4th generation alternator battery


New Member
1991 Civic hatchback. As per a technical service bulletin, which I saw a reference to, the car sometimes charges at 12.5V and sometimes at 14.34V or thereabouts. It's an automatic feature. The battery starts the car well, even after 4 days rest, and the voltage drops to around 9.57V while crank. No problem. If I charge the battery manually with a 100mA charger for about 18 hours, it will register 12.24V after resting 4-5 hours. So far so good, and then drop about .03V each day to around 12.07V. Fine, and starts fine as described. But when I drive the car 10 miles, and measure it at the end of the day, the battery is still at 12.07V. It seems like the alternator is not charging the battery much at all.

Having no error codes or battery/charging lights on, it seems there's no problem. But is the alternator underperforming? Or is the battery not accepting the charge? Or is the marginally low battery voltage causing the alternator to under charge? I think it's worked this way for years, the few times I measure battery at rest it's always 11.9v to 12.1V. I'm thinking the alternator is beginning to not charge as well as when it was newer, but then there is that 14.34V charging voltage that seems fine.

Any insights appreciated. Thanks.