Measure the battery voltage with the engine off and with it running. With it off it should be 12.6 volts. If it's closer to 12.2 volts it's good but still discharged. Charge it at a slow rate for an hour. If you find around 11 volts it has a shorted cell and must be replaced.

The voltage with the engine running must be between 13.75 and 14.75 volts. If it is low, raise engine speed a little and recheck it. If it's still low, suspect the generator.

Three weeks is about the longest a good battery can sit and still start the engine. That's because of the many computer memory circuits that draw a small amount of current. An older battery won't last that long so most likely there's nothing else wrong except your battery didn't have enough time to recharge. It should be charged at a very slow rate for an hour. Battery chargers on a high rate can damage an older battery.

Answered by caradiodoc (expert)
17,297 answers provided