This is the calculation that exposes the single most dangerous assumption in a lithium conversion: that the existing alternator can simply keep doing what it was doing.
Why lithium is hard on an alternator
A lead-acid bank's charge acceptance falls away as it fills. The alternator delivers high current briefly, then tapers, and spends most of the engine's running time working lightly. Automotive alternators are designed around exactly that duty cycle.
A LiFePO4 bank keeps accepting high current through most of its charge. From the alternator's point of view, that means full output, sustained, for as long as the engine runs — a duty it was never designed for. The result is heat, and heat destroys alternators.
This is not a marginal concern. It is a well-documented failure mode, and in some installations a fire risk.
The two numbers
Charge time tells you whether the charging strategy is viable — whether a realistic amount of engine running actually replaces what you use.
Sustained current tells you what the alternator must hold. Use the current the alternator can deliver continuously without exceeding its thermal limits, not its nameplate rating. Those are very different figures, and the nameplate is usually a cold, short-duration number.
What to do about it
The options are well established, and all of them involve limiting or controlling the current rather than hoping:
- A DC-DC charger between the engine's charging system and the lithium bank, sized to what the alternator can sustain.
- An external regulator designed for lithium, with alternator temperature sensing that backs off output before damage occurs.
- Both, on larger systems, along with a defined behaviour if the BMS opens the charge path while the engine is running.
That last point deserves emphasis. If the BMS disconnects the battery while the alternator is charging into it, the alternator can be exposed to a load dump. The system needs a designed answer to that, not an accident.
The engine pays for it too
An alternator is not a free energy source. The mechanical power it absorbs comes from the engine, and at typical alternator efficiency the engine gives up meaningfully more power than the electrical output suggests. On a small auxiliary this is not negligible.