Battery state of charge by measuring voltage

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Who says you can't tell the battery State of Charge (SOC) by looking at the voltage? All you need to know is the total AH capacity and the rate of discharge/charge. Use the following formula to calculate the "C rate" and then use the chart in the link to determine SOC

C rate (C/xx) = Battery AH capacity/xx = amps out/in
or more usefully
Battery AH capacity/amps in/out = xx where the xx is C/xx
this will tell you the C rate curve to use.

http://www.arttec.net/Solar_Mower/4_Electrical/Battery Charging.pdf

Thoughts?
 
Oct 14, 2005
2,191
1983 Hunter H34 North East, MD
Don't know, Bill...

about the math. I use a digital voltmeter to determine my batteries' SOC: 12.6 = Charged; 12.2 = need to be recharged. :)
 
May 11, 2005
3,431
Seidelman S37 Slidell, La.
Me Too

I'm kinda with Dan on this one. !2.7 is fully charged. I use a DVM when I wanna check it, but lately have been getting real lazy. When the solar controller goes to float, I just figure they are all up to snuff and fine.
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Got it, with no, or low discharge....

The voltage curve is almost flat so you can just do the extrapolation for how charged the batts are.
When you start drawing current though, the relationship changes shifting all the voltages downward. For an LED lamp or bilge pump your solution works fine. When you have a reefer, freezer, and kids leaving on all the incandescents all the time the relationship is not so straight forward. I was looking for a chart to relate my current discharge rate and voltage to the SOC and this does just that. FWIW.

As for surface charge it only lasts for a few seconds at high current flows and ~ 10-15 seconds at low current flows so it really does not affect the curve. The curve is based on a constant charge or discharge so there should not even be any surface charge on the batts.
 

Johnb

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Jan 22, 2008
1,420
Hunter 37-cutter Richmond CA
I don't doubt that you can get plenty of useful information by reading the voltage, say during a weekend, but to really know what the condition is you need to factor in temperature, especially if it has been in cold storage for a while.
 
Jul 1, 2004
567
Hunter 40 St. Petersburg
Bill, I would've hated you when I was in school.

I always struggled in math while guys like you ruined the curve.

Just kidding (not about struggling in math, though.)

I suspect your method has merit and even though I have a Link 10 it sits in voltage mode most of the time which I consider a reasonable "down and dirty" quicky check of battery state.

I do know that things get pretty complicated when loads are increased or when the batteries are a little older and not quite what they used to be (kinda like me.)

Interesting post though.

Now, get down here and buy me a cold one for making me look bad in Mrs. Fitzgibbons algebra class . . .
 
Dec 1, 1999
2,391
Hunter 28.5 Chesapeake Bay
I mostly agree with Bill on this. While it is useful, and easy, to read how many volts yoiur batteries are putting out with a DVOM, such a test alone doesn't tell you much about cranking amps -- and that's what starts, or doesn't start, your engine. It is quite possible to have battery that can read 12.6-12.8 vdc ("fully charged") at rest, that also cannot put out the needed amps to start you engine. So, as Bill notes, you've got to go further than measuring volts to get a real sense of the health of your batteries.
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
Bill,

I have a considerable amount of time invested in bench testing to try and figure out how to best determine a battery SOC via volts. The only way I have found that is close to accurate, and I was benchmarking this with a battery monitor, is to let the batteries rest for a good long time, completely disconnected. I have tried high current discharges, low current discharges on, and on and on and still was never able to get the volts to agree with the monitor unless I had a 12-24 hour resting voltage. Deep cycle batteries have thick plates and can holed a surface charge for quite a while.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
I think the method Bill linked to would work fine - but it is not just looking at voltage. You need to also measure current (and know the battery total capacity).

Ie,
rate of discharge/charge
is another name for current in or out of the battery.

Interesting that the link uses voltage and current to determine the state. Puekert uses current and time to determine state. Both would be temp sensitive.
 
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