Dual Battery Setups: Why do I need a DC-DC Charger?

Posted on October 29, 2021

Electricity In Your Car – The Basics

After it’s been used to start the car, your starting battery isn’t used to run the electronics in your car. That’s actually the job of the alternator. Your vehicle’s alternator is a small generator, generating electrical power from the mechanical energy of the motor – much like a wind turbine. This electrical power is used to run your vehicle’s accessories, like the lights and radio, as well as recharging the starting battery.

So, the alternator is the source of your car’s electricity while it’s running. How do we harness that power to charge an auxiliary battery?

Devices For Charging Your Auxiliary Battery

There are two options if you want to charge a battery from your alternator: using an isolator or a DC-DC battery charger.


An isolator is a simple device that ensures you won’t flatten your vehicle’s starting battery. It does this by only allowing the auxiliary battery to be charged from the alternator after the starting battery is adequately charged.

However, that’s where the positives stop. Isolators rely on the alternator to manage the current being delivered to the auxiliary battery – and the alternator is only designed to charge a starting battery. Starting batteries have different structure and chemistry to auxiliary/deep cycle batteries, which require multi-stage charging to fully charge. Therefore, with an isolator, your auxiliary battery won’t fully charge, and over time, this will reduce the battery’s lifespan.

Isolators also don’t work well with newer car models that have smart alternators. Smart alternators are designed to produce minimal extra energy, and therefore, there’s less energy to charge your auxiliary battery.

So, using an isolator isn’t a great option, especially if you have a smart alternator. You also can't use an isolator with a lithium battery.

DC-DC battery chargers (a.k.a. B2B or battery-to-battery chargers)

Like an isolator, a DC-DC charger also ensures you won’t flatten your vehicle’s starting battery. However, it has some advantages over isolators. DC-DC chargers prompt the alternator to generate more energy, boosting the current coming from the alternator and therefore charging your auxiliary battery faster.

They also protect your alternator from overworking, and deliver the multi-stage charging needed to full charge your auxiliary battery. This maximises your auxiliary battery’s lifespan.

The downside to DC-DC chargers is that they are more expensive than isolators, but we recommend them as they are a much better performing product over the short and long-term.

How Many Amps Do I Need In A DC-DC Charger?

Either take this directly from the recommended charge current on your battery’s specs, or you can calculate it:

  • AGM/deep cycle batteries: Ah capacity of the battery x 0.2 For example, a 100Ah AGM will match with a 20amp DC-DC charger.
  • Lithium batteries: Ah capacity x 0.5 For example, a 100Ah lithium with match with a 50amp DC-DC charger.

If you have multiple batteries connected in parallel, you add together the Ah capacity of all the batteries and the multiply it by either the 0.2 or 0.5, depending on which type of battery they are.

Check out our Dual Battery Kits here.