Battery Voltage vs. State of Charge

Feb 6, 1998
Canadian Sailcraft 36T Casco Bay, ME
Maine: For simple people like me when conducting the light bulb test: how does one add/subtract light bulbs load as the discharge hours transpire?

Usually just some cheap toggle switches or even a PWM adjustable dimmer.... You can build a home made dummy load pretty easily...

Lets say you're testing a 100Ah battery at 75F. Your dummy load should be 5A.

As battery voltage drops the current being drawn by the light bulbs also drops thus artificially inflating your true capacity if you don't add in more bulbs to maintain the 5A load..

A DC bulb drawing 1.4A at 12.72V is burning 17.8 Watts. As battery voltage approaches 10.5V that same bulb is now burning 13.1 watts and the current it is pulling from the battery has dropped to 1.25A from 1.4A. As the battery voltage drops you need to add more small lights to the circuit to maintain the desired 5A load. Course if you want a close enough figure just apply a 5A load at full charge and let it go to the 10.5V cut off. You will have a slightly exaggerated Ah capacity but at least you'll have a rough idea..
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Feb 5, 2004
Tartan 3800 Westport, MA
The 3% - 6% number we hear bantered around was initiated by battery monitor manufacturers who recognized most boaters will not run charging equipment long enough to allow current to taper to the proper tail-current level.
I traced back to where I got that range into my head, and it is from Battery University (on the web). It's also all over the place in my notes, as I tried, for years, to get my old Xantrex InCharge smart regulator to fully charge my batts before it went to float. To get to .5% at acceptance I guess I could just crank it to the max accept time of 4.5 hours and watch the Ammeter, and forcing it into float when the current is at 0.5%C, which for that boat is now exactly 1 Amp. (But will likely never do that, especially when that boat sells).
Dec 29, 2008
Treworgy 65' Custom Steel Pilothouse Staysail Ketch St. Croix, Virgin Islands
I have 2 East Penn 8A4D AGM house batteries (400 amp hours)
We have the same batteries. We bought ours in 2007. We've always been nervous when they get below 12.5V. But, they have served us well. We have a 123W solar panel on our davit, and only a small pwm controller. But, up to this point we haven't had a major load on the batteries, as we had an AC only refrigerator. We will soon be converting to 12V Seafrost refrigeration, and will need to add about 300-350W of solar and an mppt controller. At that point, we anticipate we will need to replace those 10 year old batteries.

Like you, down here at St. Croix, we aren't going to have the occasion to do load tests, disconnect for 24 hours, plug into shore power, etc. Heck, we can't even comply with Maine Sail's list of steps to do the load test farther down in this thread, "#5 Allow the battery to rest for atleast 24 hours at 75-80F (40 hoursor more with AGM or GEL ismore accurate."! It is rare indeed that the temp here is only 80!

When we run the engine, it charges the batteries. When we run the generator, the Xantrex charges the batteries. The rest of the time, the solar panel seems to keep them up over 13.1V, and the settle down to maybe 12.9 overnight. What we get is what we get.
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