Will my truck charge my trailer battery?

In this blog post, we will answer the following question: Will my truck charge my trailer battery? We will explain how you can recharge your trailer battery and why you may need an alternator. 

Will my truck charge my trailer battery?

Yes, your truck can charge the trailer battery while driving, but be careful; it will not be able to recharge a drained trailer battery!

Here’s the process of recharging a trailer battery with a truck explained in a few key points: 

  • First of all, a “7-socket connector” is the socket that brings electricity from your pickup to the front lights of the trailer, the reversing lights, as well as the taillights, stoplights and cornering lights. 
  • Second, there is also a connection between the “ground” and the “battery”. This last connection is there to allow the truck’s charging system to direct the voltage to the “house” battery of the carried trailer- the one used for the power of the interior lighting, the operation of the water pump, and the battery’s other low voltage equipment.
  • The circuit (if properly wired and fitted) will feedback current from the truck’s charging system and recharge the trailer’s battery. But due to the “length” of wire from the truck’s engine bay to the trailer battery’s location, this could be a really bad way to recharge the battery, as the distance would be too long to bring a recharge of any value.

We can illustrate it this way: suppose you want to fill your tank with freshwater; do you prefer to use a garden hose to fill it or a multitude of interconnected straws? If you had plenty of time, you might be able to fill the water tank with the straws, but the small diameter would seriously limit the speed at which the water would flow!

It’s the same with electricity. The wiring from the alternator to the 7-way connector on the truck’s bumper is thin, thin, thin! You, therefore, need a larger diameter “hose”! This is generally true for the wiring from the main connector to the battery of the trailer.

How to charge the trailer battery using an alternator

Here is what some RVers have done to conveniently charge the battery with the alternator:

  1. First of all, use a battery isolator: this device allows current to flow between the loading system from the truck to the trailer carried when the engine is running. This prevents the mounted trailer from “siphoning” the battery from the truck, leaving you in a secluded area with no way to start!
  1. From the insulator of the trailer, install a 12-volt circuit breaker to protect the circuit. This protection should be based on the output current of your truck alternator.
  1. Pass a single large gauge wire (8 AWG or larger) from the circuit breaker (circuit breaker) to the truck frame to the bumper. Secure it well with metal ties and keep it away from moving parts and components of the exhaust system.
  1. At the bumper, attach a solid electrical outlet with a twist lock rated at 30 amps or more, for the “current”. At the same time, connect another wire to one of the other terminals in the 7-way connector and connect the free end of the wire to a part of the truck that acts as a good ground.

A corresponding electrical plug is used to connect to the side of the trailer being carried. Use this wire of the same size, you will need two conductors and run them to the “house” battery of the carried trailer.

  1.  Ensure the current polarities by keeping the positive to the positive and the negative to the negative! It would be good to find a “blanket” that rests on the plug pins of the trailer because it will always be “on”. Covering the pins will prevent them from “shorting” against something. Your imagination will help you find the outlet cover according to the size of the plug.
  1. Now test your new installation with a digital measuring device: with the engine stopped (off) and the plug connected, test the voltage of the “house” battery. Now start the truck’s engine and within a minute or two, the voltage at the “house” battery should show a higher reading.

How are trailer batteries recharged?

You can recharge the battery of your trailer:

  1. While driving (thanks to the engine alternator)
  2. By plugging the motorhome into a 220-volt outlet (in a service area for example)
  3. By alternative means such as solar panels, a fuel cell, or even a wind turbine.

Note that ideally, the trailer battery should be recharged from the mains once a month minimum, for 24 to 48 hours to optimize the life of the battery charge.

When we start the trailer engine, the motor automatically charges its battery and when it is charged, it sends the current to the auxiliary battery in the passenger compartment.

Assuming (as usual) that the engine battery is always charged, the full recharge time of the auxiliary battery is approx. three-quarters of an hour to an hour, (depending on the type of vehicle, the type of battery, the alternator, etc.). and if we have two auxiliary batteries, the time would be about an hour.

It should be borne in mind that if you drive at night it will take a little longer to recharge the auxiliary batteries since part of the current produced by the vehicle’s alternator is spent on the main or population lights, since as we said before, the engine preferentially charges the Its own battery and when it has already been charged, it sends the current to the auxiliary or auxiliary ones.

A second possibility is when we are at a camping site, they are recharged with a battery charger inside the trailer (some trailers have it already installed and if it cannot be installed easily) to use it when we connect the trailer at a camping site.

Now you must bear in mind that these chargers take much longer than the vehicle’s engine to charge them, they are usually 6 or 8 A. So the charging time for a 100Ah battery is 12 to 16 hours minimum and if we have two batteries, double it.

If you want to recharge faster, you must install a special 20A electronic charger.

What is important to know is that the vehicle’s alternator charges the passenger compartment auxiliary battery (s) when the engine is running Provided that these batteries have some charge. 

If they have been completely discharged for some strange reason, there is a Relay that does not open and then they do not recharge, with which you run out of Light inside the trailer. 

It can be a problem because if you are travelling, you have no choice but to enter a campsite and connect the motorhome to 220 V. The charger inside partially charges the auxiliary battery, takes a little charge and can with the engine start to continue charging it.

We recommend that you install a manual Bypass, so if this happens to you, it passes through the Bypass for a couple of minutes or three currents to the auxiliary battery and you can start the engine and recharge it normally with it.

Final  thoughts and tips

We advise you to check the condition of your trailer battery frequently. If you spend a specific time without driving the trailer, it is recommended to disconnect the terminals so that there is practically no consumption.

Suppose you do not want to disconnect the terminals. In that case, you have to start the engine occasionally and it is important to check that the entire passenger compartment electrical system is switched off.

If you need personalized advice on equipping your trailer with a battery, converter, generator or even a solar panel kit, please feel free to contact us! 

FAQ on Will my truck charge my trailer battery?

How long does it take to charge an RV battery with a truck?

Charging an RV battery to its full capacity with a truck can take up to 10 hours for a minimum capacity, and up to 40 hours for a full charge. Of course, this assumes that the battery was completely empty. It will also vary depending on the RV battery’s capacity and use. The best way to recharge the RV battery is via AC power.

How long does it take to recharge a trailer battery?

Recharging a battery to its full capacity can take up to 40 hours. Of course, this cooldown assumes it’s completely empty. It will also vary depending on its capacity and use. The best way to recharge the battery is via AC power.

What is a smart battery charger?

Smart battery chargers are based on the latest developments in technology and aim to simplify the task of users. Also, this type of charger analyzes the type of battery and its condition throughout the charging process.

What battery to put on a trailer?

You can use a 12-volt battery or two 6-volt batteries installed in series or in parallel for a trailer. In both cases, they are deep cycle batteries fitted with lead-acid batteries and equipped with 2.1-volt cells.

What charge for a 12V battery?

A battery must always be charged, the charge must be stopped between 2.25 V and 2.4 V per cell. Either: Between 6.75 V and 7.2 V for a 6 V battery, and between 13.5 V and 14.4 V for a 12 V battery.


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