Why lithium batteries damage inboard alternators (load dump explained)

What happens inside the alternator

A marine alternator makes its output by spinning a magnetised rotor inside a stator. The rotor's magnetism comes from current in the field winding, and the regulator controls that current to hold the bus at around 14.4 V. The field winding is an inductor, and an inductor resists any change in its current. When the battery, which is the alternator's main load and its voltage reference, suddenly disappears, the regulator cannot reduce field current instantly. For a few tens of milliseconds the alternator keeps producing into a bus that no longer has a battery to absorb the energy. The voltage rises until something absorbs it.

Automotive electronics are specified against this event under ISO 16750-2, which defines a load-dump pulse of up to 79 to 101 V on a 12 V system for unsuppressed alternators. Marine aftermarket protection modules from Balmar and Sterling are built to clamp around 60 V. Designers in the marine charging business have cited transients above 100 V on unprotected systems.

Why a lithium start battery makes it worse, not better

A lead-acid battery almost never disconnects itself. A lithium battery is designed to. Its battery management system opens the circuit on over-voltage, over-current, over-temperature, under-temperature or a cell fault. On a start battery, which lives at 95 to 100 percent state of charge, the most likely trip is high-voltage, and the most likely moment is when the alternator is producing. That is precisely the load-dump condition.

There is an irony here. The failure mode most people argue about, alternator overheating from lithium's low internal resistance, is a house-bank problem and barely applies to a start battery that gives up one to three amp-hours per crank. The failure mode almost nobody talks about, the disconnect, is concentrated on the one battery whose loss stops the engine.

What the industry has done about it

Nothing inside the battery. Every lithium cranking battery we have found on the market responds to an over-voltage event by opening its circuit, and manages the risk with a number on the spec sheet: a maximum alternator size pushed onto the installer. Load-dump protection is sold separately, as a clamp module wired across the alternator output. If the battery industry had solved the problem internally, that accessory market would not exist.

What a battery can do instead

The architecturally correct answer is to control charging and discharging on separate paths. When the pack must refuse charge, only the charge path closes. The discharge path keeps conducting, so the alternator still sees the boat's electrical system as a load and the engine still sees its battery. No spike, no stall. A warning should go out before any of that happens, with enough margin for an external regulator, a DC-DC charger or an engine controller to react.

This is the behaviour WakeTender is built around, and it is the reason the alternator limit on its datasheet is a real engineering number rather than a liability footnote: 110 A per battery, which comfortably covers the 95 A alternators fitted to stock Indmar, PCM and Ilmor engines.

What you can do on an existing boat

If you already run a drop-in lithium battery on an inboard, an alternator protection module across the alternator output is cheap insurance. Keep the lithium out of the engine circuit where you can: lithium house bank, lead-acid or an engineered lithium start battery at the engine, joined by an ACR or DC-DC charger. And do not add a high-output alternator to compensate for a battery that disconnects; a bigger field makes a bigger spike.

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