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Old 21-07-2010, 07:23   #1
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Voltages vs Percentages

can someone list for me the types of voltages we should be seeing vs the percentage of charge on or deep cell batteries. Ie if 13.25 = 100% then what is 12.25?
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Old 21-07-2010, 07:32   #2
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Battery SOC chart

I think you're looking for the SOC chart. It varies some based on temperature and if you're charging or discharging. This link provides some info.
http://www.scubaengineer.com/documen...ing_graphs.pdf
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Old 21-07-2010, 08:15   #3
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About 50%

OCV vs SOC - Remove the surface charge, measure the Open Circuit Voltage (OCV) with an accurate (.5% or better) digital voltmeter across the terminals, temperature compensate, and... Cruisers & Sailing Photo Gallery
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Old 21-07-2010, 08:16   #4
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My understanding is that this does not work reliably. The only good way to assertain the state of charge of a battery is to measure charge in and charge out.
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Old 21-07-2010, 08:18   #5
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How does this work if an unknown percentage of the charge is converted to waste heat? ..or is it known?
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Old 21-07-2010, 09:25   #6
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Originally Posted by Broomfondle View Post
My understanding is that this does not work reliably. The only good way to assertain the state of charge of a battery is to measure charge in and charge out.
My understanding is that resting voltage is the best you can do for sealed batteries, testing specific gravity is better for wet cells, and measuring amps in and out is not reliable.

Here is a good explanation of the problems with amp-hour counters.
SmartGauge Electronics - SmartGauge compared to Amp Hours Counters
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Old 21-07-2010, 11:00   #7
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Here is a good explanation of the problems with amp-hour counters.
SmartGauge Electronics - SmartGauge compared to Amp Hours Counters
thanks - that's a really useful link . we've got an AH counter, and despite it's imperfections, i think it still allows me to take better care of the batteries than i would with nothing (or just a voltmeter). that link's great though, and better understanding the shortcomings of the AH counter will help me to be even more cognizant of the true battery state.

if the OP's trying to ascertain battery discharge level for the purposes of battery care (as opposed to just asking a general question about battery attributes), i'd still recommend some sort of meter. they're pretty cheap, and even if they're imperfect, they're better than guessing or taking a voltage reading. the few bucks you'll shell out will probably be returned to you in extended battery life.
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Old 21-07-2010, 11:08   #8
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Originally Posted by JayH View Post
My understanding is that resting voltage is the best you can do for sealed batteries, testing specific gravity is better for wet cells, and measuring amps in and out is not reliable.

Here is a good explanation of the problems with amp-hour counters.
SmartGauge Electronics - SmartGauge compared to Amp Hours Counters
Keep in mind that these folks have a product to sell so their explanation might be a bit biased.
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Old 21-07-2010, 11:28   #9
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State of charge (SOC) can be pretty accurately measured with a digital voltmeter, provided that there has been no charging or discharging for several hours. I've done this on my boat for over 20 years, with no surprises. However, you do need to be able to "rest" the batteries for several hours -- best overnite -- in order for this method to be accurate.

However, and it's a BIG however, SOC is only one of the things you need to know. As important....or, arguably even more important....is the capacity of your battery bank. It's perfectly possible to have a fully charged battery (100% SOC) which is totally incapable of, say, starting your engine. We see this often in cars, where the battery has reached the end of its useful life.

Over time, battery capacity (i.e., the ability to accept and hold and deliver electrical energy) deteriorates due to a number of factors, including: plate sulfation, corrosion, contamination, stratification, physical damage, excess heat, overcharging, undercharging, repeated deep discharge to levels below 50%, etc., etc. Figuring out how much capacity remains in your batteries can be a tricky exercise.

Properly used, battery monitors (AH counters) can approximate battery capacity, but the only ways to really get a good estimate is either thru:
(1) a time-measured discharge with a known load calculated to the 20-hour rate; or (2) a relatively costly measurement device which measures internal battery conductivity/resistance (like the Midtronics testers).

The first method is by far the most accurate. The second method has become the "standard of the industry" since most mechanics/engineers don't have 20 hours to wait around for the battery to fully discharge.

FWIW,

Bill
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Old 21-07-2010, 11:54   #10
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Originally Posted by btrayfors View Post
State of charge (SOC) can be pretty accurately measured with a digital voltmeter, provided that there has been no charging or discharging for several hours. I've done this on my boat for over 20 years, with no surprises. However, you do need to be able to "rest" the batteries for several hours -- best overnite -- in order for this method to be accurate.
As usual Bill is spot on with his advice.

For a visual explanation of measuring SOC via a DVM feel free to read this.

Measuring A Lead Acid Battery SOC (LINK)
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