DC-DC converter and B2B charger: what you need to know
A DC-DC converter converts one DC voltage to another. In camper vans they serve two purposes: charging via the alternator (B2B charger) and powering 12V equipment from a 24V or 48V system. This guide explains both applications.
Calculate your DC-DC converter
B2B charger: charging via the alternator
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).
DC-DC step-down: 24V or 48V to 12V
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.
How to choose the right power rating?
For a B2B charger: the rule of thumb is that you should use a maximum of 50% of the alternator's output for the B2B charger. A standard alternator provides 90-150A at 12V. With a 120A alternator you can use a B2B charger up to 60A, but account for the vehicle's own power draw (air conditioning, headlights, etc.).
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.
Installation considerations
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.
Common mistakes
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.
Calculate your DC-DC converter
Use our free tools to calculate the right power rating and wiring for your DC-DC converter.
Frequently asked questions
- Do I need a B2B charger if I already have solar panels?
- Highly recommended. Solar panels depend on weather and season. A B2B charger gives you a guaranteed charge source on driving days, regardless of weather. The investment quickly pays for itself in extra energy security.
- Can I combine a B2B charger with an MPPT charge controller?
- Yes, that is even the standard setup. The MPPT controller manages solar charging, the B2B charger manages alternator charging. Both charge the same battery bank simultaneously. With Victron equipment, they communicate via Bluetooth (VE.Smart) to coordinate charging.
- What is the difference between a B2B charger and a charge relay?
- A charge relay (VSR or split charger) simply connects the two batteries in parallel when the engine runs. A B2B charger is an active DC-DC converter with a programmable charge profile, current limiting and protections. Comparison: - Suitable for lithium: relay no, B2B yes - Charge profile (3-stage CC/CV): relay no, B2B yes - Works with Euro 6 smart alternator: relay no, B2B yes - Protects alternator from overload: relay no, B2B yes - Price: relay 20-50 euros, B2B 150-400 euros In short: a relay is only an option for lead-acid batteries in older vehicles. In all other cases, a B2B charger is the safe choice.