Ash and I have been living fulltime in a van now for the best part of five years. Over the course we have slowly refined what is essential to a way of living that suits us. Not everyone wants the same and there are many options to choose when it comes to having power when you need it.


We started off without a secondary battery or any electrical in our van whatsoever, except the door light that would turn off every 2 minutes or if left on, might run the main battery flat. There was always a torch somewhere too if you could be bothered digging around for AA+ batteries that might have some charge left in them. We eventually got our hands on a auxiliary/service battery and had to figure out how to keep that battery charged. We did some research and initially decided on an isolator switch over a DC-DC battery charger. Price was a factor and we picked up an isolator switch for one hundred and twenty dollars. At the time we had just purchased a little 35L chest fridge. Over the next year it became apparent that the service battery could not hold the charge required due to the fridge running all the time. Our perishables did exactly that. Perished. Over and over. After chatting with specialists, friends and spending time on forums, we decided to invest in a DC-DC charger.

What’s the difference between an isolator and a DC-DC charger?

An isolator switch is cheap and simple. It sends a charge to your secondary battery while you drive. They come in automatic and manual, where you have the option to turn it on and off when you are needing to charge up the service battery. On the negatives, it requires larger cabling to your service battery and will never charge your secondary battery to 100%, especially if it is AGM. The alternator sends one level of charge to your secondary battery, this limits the level of charge your service battery receives because the isolator can only transfer the charge it receives. The service battery might only charge up to 85% of it's full capacity. Meaning it will drain more quickly and it also takes longer to the recharge battery.

A dc/dc charger acts as both an isolator and a smart charger. It allows a smaller cable to be run (6-8mm B&S twin) and has the ability to step up and down voltage. The dc-dc charger will adjust the amps down in the power that it delivers to your battery if it approaches full charge (same as a multi stage charger does that you use at home). This allows it charge up to 100%. It can also tailor the charge to the type of battery whether lead acid, gel or AGM. They are usually a 3 or more stage charger which will charge the battery faster and be better for it in the long run.

We decided on CTEK’s D250SA DC-DC BATTERY CHARGER, which represents the latest in technology from Sweden, providing maximum performance for our dual battery set up and comes with a two year warranty. The D250S is suitable for any dual-battery setup including vans, RV, cars, 4WDs and boats and is an ideal charger for vehicles without access to mains power.


We have Mercedes Sprinter with 2 X 130 Amp hour deep cycle AGM Batteries. We want these batteries to stay charged up so they have a longer shelf life and we can maximise the power usage. The D250S also has a temperature sensor that optimises the charge voltage by increasing the charge voltage in cold temperatures and reducing it in hotter temperatures. This function is always active and handy in Australia’s diverse climate conditions. The D250S charges your batteries while you are on the move and has a fully automatic 5 step charge that charges, conditions and maintains any 12V lead acid service battery from 40-300Ah. It can charge the secondary battery either from the alternator, solar or a combination of both and provides a little eye candy not just for the tech nerds.

Here is a basic run through of how to install.

- Firstly run your 12mm B&S cable from the main battery to where you auxiliary battery/ies are housed. (Honestly, running the wire cabling throughout the van is the hardest part of installing this charger.)

- Mount the D250S charger in a location as close to your the service/auxiliary battery as possible. (The closer the charger is to the service battery, the less voltage will drop in transfer, charging the service battery quicker)

- Run out a fused cable from the main starting battery ‘+’ to the ‘IN’ terminal of the D250S.

- Run a fused cable from the ‘+’ terminal of your service/auxiliary battery to the ‘OUT’ terminal of the D250S.

- Run wired cable from the ‘-’ terminal of the starting battery to the ‘-’ service/auxiliary battery then onto the ‘-’ terminal of the D250S.

- Place the battery temperature sensor close to the service/auxiliary battery.

- Connect the solar panel inputs.

- Attach all positive and negative points.

- Once it is hooked up start the vehicle and check that the indicator lights on the D250S are illuminated as described in the instructions. It’s a super easy and straightforward setup of a very efficient dual-battery system. No need for relays or solenoids, just a couple of fuses, some length of 8mm B&S wire cabling and you’re away.

(We recommend using only a licensed auto electrician to do this work for safety purposes)

Below is a diagram of a basic CTEK D250S setup.


A dual-battery system really doesn’t get any easier than this. We run a fridge, pump, hot water heater, exhaust fan and need 240 volt power to keep our online remote work functioning and sustainable. We haven’t had any dramas besides the gyozas taking a little longer to thaw out!

We haven’t installed any solar as of yet seeming we travel for a least an hour every one to two days. This is more than enough to keep our fridge and air vent fan running day and night. Once our batteries are fully charged, all our electrical will run for roughly two - three days before needing to be charged again.


Until CTEK brings out an updated version of this model battery charger, I don’t believe you can get a better unit for the price. These retail for around the $350-$400 which makes it a great value, dual-battery DIY option. Especially if you value your battery life and keeping them working at peak performance than this is a must have. An easy install and operating system means you can let it do it’s thing without any fuss. The fact that it includes a solar power input is possibly excessive, unless you are the type of person who likes to be stationary for more than a week. Just make sure you take some sunscreen.

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Jared Campbell1 Comment