Here's the photo after 24 hours rest. The battery is back to just about where I had calculated it would be. I also waited longer and measured it at 34 hours and it was still at 12.26 volts and had obviously reached a steady resting voltage. Temps in the shop were a steady 74F..


Using a digital volt meter (DVM) or even an analog one to measure your batteries state of charge can be quite misleading with some rather wide margins of error unless you let it attain an honest resting voltage. Live loaded voltage readings can be used, to get an approximation, but it really requires some home work on your part in order to learn how to track your bank, at specific loads and what type of voltage you should expect. Most boat owners will never spend the time to go through this exercise..


EDIT 2/13/11: As an update I have since applied many different loads to remove the surface charges or about 6 or seven different batteries, ages and types.. 5 amps, 10 amps 20 amps etc. etc., and all for varying times.


The problem is that none of the surface charge removing loads give me an accurate reading that agrees & matches up with a 24+ hour rest. I suppose you could do some rather lengthy experiments and find the exact load to remove a surface charge, that works for your bank, at a certain state of charge, but this would be complicated at best. The closest I got was within 8-10%. A 10% variance on a 400 Ah bank is 40 Ah's or nearly a days worth of power for us. Having & interpreting my bank, reading the equivalent a whole day off, can be and would be rather frustrating.


TEMPERATURE: I have also experimented at length with resting voltages and temperature. At 95F a battery can come to rest in as little as 12 hours. When cold during the winter this can take 10+ days!! Temperature plays a huge role in when you will achieve a "resting" voltage.


For more information on the efffects of temperature on self discharge see the link below:


Effect of Winter on Self Discharge (LINK)


Measuring the battery state of charge with a DVM can be misleading if not done correctly or with some real in-depth knowledge of your bank. When just 0.1V represents approx 10% of the capacity it is easy to see how resting vs. not resting can trick a user into believing their bank is more full than it really is...