DC-DC converter and B2B charger: what you need to know

A B2B charger (battery-to-battery) is a DC-DC converter specifically designed to charge your leisure battery from the starter battery while driving. The engine drives the alternator, which charges the starter battery; the B2B charger takes a portion and sends it to your leisure battery with the correct charge profile.

Important: with lithium batteries (LiFePO4), a B2B charger is mandatory for two reasons. First, the alternator provides a charge profile for lead-acid batteries which can damage lithium. Second, lithium batteries have very low internal resistance and without current limiting can draw so much power from the alternator that it overheats and burns out, especially at idle.

In modern vehicles (from around 2015, Euro 6), a B2B charger is extra essential: the smart alternator stops charging once the starter battery is full to save fuel. A traditional charge relay no longer works. Only a B2B charger can still activate the alternator.

Common ratings: 20A, 30A and 50A. A 30A charger at 12V delivers approximately 360-400 Wh per hour of driving (depending on actual charge voltage).

If you have a 24V or 48V system, most camper van equipment won't work directly. Lighting, fridges, water pumps and USB chargers expect 12V. A DC-DC step-down converter transforms the higher voltage to stable 12V.

The converter must provide enough power for all connected 12V consumers simultaneously. Add up the total wattage and choose a converter with at least 20% headroom.

Example: 5A LED lighting + 6A fridge + 2A water pump = 13A at 12V = 156W. Choose at least a 200W converter.

For a B2B charger: reserve no more than about a third of the alternator's output. A standard alternator provides 90-150A at 12V, so a 120A alternator lands you on a charger of about 40A. The figure on the alternator is a peak at high revs and cold; continuously and hot it delivers less, and the vehicle itself eats from it too (air conditioning, headlights, glow plugs).

For a step-down converter: add up all your 12V consumers and add 20% headroom. Don't forget that some appliances have higher inrush current (compressor fridge, water pump).

Our DC-DC Calculator helps you calculate the right power rating for both applications.

A B2B charger takes its current from the alternator, and the alternator has a limit. The label often says 140, 180 or 220 amps, but that is the peak at high revs and cold. Continuously, at idle and hot under the bonnet, it delivers considerably less.

On top of that, the starter battery, the glow plugs, the lights, the wipers and the engine electronics all eat from the same plate. What is left for your house battery is less than the label suggests.

A workable rule of thumb is to reserve no more than about a quarter to a third of the alternator's nominal output for your B2B charger. On a 180 amp alternator that puts you between 45 and 60 amps.

On newer vans something else comes into play: the smart alternator. It regulates its voltage to whatever the starter battery needs and sometimes deliberately lets it drop to save fuel. A simple split charge relay no longer works reliably there; a B2B charger does, because it makes the correct charge voltage itself from whatever arrives.

A B2B charger usually sits a long way from its source. The starter battery is up front, the house battery at the back, and a thick cable runs the length of the vehicle between them. That makes the wiring the part where most of the mistakes live.

Three things that always apply:

• A fuse at both ends, close to each battery. The cable must be protected against a short from the side the energy comes from, and with two batteries that is two sides
• Cross section calculated on the length there and back, not on the straight line distance. Six metres one way means calculating with twelve metres of cable
• A separate negative cable of the same size, or a demonstrably sound earth through the chassis; the negative side matters as much as the positive

The fuse rating follows the charger's output current plus margin, and the cable follows the fuse. Never do that order the other way round: a cable thinner than its fuse is exactly the situation in which the cable melts before the fuse notices.

Not every camper needs one, and it is one of the items where hundreds can be saved if it suits the way you travel.

You probably do not need one if you stay in one place for months with enough solar to cover your use, or if you are almost always on shore power. On a small installation with a light load or two, the price does not always earn itself back either.

You do need one if you drive a lot and have little sun, if you travel in winter, or if you have a lithium battery. That last one matters most: a lithium battery accepts so much current that a directly connected alternator can be overloaded, and the voltage an alternator delivers is not the voltage lithium wants to see.

If you are unsure, look at how many hours a week you drive. Under two hours a B2B charger returns so little that an extra solar panel usually does more for the same money.

Place the B2B charger as close to the leisure battery as possible to minimise voltage drop. Use cables rated for the maximum charge current (our cable calculator helps with this).

Ensure adequate ventilation: DC-DC converters produce heat, especially at full power. Do not mount them in an enclosed space without air circulation.

Use appropriate fuses on both sides: on the starter battery side and on the leisure battery side. The fuse rating depends on the maximum power and cable thickness.

Bear in mind that some B2B chargers need a signal wire (D+ or ignition feed) to detect that the engine is running. Other models detect this automatically based on starter battery voltage (voltage sensing). Check the specifications of your B2B charger and vehicle.

With a 24V system and B2B charger: your alternator outputs 12V, the B2B charger must step this up to 24V. Not all B2B chargers support this; check the specifications.

Connecting the alternator directly to a lithium battery: this damages the battery and can be a fire hazard. Always use a B2B charger.

Undersized wiring: at 50A on 12V, thick cables are needed (minimum 10mm2 at short distance). Undersizing leads to heat build-up and voltage drop.

Converter without fuses: a short circuit in the 12V circuit without a fuse can damage the converter and wiring.

Undersized step-down converter: if the converter is overloaded, it shuts down and your fridge stops. Always allow headroom.