How this calculator sizes the charger
- Battery limit: capacity × maximum charge rate (0.5C for LiFePO4, 0.3C for AGM, 0.2C for flooded).
- Alternator budget: a cautious rule of thumb keeps the charger's input current to about 40% of the alternator's rating, leaving the rest for the vehicle. Input current is about 20% higher than output, because the charger boosts the voltage and isn't 100% efficient.
- Daily need: the current that would replace your daily use in the time you drive.
The suggested size is the smallest standard charger that meets your daily need without exceeding either limit.
Needed amps = Daily Ah ÷ (Driving hours × 0.95)
Input amps ≈ Output amps × 1.2
Why use a DC-DC charger instead of a split-charge relay?
- Modern "smart" alternators drop their voltage once the starter battery is full, so a relay alone often barely charges the leisure battery.
- Lithium batteries need a proper charge profile and can draw too much current through a relay.
- Long cable runs to the back of a van cause voltage drop, which a DC-DC charger compensates for.
Frequently asked questions
What size DC-DC charger for a 200Ah lithium battery?
Usually 30–50 A, provided the alternator is rated 150 A or more. A 30 A charger adds about 28 Ah per hour of driving.
Will a DC-DC charger damage my alternator?
Not if sized sensibly. Large chargers on small alternators can run them hot during long idle periods; many chargers have a reduced-current mode for this.