What a DC to DC charger actually does
A DC to DC charger sits between your vehicle's starter battery and a second battery, and it does one job: it takes whatever voltage the alternator is putting out and turns it into a proper multi-stage charge for the auxiliary battery. Bulk, absorption, float. The same charge profile a good 240V charger runs at home, except it happens while you drive.
That matters because an alternator is not a battery charger. It is designed to keep the starter battery topped up and run the vehicle's electrics. Left to its own devices it will push a second battery to somewhere around 80 per cent and then back off, and on a modern vehicle with a smart alternator it can drop as low as 12.5 volts once the starter battery is happy. A lithium battery at that voltage barely charges at all.
Most DC to DC chargers sold in Australia also have a solar input. Plug a panel or blanket into the solar side and the charger picks the best source available, or blends both. That single box replaces a separate solar regulator and a voltage sensitive relay.
Do you actually need one?
You need a DC to DC charger if any of the following apply to you:
- Your vehicle is roughly 2012 or newer. Almost every 4WD and ute from that era onwards runs a variable-voltage or smart alternator to save fuel, and a plain isolator will never fully charge a second battery behind one.
- You run a lithium auxiliary battery. Lithium needs a charge voltage of around 14.2 to 14.6 volts and a controlled current. A DC to DC charger delivers that. A relay does not.
- Your second battery lives in a caravan or camper trailer. By the time 12 volts has travelled from the alternator to the back of a van through the trailer plug, voltage drop has eaten a chunk of it. A charger mounted in the van boosts it back up.
- You want to charge from solar and the alternator without two separate systems.
You can probably get away without one if you have an older vehicle with a fixed-voltage alternator, an AGM battery mounted under the bonnet on short, heavy cable, and you drive for long stretches. Plenty of people toured like that for years. It works, it just does not charge the battery all the way.
How to size a DC to DC charger
Chargers are rated by output current: 20, 25, 30, 40 and 50 amps are the common sizes. Bigger is not automatically better.
- Match the charger to the battery. A rule of thumb is a charge rate of 10 to 30 per cent of the battery's amp-hour capacity for lithium, and 10 to 20 per cent for AGM. A 100Ah lithium battery is comfortable on a 25 to 30 amp charger. A 200Ah lithium bank is where a 40 or 50 amp unit starts to make sense.
- Check what the alternator can spare. A 50 amp charger draws closer to 60 amps from the vehicle side once losses are counted. On a small alternator with a winch, driving lights and a fridge already on the load, that is a lot.
- Think about drive time. If your typical day is two hours between camps, a bigger charger recovers more of what the fridge used overnight. If you drive all day, a smaller one gets there anyway.
Where to mount it
The charger should be mounted as close to the auxiliary battery as practical, not next to the alternator. The long cable run should be on the input side, where the charger can compensate for voltage drop, not on the output side where it cannot.
Under the bonnet is fine for most units as long as it is rated for the heat. In a canopy or drawer system it needs airflow. In a caravan it goes near the battery box, with the vehicle feed arriving through an Anderson plug on the drawbar.
Cable size and fusing
This is where most DIY installs fall over. A 40 amp charger on 6mm² automotive cable over a six metre run will drop enough voltage that the charger throttles itself, and you will never see the current you paid for. Use the cable size the manufacturer specifies for your run length, and if you are in doubt go one size heavier.
Every long cable needs a fuse or circuit breaker at the battery end, on both the input and output sides. A fuse at the charger protects the charger. A fuse at the battery protects the vehicle, which is the one that catches fire.
Ignition triggers and smart alternators
Modern chargers detect when the engine is running by watching the input voltage. On a smart alternator that voltage can sit low enough that the charger thinks the engine is off. The fix is an ignition-triggered wire, a small feed from a circuit that only goes live with the key on. Most chargers have a terminal for it. Use it on any vehicle with start-stop or a variable-voltage alternator, and you avoid the charger draining the starter battery while parked.
Common mistakes
- Buying a bigger charger than the alternator or cable can support, then wondering why it only ever shows 18 amps.
- Setting the charge profile for AGM when the battery is lithium, or the reverse. The profile switch is usually a small dial or dip switches. Check it twice.
- Mounting the charger at the front of the vehicle and running thin cable to a battery at the back.
- Skipping the ignition trigger on a vehicle with a smart alternator.
- Leaving the solar input unfused because "it is only a blanket".
What it means for your insurance
A DC to DC charger, a second battery and the wiring that connects them are modifications, and like a bull bar or a suspension lift they add to the value of the vehicle. If they are not listed on your policy they may not be covered if the vehicle is written off or stolen. Club 4X4's 4WD insurance lets you declare modifications and accessories so they are included in the agreed value, and the same applies to the 12V gear in your caravan or camper trailer.
Keep receipts and a photo of the install. It makes any claim faster.
Quick answers
What is the difference between a DC to DC charger and a battery isolator? An isolator is a switch that connects two batteries when the engine is running. A DC to DC charger actively converts and regulates the voltage to charge the second battery properly. On a modern vehicle or a lithium battery, only the charger does the job.
Can a DC to DC charger drain the starter battery? A correctly installed one, with the ignition trigger wired, will not. Without the trigger it can on a smart alternator vehicle.
Can I run a DC to DC charger and solar at the same time? Yes, if the charger has a solar input. That is the point of a dual-input unit.
Do I need an auto electrician? For an under-bonnet install on a vehicle you know, a competent DIYer with the right cable and crimping tools can do it. For anything involving the vehicle's ECU wiring, an ignition trigger you cannot identify, or a caravan install, get an auto electrician. It costs less than a fire.
