DC-DC Charger Sizing (Alternator)
Size a DC-DC charger from alternator output, vehicle reserve, cable run, and battery chemistry limits.
Why not use all the alternator amps?
Alternators are **air-cooled** and sized for intermittent peaks. Running near 100% for long periods overheats diodes and windings. Leaving headroom (e.g., 20–40%) prolongs life, especially in hot engine bays.
What C-rate should I assume for LiFePO₄ vs AGM?
A conservative rule is **0.5C** for LiFePO₄ (100 A per 200 Ah bank) and **0.2C** for AGM (40 A per 200 Ah). Always check your battery’s datasheet/BMS limits; some LFP cells allow 1C, but cooling and cabling must support it.
Do I really need big cables for DC-DC? It’s “only” 12 V.
Yes—low voltage means **high current**. Voltage sag wastes power and can cause chargers to throttle. Size for **≤3% drop** on the round-trip length and verify **ampacity** for heat and bundling.
Where should I place the fuse and what size?
Put a fuse **near the source** on **both** sides if sources are independent (starter and house). Start from charger rated **A × 1.25**, then round to a standard size that **doesn’t exceed cable ampacity**.
Alternator-limited result feels low—can I raise it?
You can: improve cooling, use a **smart regulator**, or **increase engine RPM** during charge. Otherwise accept the limit or move to **24 V** systems to cut current for the same watts.