Can One Alternator Charge Two Batteries?

Due to how fast your car runs out of power because of the car’s electrical component, you may be wondering if you can add an extra battery to the car and how to keep the battery charged. The big question is can one alternator charge two batteries?

The answer is yes, the alternator in your car can charge two batteries. The alternator protects itself from the excessive current drain, registers the two batteries as one large battery, and charges both equally. This means the alternator is effective for multiple batteries.

Running a dual battery system will require a battery isolator, which helps ensure that the alternator charges both batteries smoothly.

What is a battery Isolator, and why do I need it?

A battery isolator is a device that disconnects the batteries when the alternator is not charging the battery. This helps keep the batteries from draining each other. Different conditions can allow batteries to drain each other such as the size and age of the batteries and the discharge rate of the batteries. An isolator stops the main battery from draining the idle battery overused. Also, it helps the batteries work the way they are supposed to and thereby lengthen both batteries’ lifespan.

car alternator

Ways of setting up the two batteries

Knowing how to set up the two batteries effectively can save you time and money. The ways to set the batteries up are highlighted below:

  • Add the second battery to the car: choose a good spot for the second battery. You should install a battery tray to place the battery on to hold it securely in the engine.
  • Add an Isolator to the car: An isolator has three terminals that help it to connect the two batteries and the alternator. Disconnect the original battery from the car and plug in the battery positive cable to one of the terminals on the isolator. This creates a three-way system of isolator, battery, and alternator. 
  • Connect the second battery to the alternator and isolator: connect a cable to the isolator’s second terminal and connect it to the alternator. After that, use another cable to connect the positive side of the second battery to the last terminal on the isolator. This means the three terminals on the isolator are connected to both batteries and the alternator.
  • Connect the two batteries: finally, to complete the circuit and make current flow in the system, connect the first and second batteries’ negative terminals to allow the alternator to deal with both batteries as one. Make sure all the connections are firm and secured. This helps the two batteries with the isolator and the alternator.

Do the two batteries have to be the same?

While this is not recommended, the two batteries should be the same type and age. First, batteries of the same type and age have similar resistance, which helps them charge equally and does not pose extra stress on the alternator due to the difference in resistance. Also, batteries of different types have different properties, which carries a greater risk of the batteries discharging each other over time. Lastly, batteries connected without an isolator have the risk of catching fire under a load of they are of different types and ages.

How to charge the extra battery while driving

There are a few ways to charge the extra battery, but the easiest is the alternator while driving. You can charge it using a voltage sensory relay or an isolator switch, which is sometimes referred to as a split charging setup.

How quickly does the alternator charge the battery?

Most car batteries range from 40AH to 110AH, and alternators range from 45amps to 200 amps. Depending on the car’s RPMs, an alternator can charge a battery full from 30 minutes upward.

car alternator in an engine

The merit and demerits of having two batteries

Although having two batteries helps out a lot, some demerits are also associated with using them. They are listed below:

– Boost in-car general performance
– No trouble with the ignition
– Enough power for electrical components
– Battery lifespan extended
– Easy to jumpstart a car with the second battery
It can be very expensive
– Incorrect setups can lead to a fire
– There is additional weight to the car
– More strain on alternator, especially without isolator

Tips to avoid damaging the alternator with the extra battery

The alternator will have extra work to do with charging two batteries, and this can cause alternator damage. There are some tips you can use to help prolong the life of your alternator. They are:

  • Make sure the batteries are not always in a state of low charge and don’t run the devices that use the batteries all the time
  • Using a battery isolator
  • Using a high-powered alternator to help distribute high power charge more evenly to the batteries
  • Don’t run the alternator for very long hours at a time.


An alternator can charge two batteries at the same time. However, it can be risky if the setup is not done properly or uses different batteries. Also, an isolator can help prolong the life span of the alternator and the batteries, so it is better to use it. Consider the merits and demerits before setting up a second battery and follow the necessary setup based on your decision. 

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