How an alternator (DC-DC) charger tops up your campervan battery while you drive, and why it protects both batteries.
A DC-DC charger is a small unit that charges your leisure (home) battery from the vehicle alternator while you drive. Unlike a simple solenoid or relay, it regulates the charge to the correct voltage and stages for your battery chemistry, and it protects the starter battery from being drained.
The alternator in most vans is set up to charge a starter battery with a constant voltage. A modern LiFePO4 leisure battery needs a different, carefully controlled charging profile. A plain relay can overcharge lithium or undercharge AGM. A DC-DC charger takes the alternator's power and delivers the right charging stages.
DC-DC chargers are rated in amps (for example 20 A or 60 A). As a guide, 20 to 40 A suits a 100 to 200 Ah bank; heavier users or larger banks may want 60 A. Match the charger to your battery size and your alternator's output, and fuse everything to local standards.
DC-DC charging pairs naturally with solar: solar recharges you when parked in the sun, and the alternator tops you up on travel days even in cloud. The combination lets you run a smaller solar array with confidence.