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Posted by David Sherman [72.47.9.228] on Monday, January 17, 2011 at 21:08:27 :
In Reply to: Re: voltmeter suppose to read at 13 1/2 volts? posted by gmharris [71.105.178.157] on Monday, January 17, 2011 at 20:41:41 :
You're right that 2.1 volt is the nominal open-circuit resting voltage of a lead-acid cell at room temperature. Putting that voltage onto a cell will not charge it fully, though. Lead-acid batteries are fairly easy to charge because a constant voltage applied across them will charge them but not over-charge them. A lot of the newer lithium type batteries are way trickier that way.
It's safe to apply 2.3 volts per cell indefinitely as a "float" charge. This will eventually charge even a dead battery, but it may take the better part of a week. Backup power supplies such as emergency light and computer backups often leave 13.8 volts on the battery continually. It makes for a simple circuit and it does the job of you don't need to charge it quickly.
For fast charging without risk of damage, you can go up to 2.4 volts per cell pretty safely, provided you use some juice out of the battery fairly often. This is a good value for most automotive applications.
I've put insanely high voltages, up to maybe 5 volts/cell on a "dead" sulfated battery sometimes to get it to start charging. The only trick there is to limit the current to 5 amps or so, which means watching the meter on the charger and turning the voltage down as the battery begins to accept a charge. I've often gotten another year or two of good life out of "dead" batteries by force-feeding them that way. I'll usually then leave it for a week or more with an amp or two going into them to help all the cells equalize and get as charged as they can be.
The short answer to your questions is that 13.5 is too low to fully charge the battery. Check it right at the battery posts with a good digital meter. Maybe your panel meter is wrong. But if you're only getting 13.5 at the battery posts, with the engine revved up, you're going to keep having battery problems, wondering if it's the alternator, re-cleaning all the connections, etc. You'd be better off trying to get the voltage up to 14-14.4 if you can. Then check the electrolyte now and then to make sure it's not over-charging.
On the other hand, if your alternator is maxed out in terms of current (check with an ammeter or see if it feel very warm) and is only able to get the voltage up to 13.5, you may have a shorted cell in the battery. You could also try swapping in a known good battery to see if the charging voltage gets higher.