An engine that runs perfectly can still put nothing into the battery bank. The alternator is only one part of the charging path, and the parts around it, the belt, the field or sensing wire, the split charge relay or diode, the main charge cable and its fuse, fail at least as often as the machine itself.
The tests below are ones an owner can safely do with a multimeter while the engine is running. Anything involving dismantling the alternator, altering regulator settings or working on lithium charging profiles is work for a qualified marine electrician.
Start with the belt. A glazed, slack or dusty belt slips under load and gives a charge that fades as soon as the engine warms up. Black dust on the pulleys and a squeal on start up are the giveaways.
With the engine at fast idle, measure voltage directly at the battery posts. A healthy lead acid charging system usually sits noticeably above resting voltage. If the reading barely moves off the resting figure, the alternator is not delivering.
If the alternator output stud reads high but the battery posts read low, the loss is in between: the main charge cable, its fuse or isolator, a split charge relay, a battery combiner or a corroded terminal. That is a wiring fault, not an alternator fault, and swapping the alternator will not fix it.
If both readings are low and equal, the alternator or its regulator is the likely cause.
Many marine alternators need an ignition feed to excite the field. A failed warning lamp, a broken feed or a corroded connector can leave a perfectly good alternator producing nothing at all. On externally regulated and lithium ready systems, a sensing wire connected to the wrong point will also cause chronic undercharging that looks like a dying bank.
This is where boats differ from cars. Split charging, DC to DC chargers and lithium battery management can all interrupt charging deliberately, and the symptom looks identical to a failure.
A lithium bank whose battery management system has disconnected on high voltage, low temperature or cell imbalance will show as no charge acceptance while every component tests fine. Reading the BMS status first saves hours.
Alternators charging lithium without a DC to DC charger or proper regulation can also overheat. If the alternator is very hot and output has collapsed, stop and get it looked at rather than continuing to run it.
Physically it is often straightforward, but marine alternators, regulation and lithium charging need to be matched to the bank. Fitting the wrong unit or regulator can damage batteries or overheat the alternator, so specification is worth checking before you buy.
Usually belt slip as things warm up, a thermally protected regulator backing off, or a lithium BMS ending the charge cycle. Each leaves a different voltage signature, which is why we measure rather than guess.
On a boat that relies on engine charging, especially with lithium, an external regulator gives a fuller, faster and safer charge. On a boat that mostly sits on shore power it usually is not the first upgrade to make.
We diagnose the whole charging path, alternator, regulation, split charging and cabling, then quote in writing before any work starts.