a full charge state? Any advice on this is appreciated.

Jon G.

10/1/2020

Even if you used 2 gauge wire, the voltage for the battery would drop enough that you wouldn't get a sufficient charge. Plus it would be more like a trickle charge that just keeps your trailer battery from dropping. You will need something like the Redarc BCDC to completely charge your trailer battery because it is able to determine how much power is needed and then push through enough charge for your trailer battery to reach 100%.

Brett C.

10/1/2020

@JonG thanks for responding. Your comment spurs another question if you don't mind. In the design of the charging system I was planning on putting the DC charger in the trailer. Is that a bad idea because it may have the same voltage drop problem? Should I put the charger in the yow vehicle?

Brett C.

10/1/2020

@JonG Thanks for the reply. Is putting the DC charger on the trailer okay or does it need to be close to the truck (source) battery? There just isn't a whole lot of room around that PowerStroke.

Jon G.

10/2/2020

@BrettC Good question! According to Redarc the best place to install the BCDC (like part # 331-BCDC1225D) is as close as possible to the auxiliary battery bank. If this was near engine heat you'd want to avoid that but I don't think that's the case for your situation. The BCDC is able to take that low voltage from the start battery and adjust it so that it can fully charge the battery bank on your trailer.

Brett C.

10/2/2020

@JonG Awesome. Thanks so much for your guidance. I'll check this unit out as well as 331-BCDC1240D since the refrigerator pulls 438 amps plus the inverter overhead and I want to be able to run that without draining the batteries. I also see that this has dual inputs and a built in MPPT charge controller. Could I use this or 331-BCDC1240D as a full-time charge controller when parked if I wanted and not have to buy a stand-alone solar charge controller? Just thinking of future solar options.

Jon G.

10/2/2020

@BrettC Most definitely! That's one of my favorite things about the BCDC is that it can pull power either way, and it will actually prioritize solar power (as well as act like a charge controller). When you are hooked up to your tow vehicle it will pull as much as it can from solar and then the remainder from your truck. If you're looking to really trick out your battery system you might even like the Manager30 # 331-BMS1230S2-NA. This is definitely a top-of-the-line product so the price reflects this, but it also does more than the BCDC if you're looking for an end-all solution.

Brett C.

10/5/2020

@JonG Thanks for that link. I'll check it out. The other 2 items of concern are my RV batteries and my truck alternator. I'm assuming I have enough power in my alternator on my 2017 F350 PowerStroke to run the 50 amp charger. I have the upfitter switches and all those total something like 180 amps so I wouldn't think I should be fine powering a 50 amp charger. I'm going to go through a constant duty solenoid and just use an upfitter switch to through the solenoid on. Even the wire from the 40 amp switch looks pretty small to be of any real use. I could be wrong here. The other item is my RV house batteries. I currently run a single 12volt lead acid. If what I'm reading is correct, the 50 amp charger won't actually be trying to charge my battery at a full 50 amps as long as I have my inverter and fridge on. Should I be concerned about boiling the battery if I had the charger on with no load? Eventually I will go to lithium but small steps here financially unfortunately. Until then I'm considering installing 2 6volt lead acid golf car batteries from Sam's. I had those on another RV and I liked them. -Thoughts? And thanks again as always.

Jon G.

10/5/2020

@BrettC The alternator in your F-350 will be powerful enough to charge your trailer battery bank. Just be sure you follow the installation instructions as far as wiring and such goes to make sure everything is rated properly. If you have a relatively small battery bank then the 50A BCDC will be a little overkill but it won't fry your battery as it has the ability to sense what your battery bank needs and then charge it according to that. Hope this helps!

Brett C.

10/5/2020

@JonG I know it's overkill but the goal was to have enough power to run the absorption refrigerator on the inverter and not kill the battery in a few minutes. Thanks for answering all the questions. You've been a big help.

See All (9) Replies to Brett C. ∨

Tim M.

7/29/2020

Can the BCDC be mounted permanently on tow truck? I have a removable winch set up for my two trailers, one enclosed and one open. I'd like to be able to move battery and winch to either trailer, and charge battery. Thanks

Jon G.

8/6/2020

Ideally the BCDC will be mounted to the trailer, however it can be okay to put it on the tow vehicle if the circumstances are warranted.

Martin R.

7/5/2020

I have a 2018 F350 with (2) 220 amp alternators and a factory installed power inverter. Will this sufficiently charge my batteries in the 5th wheel while towing? FYI. I have a solar charger for dry camping, just wondering if the system in the truck will help maintain the charge through the 7 pin hook up?

Chris R.

7/9/2020

Yes, the current setup on your F-350 will work to maintain the charge on your trailer batteries via the 12V circuit coming through the 7-Way connector.

Martin R.

7/12/2020

@ChrisR Thanks Chris!

Chris R.

7/17/2020

@MartinR No problem!

See All (5) Replies to Martin R. ∨

Natalie R.

5/9/2020

I am a new RVer and I do not know how to charge RV battery from a vehicle. So your information provided a lot for me. Thank you very much!

Chris R.

5/12/2020

Awesome! I'm really glad we could help. Please don't hesitate to reach out if any questions come up.

Glenn M.

3/20/2020

I was thinking of somehow hooking up a tender through a plug to charge the trailer battery while disconnected from the towing vehicle. basically so it would always be charged. any ideas?.. thx,

Chris R.

3/20/2020

Is your trailer stored somewhere where shore power is available? If so, you can use a standard charger like the NOCO # 329-GENIUS10 to keep the batteries maintained. Otherwise you'll need a solar power setup to provide power for the charger/batteries.

Helpful Links

Battery-Chargers/NOCO/329-GENIUS10.html

Glenn

3/19/2020

didnt see this above, so Ill ask. can I keep my trailer battery charged via the trailer plug with something similar to a battery tender? would that work?.. I just dont want to put a hole in the trailer to run a cord through if that makes sense.. thx

Chris R.

3/19/2020

I'm not quite sure I can picture the setup you're describing. Would you be wiring the tender to the trailer's 7-Way for power? If you can give me some more details of what you have in mind I will be happy to verify it it'll work (or maybe offer a similar system that could be better).

Jeremy V.

3/16/2020

I have a 2016 f150 and am pulling an R pod with 2 -12 volt batteries... can not run the 12 volt refrigerator with out draining the batteries when towing.. a common problem with the f150s ... what do you recommend?

Chris R.

3/17/2020

The best option here is to add a BCDC charger like the Redarc # 331-BCDC1225D to your setup. The 12V circuit coming from your F-150's 7-Way is likely just enough to keep the battery maintained, but not nearly enough to actually charge up a battery and keep up with what it's losing to the fridge during travel. The Redarc unit will actually boost this power so that you can charge up the batteries to prevent them from being drained.

Helpful Links

Battery-Chargers/Redarc/331-BCDC1225D.html

George P.

3/14/2020

My utility trailer came equipped with the required DOT breakaway system. Shouldn’t the 7-way connection keep the battery charged ??

Chris R.

3/15/2020

Yes, the breakaway system is likely already wired so that it receives charge from the 12V pin on your tow vehicle's 7-Way during travel. This article is primarily talking about the larger auxiliary batteries on travel trailers that are used to power other accessories on trips. The 7-Way is sufficient for maintaining smaller breakaway batteries.

Frank G.

3/13/2020

On one of our trailers, we have a solar charger,in conjunction with the trailer emergency brake battery. Havent seen another like it. What might be out there?

Chris R.

3/14/2020

I could be wrong but it sounds like you're describing something like the Performance Tool Solar Battery Charger # PTW2997. This is a small 2.5-Watt panel that could hook right up to the breakaway battery and is easily mounted wherever you have space. Just let me know if I'm way off here and I'll be happy to keep looking!

Helpful Links

Battery-Chargers/Performance-Tool/PTW2997.html

Kemp B.

3/13/2020

How can I keep my rv batteries from dying when not in use. They totally discharge and I have to replace them yearly

Chris R.

3/14/2020

Great question. If you have access to shore power when the trailer is in storage (or are able to actually bring the batteries home) a great solution is a charger like the CTEK # CTEK56353, which provides long-term maintenance charge so they're ready to go when camping season starts again. If the batteries will be left in the trailer and you don't have access to shore power your best bet is a solar panel setup like with the Go Power # 34273836.

Helpful Links

Battery-Chargers/CTEK-Power-Inc/CTEK56353.html

RV-Solar-Panels/Go-Power/34273836.html

Joe J.

3/13/2020

it seems easier t’o plug trailer power cord into my small generator and recharge batteries that way. Is there a reason this would not work?

Chris R.

3/13/2020

Just in terms of the initial setup, connecting a portable generator to the trailer power cord is indeed simpler. It's a feasible solution but you have to be sure the generator is somehow stored outside or in a ventilated area for exhaust purposes and that it's tied down to limit as much movement as possible. Too much moving around could trigger a low-oil shutoff or lead to similar issues. It's also of course a good way to run through your gas supply.

Jerry S.

3/13/2020

above it states not to let your battery below 50 percent think some people would think that 50 would be half of 12 volts = 6 volts. when 1/2 is actually around 12.2 volts.Suggestion, include the correct numbers so customers are not confused

Chris R.

3/13/2020

That's a really good (and correct) point. We really appreciate this kind of feedback.

Claude N.

3/13/2020

Can a portable generator, running in the the back of a pickup be used while traveling to provide A/C power for a battery charger or power for electric heat while driving?

Chris R.

3/13/2020

You could technically run a generator in the pickup during travel, but it's not something I recommend. For starters the movement/shaking while on the road could negatively affect the generator itself. If it has a low-oil shutoff this could be triggered from the movement it undergoes. The best solution would be an inverter like # WC3720 to provide AC power and you could use a BCDC charger like the Redarc # 331-BCDC1225D to keep the aux battery charged.

Helpful Links

12V-Power-Accessories/Wagan/WC3720.html

Battery-Chargers/Redarc/331-BCDC1225D.html

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