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What is a DC to DC battery charger?

Tom Vondrasek | 6th May 2023 | 5 minutes to read

The DC-to-DC battery charger is gaining in popularity as more people add auxiliary batteries to their vehicles to run winches, fridges, and a host of other 12V devices. They are in most caravans and camper trailers that have 12V batteries. These 12V batteries need to be maintained and are used to provide power when 240V mains power is not available.

If you’re looking to charge an auxiliary battery, then a DC-to-DC charger is definitely worth looking into. However, there are a number of considerations when purchasing a DC-to-DC charger and it’s important to know how to distinguish it from other similar devices.

Let’s take a look at how a DC-to-DC charger works and how to choose one:

  1. How Does a DC-to-DC Charger Work?
  2. Is a DC-to-DC Charger the Same as a Power Inverter?
  3. How Do You Select a DC-to-DC Charger?
  4. Solar Panel Recharging
  5. Do You Need a Battery Isolator or VSR (Voltage Sensitive Relay) When Using a DC-to-DC Charger?

How Does a DC-to-DC Charger Work?

DC-to-DC chargers take a DC input from a vehicle alternator or solar panels and convert it to a DC output used to charge your auxiliary battery. The auxiliary battery is normally a deep cycle battery. This output can be higher or of greater strength than the input voltage and current. It is not a constant output and is based on the battery’s need at the time. It monitors the battery and adjusts its output accordingly. Whether it needs a fast charge to get the half-flat battery up or just keep it topped up if it is already charged.

They came about as auxiliary batteries got bigger and needed a greater charge to keep them topped up. The other issue was a vehicle’s alternator can only produce so much charge. The alternator’s main role is maintaining and keeping the starting battery ready to go, so it needed some help when an auxiliary battery was connected as well — the DC-to-DC battery charger fulfils this role perfectly.

DC-to-DC chargers need to be mounted close to the battery they are charging. This allows the auxiliary battery to be mounted anywhere in the vehicle or caravan as they can work on lower voltage and current caused by distance from the alternator on the charger’s input. It then allows the full output charge to go to the battery. The further the distance from the alternator, the more attention needs to be paid to the diameter of the wiring connecting it up —too thin of a cable can choke the power getting to the charger. 

Is a DC-to-DC Charger the Same as a Power Inverter?

Don’t confuse a DC-to-DC battery charger with a power inverter. Power inverters convert the 12V DC to 240V AC so you can plug in 240V household appliances. Most large caravans and campervans will have one of these as well as the DC-to-DC charger.

How Do You Select a DC-to-DC Charger?

The important facts you need to know are your battery type (Lithium, AGM, Flooded) and its ampere per hour (AH) rating. Different battery types like a gel or AGM battery can handle a higher amp recharge than a normal wet cell or flooded battery. The usual figure to use is 20% of an AGM or gel battery's capacity or 10% of a flooded or wet cell.

A Lithium battery is completely different again and you need to have a charger that is capable of recharging them. Do not use a lead acid DC to DC charger on a Lithium battery unless it has a specified setting for Lithium batteries.

Solar Panel Recharging

Most good DC-to-DC battery chargers allow an input for solar panels to help recharge your auxiliary battery. Most caravans or campervans have solar cells to help keep the auxiliary battery charged during the day. They are also getting popular for in vehicle auxiliary batteries. Some of the smart DC-to-DC chargers won’t take power from the alternator if the solar panel is recharging the battery.

You can mount solar panels to your vehicle roof which is gaining in popularity. A lot of people prefer portable solar panels that can be moved around to follow the sun. They can be plugged into your vehicle through an external connector or use a lead that unwinds to reach the solar panels. There are a lot of different ways to do this, the important thing is to make sure you use the correct size cabling.  The other factor is cable length. The shorter length the better (less electrical resistance) though it still needs to be usable and practical.

DC-to-DC charger from RedArc with instructions

A DC-to-DC battery charger from RedArc.

Do You Need a Battery Isolator or VSR (Voltage Sensitive Relay) When Using a DC-to-DC Charger?

The answer is no, you don’t. The DC-to-DC charger performs the role of both of these units. It will not drain the start battery to keep the auxiliary battery topped up.

Wiring Kits

For those that are not that confident with sorting out electrical wiring from a parts list on a website, Redarc can help. Redarc makes a number of different kits to wire in a DC-to-DC charger and auxiliary battery. These simply cover wiring from the start battery to the auxiliary battery incorporating the DC-to-DC charger. One is rated to 25 amps and there is a more heavy-duty version that goes up to 50 Amps.

They are for in-vehicle installations and both versions cover:

  • Having the auxiliary battery on the same side as the start battery in the engine bay.
  • Having the auxiliary battery on the opposite side of the engine bay to the start battery.
  • Having the auxiliary battery in the back of the vehicle.

You still need to wire from the auxiliary battery to your device, such as a winch, fridge, etc.

Repco Have Your Covered for Battery Chargers

At Repco we have a great range of DC-to-DC battery chargers to suit your needs and budget. We stock chargers from popular brands like Redarc, Projecta, and CTEK, so you are getting quality products to use on your auxiliary battery project. Check out our range of deep cycle batteries as well. Shop online or in store to and keep an eye out for our specials. If not sure, contact our friendly store staff before purchasing your DC-to-DC battery charger.

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