Re: Voltage puzzle
denverd0n, Hellosailor, and MaineSail are absolutely correct.
Accurate open circuit voltage measurement, done correctly, is an accurate method of determining the state of charge (SOC) of all lead-acid batteries....flooded, AGMs, gelled, etc. Much, much better than attempting to use a hydrometer, for the reasons cited by MaineSail.
For the OP: 12.9 or even higher OC resting voltage is very common with AGMs and gels, but not with flooded batteries. Your battery appears to be a flooded "no maintence" type, so I'd guess that your voltage measurement device is inaccurate. Or, just maybe, there's some charging device still connected, like a solar panel?
Note that the SOC is not the same as capacity. Batteries typically begin to lose their rated capacity practically from the date of manufacture, and they continue to lose capacity throughout their useful lifetimes.
The best way to measure capacity is with a controlled, purely restive 20-hour load test, with the load being 1/20th the ampere-hour (AH) rating of the battery and the load test continuing until the battery reaches 10.5VDC. This is not possible for most users, lacking the time and/or equipment necessary.
The industry standard for measuring battery capacity these days is with a device which measures internal battery conductance and resistance, such as one of the Midtronics series. These cost upwards of $600 and more, so are not available to most users.
For measurement of SOC, however, IMHO you'd be hard-pressed to find a better way than an OC voltage measurement directly at the battery terminals with a calibrated voltmeter after a suitably long resting period.
BTW, this includes the fashionable-these-days "battery monitors", which are only accurate if they have been properly installed and calibrated -- and recalibrated as the battery ages.
Last edited by btrayfors; 05-31-2012 at 11:09 AM.