Charging Trailer Battery From 7 Pin Plug: What To Know

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Charging your trailer battery with a 7 pin plug is a great way to keep your battery topped off as you drive your tow vehicle. It can also work even if you are not driving your truck and are connected to a trailer, camper or travel trailer. In this post, we will review:

  • How you charge with a 7 pin plug
  • Charge your battery while driving
  • Charge your battery while not driving

We will review the advantages and disadvantages of each method and how well they work.

Table of Contents

How you can use 7 pin to charge your battery

Each pin of your connector has a special function and job related to giving power as needed. For starters, the 7 pin connector uses various pin connectors to wire certain components. This means it’s possible to power functions such as reverse lights, brake lights and brake controller activation without having to rewire your vehicle or trailer separately.

One of the pins is specific to providing constant 12V power to your battery. You can check this with a volt meter to show you are getting power even when the truck is not turned on.

Pin with constant 12V power supply

This pin is powered directly from your truck battery and is constant power as opposed to brake and running lights. These are pins only used as needed when driving. So when you connect your trailer to your truck, you create a battery charger. This may sound great, and it will fix all of your RV battery issues. However, there are limitations in voltage that we will discuss further.

Voltage

Both your tow vehicle and RV battery are 12V. Whether you have a deep cycle battery or even an AGM battery, your voltage is the same at 12V.

When you are charging a battery from another battery, shore power, or even a battery charger, power flows to the dead battery by a wire. In the case of being connected to a charger or shore power, there is a constant flow of power going to the weak battery.

When you are connected only to another battery and do not replenish that power, the weak battery will eventually take only so much before both batteries are equal in voltage. So depending on how weak your battery is, you could end up with two weak batteries.

For example, if you have a good 12v battery, connect it to a battery with only 8v. Eventually, a good battery will give it power until it equals 10V (12v+8v / 2 batteries = 10v ea). And then you have two batteries now that won’t work. More on this is below.

Charge Your Battery While Driving

Charging your trailer or travel trailer battery while driving is best if you use your 7 pin to charge.

As you drive, your alternator (on your tow vehicle) provides constant power, usually around 14.5V. With a 12V system, you can safely operate from 12V to just over 14.5V. Your alternator acts as a battery charger, just like a trickle charger that you may have at home.

Advantage

Your advantage to charging while you drive is that you have constant power going to your trailer or camper battery. As you reviewed above, charging a battery transfers power from more voltage to less. So with constant voltage going to the weak battery, it will eventually charge and get the 12V to 14V it needs to be considered fully charged.

Disadvantage

Unless you drive for hours, you may not have enough time to allow your trailer battery to take charge. It takes hours to top off a weak battery, so if you are only traveling for an hour or two, that battery charging may not be enough.

Charge Your Battery While Not Driving

Can you charge your trailer battery while your tow vehicle is not running? The answer is “yes.” But there are some important things to consider so you are not stranded with no power in either battery.

Above, we reviewed that you are always getting 12V power from the power pin of your connector. This pin is connected by a wire going straight to your vehicle battery. It is a direct connection with no added power. So it is like getting jumper cables and connecting 2 batteries together. For more info on that, see my post on how to use jumper cables to charge your RV battery.

Vehicle battery designed for burst energy for starter

Another thing to consider is that your truck and trailer batteries function differently in how they provide power. Both batteries indeed provide the same 12V voltage. However, your truck is a starting battery designed to give a burst of energy to get it going. At the same time, the trailer has a deep cycle battery meant to use power consistently and to be able to drain down almost fully. A starting battery is not meant to provide constant power. That is one reason your alternator is important: it gives that constant power, not the actual battery.

Advantage

The advantage to charging while not driving is that you are not consuming gas or needing to drive around unnecessarily. It’s a quick way to get some power if needed

Disadvantage

You are taking battery voltage from a good battery and transferring it to a dead battery. And without a constant flow of power being fed to the tow vehicle, you risk having two dead batteries. Without the tow vehicle running, it won’t be long before the truck battery gives any reserve power to the dead battery and leaves you with a vehicle that won’t start.

You also only have limited capacity to charge your trailer battery. Since you are transferring from one battery to the next, there is a limited charge, and you will likely not have enough charge to do much with your trailer or camper.

Final Thoughts

Charging your trailer battery from a 7 pin plug can be a good idea and get you out of trouble. Although your camper and tow vehicle operate on 12V, it’s important to know that they function differently in how they provide power. Your trailer is a deep cycle battery, while your truck has a starting battery designed to give a quick burst of energy to start. Knowing the advantages and disadvantages of charging while the truck is not running is also important. Also, you risk being stranded without that constant power from your alternator to your tow vehicle. Safe Travels!