Why is charge light on at 14.9 volts?

Asked by Jul 29, 2010 at 12:32 PM about the 2001 Mazda MX-5 Miata Base

Question type: Maintenance & Repair

Charge light comes on a few seconds after starting engine.  14.9-15.0 volts at fuze block in engine compartment.  12.25 volts with engine off, lights on.  12.4-12.5 volts with lights off.

2 Answers

11,185

At 14.9v - 15v you are experiencing an "over-charging" effect. That is why the light is on. At idle, charge level should be between 13.8v and 14.4v depending on what accessories are running.

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Measurements of current at the battery, measured with a clamp-on ammeter on the cable: Only lights on: -13 amp, a reasonable number Ignition on accessory, no radio or other accessories actually running: -0.6 amp Ignition on, ready to crank: -3.0 amp. I assume fuel pump and other stuff are running. Cranking: more than -135 amp. The engine started before the meter reading stabilised. Charging at idle: +13 amp, continuously decreasing as battery is recharging. Charging at 2K rpm: 7-8 amp, still decreasing Charging at 3K rpm: 6 amp, still decreasing as battery recharges After about 5 minutes of running, the charge rate at idle (800 rpm) fluctuates between 1 and 2 amp, at about 14.8 volts. These numbers all seem reasonable to me, but the battery indicator light is still lit, continuously from a few seconds after the engine starts. Any thoughts???????????

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