*****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 a bench charger, stop watch 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