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Old 11-16-2012, 10:24 AM   #1
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% of battery charge by voltage

On one of the discussion I mentioned the % of the battery charge can be estimated by the voltage of the battery. The battery has to be at rest, not charging, and no high usage. The amp reading on the Eagle is normal 1 amps just some lights on, which I call rest. When the battery get down around 12.5 time to charge. Been using this method for 40+ years. I believed most expensive fancy battery monitors are just reading the volt?



13.2+ volts 100%
12.8 volts 75%
12.4 volts - 50%
12.2 volts 25%
11.8 volts dead

The % is also on some volt meters. Anyway it's close enough for me.

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Old 11-16-2012, 11:02 AM   #2
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Quote:
Originally Posted by Phil Fill View Post
On one of the discussion I mentioned the % of the battery charge can be estimated by the voltage of the battery. The battery has to be at rest, not charging, and no high usage. The amp reading on the Eagle is normal 1 amps just some lights on, which I call rest. When the battery get down around 12.5 time to charge. Been using this method for 40+ years. I believed most expensive fancy battery monitors are just reading the volt?



13.2+ volts 100%
12.8 volts 75%
12.4 volts - 50%
12.2 volts 25%
11.8 volts dead

The % is also on some volt meters. Anyway it's close enough for me.
What kind of batteries do you have? Our wet cells are full at ~12.70 volts and 50% full the voltage reads ~12.20.

AGMs and Gels are full at or just under 13.0 volts.
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Old 11-16-2012, 11:27 AM   #3
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Go to http://www.trojanbattery.com/pdf/TRJ...UsersGuide.pdf and read Trojan's User Guide for batteries. It says that measuring voltage is the least prefered way to measure state of charge. But in any case you must wait 6 hours with no in or out current to get a true reading.

On one of the forums, someone posted voltage readings vs time for a fully charged battery after the charging source was removed. It took as much as 24 hours for the voltage to stabilize. But after 6 hours as recommended by Trojan, the voltage was pretty close to its final measurement.

Table 7 in the Trojan document gives the resting voltage vs state of charge for flooded cell batteries. 12.7 is 100%, 12.1 is 50% and 11.5 is 10%.

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Old 11-16-2012, 11:33 AM   #4
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Greetings,
How about one of those battery load testers? Any wait time?

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Old 11-16-2012, 11:41 AM   #5
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What kind of batteries do you have? Our wet cells are full at ~12.70 volts and 50% full the voltage reads ~12.20.

AGMs and Gels are full at or just under 13.0 volts.
Wet! I said this was an estimate and what is on some volt meters. If you use a different % that is up to you. I use 12.5 is when to charge. If you want to use 12.2 great! I do not want to get into a contest over the %.

MY point is/was the % charge can be estimated by voltage and fance expensive monitoring is not really necessary.
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Old 11-16-2012, 11:52 AM   #6
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I agree, Phil. I started years ago with a similar chart. Even though I now have a battery monitor, I still keep the chart handy for reference, but never refer to it.
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Old 11-16-2012, 01:11 PM   #7
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The voltage values seem high to me
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Old 11-16-2012, 04:47 PM   #8
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The 'fancy' battery monitors don't use voltage...they count amps in and out and are highly accurate. When a battery manufacturer like Trojan tells you not to use/rely on the voltage 'method', it would be wise to take note.
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Old 11-16-2012, 09:01 PM   #9
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Trying to use voltage is a real crapshoot depending on how "scientific" you want to be by waiting without ANY load on your batteries (not real practical for a cruiser)....will it give a rough idea??? Sure but how "rough" is good enoug...you might be better off "guessing" based on knowing what loads have been on for how long.

RT...the battery tester shown is a great tool for testing the cranking amps of a battery...a little less useful in telling the health of a deep cycle and darn near useless telling how many amp hours are left in a battery bank.
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Old 11-16-2012, 09:16 PM   #10
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Each to his own but to the OP's question about whether voltage is what the "fancy" meters use, I think that has already been answered with a firm "NO". I think I paid just over $200 for my Trimetrics (one on the boat and one in the bus) - best $200 I ever spent. "Resting voltage" doesn't mean you turn off the loads and take a reading - it means the batteries have to sit for a while before you take the reading - that's simply not practical for most real world use.
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Old 11-17-2012, 06:49 AM   #11
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Each to his own but to the OP's question about whether voltage is what the "fancy" meters use, I think that has already been answered with a firm "NO". I think I paid just over $200 for my Trimetrics (one on the boat and one in the bus) - best $200 I ever spent. "Resting voltage" doesn't mean you turn off the loads and take a reading - it means the batteries have to sit for a while before you take the reading - that's simply not practical for most real world use.
One of many expert/battery site recommendations....

Open-Circuit Voltage Test:
You will need a Voltage Meter to perform this test (D.C. side)
For accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs.
1. Disconnect all loads from the batteries.,,,etc....etc


Not in all the reading I have done....that's why it's not practical for "in use" batteries....

that's why I mentioned using this method is a crapshoot...but do admit that once you know your battery bank you may have a reasonable idea when it needs charging.
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Old 11-17-2012, 07:13 AM   #12
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"The 'fancy' battery monitors don't use voltage...they count amps in and out and are highly accurate. When a battery manufacturer like Trojan tells you not to use/rely on the voltage 'method', it would be wise to take note."

In addition the unit "learns" just what it takes to fill and discharge your specific batt bank.

This is very useful as the batt set ages over the years and the banks capacity shrinks.
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Old 11-17-2012, 10:07 AM   #13
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In addition the unit "learns" just what it takes to fill and discharge your specific batt bank.
I'm not exactly sure what you mean by that but I don't think that is true. What does happen is that owning a SOC meter forces you to better understand how your battery bank charges and specifically what constitutes a "100%" charge. Its not quite as simple as hooking up the meter and watching the needle move up or down. Measuring watts is a little trickier than measuring water or fuel. It is however orders of magnitude better than trying to use volts to measure battery SOC.
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Old 11-17-2012, 10:19 AM   #14
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I did say the voltage is an estimate and did not imply it was the best. However, it does tell me to start charging the batteries. What I look at is the color zone the neddle is in, green 100%; yellow 75%, and red 50% time to charge. If I was concerned I would spend the money on a bigger battery bank and/or charger before monitors, which is what I have done. Anyway, works for me.

We have three 8D batteries, 600+ amps, 300 usable amps that lasts 8 hous to get us though the night with temps below freezing. We have gone with out power for weeks at a time and the voltage % works for me.
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Old 11-17-2012, 10:36 AM   #15
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Google Magnum BMK for a good description of their shunt (and related parts and pieces) and how it allows measurement of % charge and real time amps, volts, ah in/out and min/max DC volts.

If your boat is a dock fixture there is probably not a lot to be gained with a BMK, but if you cruise and anchor out a lot they are pretty informative. The BMK type devices biggest customer bases are motor homes, standby gensets/battery banks and hybrid/electric cars.
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Old 11-17-2012, 11:48 AM   #16
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Pretty much the only way to accurately determine the state of charge of flooded cell batteries:



Of course, it's pretty much a PITA to use and can't be used on sealed batteries.
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Old 11-17-2012, 12:37 PM   #17
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Ron

Right you are, along with the dial phone and tube & CRT TVs, the SG float devices have their following.

Years ago the favorite trick for a few fly-by-night gas stations was to pull an electrolyte sample with a "specially calibrated" SG device and say "your battery is shot."
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Old 11-17-2012, 01:35 PM   #18
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Ron

Right you are, along with the dial phone and tube & CRT TVs, the SG float devices have their following.
Here's my phone:
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Old 11-17-2012, 06:53 PM   #19
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I did say the voltage is an estimate and did not imply it was the best. However, it does tell me to start charging the batteries. What I look at is the color zone the neddle is in, green 100%; yellow 75%, and red 50% time to charge. If I was concerned I would spend the money on a bigger battery bank and/or charger before monitors, which is what I have done. Anyway, works for me.
And me. My voltmeter is a useful guide, by no means "state of the art". You know your boat and what you expect it to read in most circumstances if all is well.
The hydrometer is useful testing a battery suspected to be on the way out, for defective cell(s) etc. A cheap accurate test to save junking a battery if it proves not to be the real culprit.
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Old 11-17-2012, 07:46 PM   #20
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Actually, the correct answer is "all of the above", but especially the hydrometer. We have a very inverter-intensive cruising lifestyle, preferring to anchor out or take a mooring (we used to live full time for months on a mooring). We have had a full Magnum system on board for about 5 years, Ms 4024, BMK and AGS.

The issue with the BMKs, as nice as they are, is twofold: 1) battery manufacturers tend to lie about the amp hour capacity of deep cycle batteries. Magnum wisely advises you to round down when inputting your capacity. 2) As a bank ages, capacity reduces. Towards the end of its life, our most recent bank, originally 400+amp hours at 24 volts, realistically had a capacity of maybe 100 amp hours. I had to keep dialing the AH down in the BMK, based on hydrometer and voltage readings, and then triangulate from there.

The only way to know true state of charge is via hydrometer. Of course, you don't have this option with AGMs or gels. So as the bank gets old, you have to look at volts as well.

For true deep cycle batteries, open circuit 12.5 volts is way high for "need" to charge. Thats 80% SOC on a true deep cycle. 12.1 (1.17SG)or 12.2(1,2SG), i.e. 50 or 60% is more like it.
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