Charging two batteries from one alternator

If you have a pickup truck and you find that you need more reserve battery power than you currently have, then you might want to consider adding a second battery.  If you do add a second battery, then you also need to know about charging two batteries from one alternator .

If you use your truck for activities like camping or off roading, or if you have a need to use a winch on a fairly regular basis, then you should definitely consider adding a second battery to your truck.

Another situation that would benefit from a 2nd battery is a car that has a very large, powerful stereo system in it.  The large stereo systems require a LOT of power to operate, so a 2nd battery would be a MUST HAVE.  In this situation, the 2nd battery would most likely be installed in the trunk of the car or inside the cab.  Usually, there isn’t enough room to add a 2nd battery to the engine bay of a car.

Second battery located in the rear of a car.

Adding a second battery also makes sense when you use accessories like lighting, or maybe a laptop in the front seat of your car/truck for work purposes.

To be clear, adding a second battery will increased the amount of reserve battery power that you have available.  This means that you can power items for longer duration with your batteries when the car/truck is not running.  When it is not running, all powered accessories are being powered by your batteries.

IF you are camping, and you have a truck bed camper or travel trailer hooked up to your truck, you can use the trailer powered items for longer duration without ever worrying about draining all the power from your truck battery/batteries. This is VERY important as you don’t want to run out of battery power, thus leaving you stranded without any battery power available to start your truck when you want to leave.

NOTE: If you are looking to increase the amount of electrical power available while the car or truck is running, then you would need to also look at upgrading to a high output alternator

So, with all this information in mind, I will talk about charging two batteries with one alternator.

The best way to accomplish this is by purchasing and installing a battery isolator .

A battery isolator is a device that you use to connect your two automotive batteries into one system so that you can charge both batteries with your alternator while driving. However, the isolator will only allow you to use one battery for your trailer, or for accessories that need power when your car/truck is not running.

This is very handy for camping, as you can run the trailer lights & accessories off of the one battery, and it will not affect the second battery in the truck at all. That way you will ALWAYS have a fresh battery to start the truck when you need to drive somewhere.

You CAN wire the two batteries in your truck together WITHOUT an isolator, but I would STRONGLY recommend that you do not do this. Wiring the two batteries together (without an isolator) like this can lead to dead batteries, and then you are left stranded with no electrical power available to start your car or truck.

I would highly recommend a battery isolator for charging two batteries from one alternator.

There are three different kind of battery isolators available – I will list them all below.

However, you might find it easier to just watch this short video that explains the differences between the three different kinds of battery isolators.

Various battery isolators and their characteristics.

This is by far the simplest type of truck battery isolator. It is simply an electro-mechanical relay that is similar to the relay for the mechanical starter for your car/truck.

There is a switch inside this type of isolator, and this switch moves into the closed position to allow current from your vehicles alternator to charge the two batteries (when the vehicle is running).

When the vehicle is not running, the switch inside this type of isolator remains open, which allows only ONE battery to be available for the travel trailer, camper, or accessories to run off of. This keeps the other battery fresh and charged and available to give power to start the automobile.

The solenoid/relay style of battery isolator is the least expensive, however, they do not last as long as the other styles of battery isolators on the market.

2. Solid State Battery Isolator

These types of battery isolators use electrical diodes to isolate the batteries. These diodes act like a one-way valve to create the isolation effect.

These isolators are very strong and will last a long time. They are easy to install and they are available in many different amperage ratings so make sure you get one that is strong enough to work with your vehicle’s alternator.

If you are not sure what amperage your vehicle’s alternator is rated at, just go ahead and buy the highest amp rated battery isolator so that it will work no matter what.

3. Electronic Battery Isolator

This style of battery isolator is newer to the automotive market.

These are excellent units if you plan on long term travel in your travel trailer or truck camper.

They allow for the strongest current possible when charging the batteries from your car or truck alternator. Most models have an easy to use bypass switch that you can utilize to help start your vehicle if there ever was a problem with the primary ‘starting’ battery (using whatever power is left in the secondary battery).

Under normal operating conditions, the electronic isolators detect the level of charge in the primary battery (the one that is for starting your vehicle) and charge it until it is fully charged. It then switches over to charge the secondary battery until it is fully charged.

This priority charging is a VERY HANDY feature to help optimize the battery levels of both units.

They are more expensive than the solid state style of isolator, but for good reason. They provide the most optimal charging situation every time.

How to install a 2nd battery in your car or truck

If you require a 2nd battery in your car, you will most likely have to locate that battery in the trunk, or somewhere inside the car itself. I would HIGHLY recommend the trunk, since these batteries use acid to function, and sometimes they can get hot and some acid may seep out of the battery (fumes or slight spillage).

Below is a video explaining how to install a 2nd battery and an isolator in your car, for audio/stereo system purposes.

Installing a 2nd battery and isolator for a car stereo.

If you require a 2nd battery in your truck, you can most likely install it in the engine bay as most trucks have the space available for a 2nd battery there.

This makes the whole install easier as there is a LOT less cable required since the two batteries, the alternator, and the isolator, are all within a couple feet of each other in the engine bay.

If you are looking to install a 2nd battery in your truck, here is a great video that explains step by step how to install the 2nd battery with an isolator.

How to install a 2nd battery in a truck.