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Compass Marine How To | all galleries >> Welcome To >> Measuring A Lead Acid Battery State of Charge > Voltage After Resting 24 Full Hours
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Voltage After Resting 24 Full Hours

Voltage After Resting 24 Full Hours

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...

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Ray 06-May-2016 12:39
Question unrelated but enjoyed your observations Charging two cranking 4ds and a 8v deep cycle in series. This has started two 8-71s for years however when we powered an inverter with the 4ds (24v) I realized the starting bat would not fully recharge. Investigating while under charge revealed 13.8 on each 4D and 10.8 on the 8Volt with boiling water. The deep cycle was zapping all the current, I understand whats happening but the Question is why. We will eventually go to all deep cycle on this system
Guest 08-Feb-2015 21:16
A voltage reading indicating the battery state of charge in a moving boat will typically be much more accurate than these tests indicate. As the boat movement more quickly remixes the stratified battery electrolyte near the plates , the voltage at the terminals more accurately reflects the true state of charge.

With waves rocking the electrolyte, accuracy will improve greatly.
JoBo 12-Jan-2014 20:33
Very interesting results.
Thank you for taking the time to record and post your results.
D.P. Wetick 24-Jun-2013 17:25
A most useful dataset for the Gp. 24 Battery...committed it to my chartbook and kept onboard my boat. I've noticed a drop of approx. 7% when charging ceased to 8 hours later on a 100% charged battery. I think this is normal.
jim 11-Dec-2009 22:15
What time is needed after its loaded for a few hrs discharge to see
voltage as they recover slightly from not being loaded?