I have seen the discussion ensue many times where folks say they monitor their batteries state of charge (SOC) via a volt meter. While this can be quite accurate when the battery has been allowed to come to a resting voltage, the reality is that when you are on the boat, and cruising, attaining a 4-12+ hour resting voltage can be very difficult if not very inconvenient.
I wanted to do a photo/time/voltage experiment to illustrate why using a simple voltage test on a boat may not be the most accurate way of checking the resting voltage of a battery especially with short resting times.
Here is what Trojan Battery says about using open circuit voltage testing of state of charge:
"Open Circuit Voltage Testing - This is the least preferred method of evaluating the performance of a battery.
• For accurate voltage readings, batteries must remain idle at least 6 hours (but
preferably up to 24 hours)
• Measure the individual battery voltage"
In order to check the state of charge (SOC) on a lead acid battery the battery should have rested, meaning no input voltage/amps or output/loads, not even the stereo memory, for a considerable amount of time. If the battery is not allowed to rest, and you have recently run the engine, or had a load turned on, the SOC will not be accurate for many hours, but accurate is a relative term because as Trojan Battery says "least preferable method".
To conduct this illustration I used an elcheapo bench charger, stop watch, temperature and a DVM. I set my camera up on the tripod and photographed the battery over 24 hours.
The battery, a Trojan group 24 deep cycle, had been on my bench and drained to about a 60% SOC. It then sat for a week before I took my baseline SOC reading. The resting voltage was 12.24 volts or about 60% of charge according to Trojan Battery. I then plugged in the 20 amp charger for about 130 seconds and then unplugged it.
This was only 130 seconds of charging and it affected the resting SOC for at least 11 hours and was off by about 10% even after sitting for 11+ hours.
*****Battery SOC at Resting Voltage and 80f (Source Trojan Battery)*****
100% = 12.73
90% = 12.62
80% = 12.50
70% = 12.37
60% = 12.24
50% = 12.10
40% = 11.96
30% = 11.81
20% = 11.66
10% = 11.51
NOTE: This battery was not "new" it was certainly broken in and used. It was also suffering from some effects sulfation hence the rather quick rise in voltage. This battery was chosen to represent the "average" condition battery I see out in the real wold of boats.
The reality is that healthy batteries are affected similarly.