DC to DC Battery Charger for Solar Systems

In this article, I will explain what a DC to DC battery charger is and why you need one.

These chargers are mainly used when you want to use your car alternator as a power source in addition to your solar panels.

What is a DC to DC battery charger?

This is the first question you might have. Let me explain it.

A DC to DC battery charger is a device that charges your auxiliary battery from your main battery.

Most of the times, a DC to DC battery charger is used by RV’s, vans, or motorhomes. When the vehicle drives, the alternator of the car will charge the main car battery which is used for starting the car.

Simplified schematic for a DC to DC battery charger
Simplified schematic for a DC to DC battery charger

Now, if you were to wire the starter battery directly to the auxiliary battery, you might drain the starter battery down until you cannot start the engine anymore.

That’s why you should install a DC to DC battery charger. It enables the starter battery to remain fully charged before charging your auxiliary battery. This means that you will always have a fully charged starter battery. The excess current that is generated from the alternator gets redirected to your auxiliary battery.

Sometimes DC to DC battery chargers are referred to as a BCDC. This is the abbreviation of: Battery Charger Direct Current, which has the same meaning as a DC to DC battery charger.

Why not charge directly from the alternator?

You can say, why would I not wire the battery straight to the alternator?

The car alternator is not made to charge a deeply discharged battery. The alternator might not be capable of delivering such a high current demand. This will lead to increased wear and overheating of the alternator.

Victron Energy, a popular solar system manufacturer, did a test where they charged a lithium battery with an alternator.

What they found is that charging a lithium battery with a car alternator is not always the best solution. At lower RPM’s the alternator was overheating. This is due to the following facts:

  • A lead-acid battery has a larger internal resistance, so the charging current will be limited.
  • A lithium battery has a low internal resistance, therefore the charging current is not limited and will lead to burning out of the alternator.
Overheating an alternator while charging lithium batteries on low RPM
Overheating an alternator while charging lithium batteries on low RPM (Source)

A DC to DC battery charger can be used to reduce the charging current without damaging the alternator.

How many amps does a DC to DC charger draw?

The amount of amps a DC to DC charger draws depends on the rating of the device.

Some chargers are 20 amps and some are 40 amps.

I recommend using 20 amps if you have one battery. Only use the 40 amps charger if you have more than one battery.

This is because 20 amps is the ideal charging current for a single battery.

Power consumption while idling

Another factor to consider is the power consumption while the device is idling.

When your vehicle is parked, you don’t need to run the DC to DC battery charger. Some chargers have a built-in feature that when the car engine is off, the DC to DC battery charger is turned off. This can be done by running a wire to the car fuse box as will be shown later in the wiring diagram.

Both Renogy and Victron DC to DC chargers have this option.

Other reasons to choose for a DC to DC battery charger

DC to DC battery chargers has custom charging profiles (Bulk, Absorption, and float) for different types of batteries.

Bulk, Absorbtion, and Float stages of battery charging
Bulk, Absorption, and Float stages of battery charging

This means that you can charge all kinds of batteries like Flooded, Gel, AGM, and Lithium. In some chargers, you can adjust the voltage levels to your liking for a custom charging profile.

How do you hook up a DC to DC charger?

In this image, you can see the circuit diagram for a DC to DC battery charger.

DC to DC battery charger circuit diagram
DC to DC battery charger circuit diagram

Recommended DC to DC battery chargers

There are two main manufacturers who make the Dc battery chargers, they are Renogy and Victron.

Renogy has a 20 Amp, 40 Amp, or 60 Amp version available.

Click here for prices on amazon.com

Victron has many versions available for 12, 24, and 48 Volts the smart edition is able to be connected to your phone through Bluetooth so you can manually program it.

Click here for prices on amazon.com

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