To measure the battery voltage, use an inexpensive digital voltmeter. I can walk you through how to set it up and read it if necessary. If you have a Harbor Freight Tools store near you, they always have one for less than ten bucks and often have them on sale for $2.99. They work fine for these tests. Sears, Radio Shack, Walmart, and some auto parts stores also have them. You don't need a real expensive one. They have features most of us never use.
A fully charged battery will measure near 12.6 volts. A good battery that is discharged will read near 12.0 volts. If there is a shorted cell in the battery, it will read less than 11.0 volts.
If the battery is fully discharged, it will take a good 15 minutes on a charger before it STARTS to recharge, then it should be allowed to charge at a slow rate for an hour or more. High charging rates can cause damage to older batteries and lead to their early failure.
Once the engine is running, measure the battery voltage again. It must be between 13.75 and 14.75 volts. If it stays near 12.6 volts or less, the alternator is not recharging the battery. There are a couple more simple tests to narrow down the cause.
Tuesday, May 10th, 2011 AT 9:07 PM