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

9,435

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.

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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