Short answer: A voltmeter, by far.
Electrical guru Mark Hamilton of M.A.D. Enterprises points out that amperage is a measure of current flow, so an ammeter is actually a “flow meter” that’s intended to measure current flow to the battery (under normal conditions) or discharge from the battery (in the case of alternator system failure). On a typical flow meter, all output must be directed through the device to obtain an accurate reading. In the ammeter’s case, that means all the alternator output used to recharge the battery must first be routed through the ammeter under the dash. Which requires a heavy-gauge cable and presents a possible fire hazard. And the ammeter itself must be able to handle all this current flow, so it must have a higher current rating than the alternator’s maximum rated output.
All this might be worth the hassle if the ammeter produced reliable information. But the ammeter can only measure the amount of current output to the battery for recharging purposes: When the alternator recharges a “low” battery, the ammeter indicates a high charge rate; with a fully charged battery the voltage regulator reduces alternator output, and the ammeter is supposed to indicate a very low charge rate. But how can you really tell the regulator has reduced alternator output because the battery is fully charged? Maybe a diode in the alternator rectifier failed, or the alternator belt slipped after it warmed up, just as if the battery were fully charged. Or maybe the meter indicates a medium charge rate most of the time-does the battery want this much or could the voltage regulator be overcharging the battery?
On the other hand, a voltmeter works like a fuel pressure gauge-but instead of measuring fluid in psi, the voltmeter measures electrical system pressure in volts. Just like a fuel pressure gauge, a voltmeter only needs to tap into a circuit; all the fuel (or electricity) does not have to detour through the gauge itself. Voltmeter installation is easy, quick, and safe: It hooks up to a fused, ignition-switched “off/on” source and does not require any modification of the circuit used to recharge the battery or any part of the alternator/regulator system. In short, the voltmeter installed at the dash will be a stand-alone circuit.
The voltmeter directly measures the result of charging-system performance. With normal alternator/voltage-regulator function, battery voltage is maintained at 14.0 to 14.5 volts-and this is reported directly by the voltmeter. In the event of alternator-system failure, voltage will be low and continue to drop as the battery discharges. In the event of an “overcharge” condition, the voltmeter will climb above its normal zone. In summary, there is no chance for misinterpreting a voltmeter’s readings as can happen with an ammeter.
Voltmeter vs. Ammeter?
AutoMeter offers both, but for most applications a voltmeter yields a safer installation while providing more useful information on charging-system conditions.