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Old 09-06-2018, 06:37   #1
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How to measure the voltage of a battery?

Voltage is indicative of the state of charge, but it should be measured at rest. Based on what I read, it is supposed to be 12+ hours. Not practical...

Load impacts voltage. What isn't clear is the speed at which voltage observed in a circuit returns to the battery's (ex. Battery voltage is 12.7 V, running the windlass brings it down to 12.2. Battery voltage will slowly return to, say, 12.65 in 12 hours.)

So, in the evening (no solar to mess with the readings), let's say that I take a reading, isolate the battery, take another reading after 5 minutes, and a third after 30. Could I use these 3 readings to approximate the battery's voltage, and therefore its state of charge? Or does it depend on too many factors so there's no point trying this?
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Old 09-06-2018, 08:46   #2
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How to measure the voltage of a battery?

OK, this is controversial, but if you go by the chart that requires 12 hours of resting, but obviously you canít let them rest for 12 hours, you will always be conservative.
Meaning that you may really be at 60% SOC when the voltage is indicating 50% SOC, but you wonít hurt your bank that way if your using voltage to determine when you need to start a generator or cut back on consumption until sunrise or to just get an idea where you are SOC wise. Lifeline has published a voltage SOC chart for their batteries under different loads. This chart is only applicable for their batteries of course.
Youíll notice the voltages are not that much differentClick image for larger version

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Old 09-06-2018, 10:20   #3
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Re: How to measure the voltage of a battery?

Hook a calibrated voltmeter up to the battery. Calibrated, because each 1/10th of a volt is 10% of the battery's charge, and cheap voltmeters can be off by .2-.4 volts on the "12" volt scale.

Charging stopped, read the voltage. Turn on a small load, some radios, some lights, watch the voltage drop. You should be able to see it stabilize after just a couple of minutes, and continue to drop very much slowly. At that point turn off the loads, you've burned off the float charge and the battery will stabilize pretty well in a few minutes more.

Yes, sitting for twelve hours is nice. But five or ten minutes will probably be more accurate than most voltmeters are going to be.
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Old 09-06-2018, 10:53   #4
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Re: How to measure the voltage of a battery?

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Lifeline has published a voltage SOC chart for their batteries under different loads. This chart is only applicable for their batteries of course.
Youíll notice the voltages are not that much differentAttachment 171323

Fantastic. Thanks for sharing this.

Yes, differences are small and predictable. Really to bad that they start at 1h. Would have been great to start at 5 mins rest...

Well. That could be a project for this Summer.
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Old 09-06-2018, 19:31   #5
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Re: How to measure the voltage of a battery?

I have a Link 2000 meter that measures both the voltage and the energy in/out of the battery adjusted for current, battery characteristics, charge efficiency, etc. Most of the time, when I compare % discharge estimated from the voltage and from the energy out counter (columb counter) the voltage based estimate is lower. I have gone all the way down to 95% discharge and still get useable power from the battery when the voltage would estimate a fully discharged battery. Basically the voltage reading is just a rough estimate.

Get a columb based gauge.
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Old 09-06-2018, 23:20   #6
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Re: How to measure the voltage of a battery?

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Basically the voltage reading is just a rough estimate.

Get a columb based gauge.
(I have a Victron.)

From what I read, it is the other way around. Battery monitors provide estimated SoC. Voltage at rest is a better measure.
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Old 10-06-2018, 09:19   #7
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Re: How to measure the voltage of a battery?

Forget it and use either a hygrometer or look at end of charge current, voltage at best only give a rough guide. Most boats have a fairly regular rate of discharge so you can have a guide that says "at anchor recharge when the battery reads X volts" but I would not use voltage to indicate full charge. I use 3% of A/hr at 14.8v as my 'end of charge' and then refer the hygrometer reading to the one I took when the batteries had been installed and cycled a few times. This is more about tracking how the battery is aging.
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Old 10-06-2018, 10:41   #8
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Re: How to measure the voltage of a battery?

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Originally Posted by gauvins View Post
Voltage is indicative of the state of charge, but it should be measured at rest. Based on what I read, it is supposed to be 12+ hours. Not practical...

Load impacts voltage. What isn't clear is the speed at which voltage observed in a circuit returns to the battery's (ex. Battery voltage is 12.7 V, running the windlass brings it down to 12.2. Battery voltage will slowly return to, say, 12.65 in 12 hours.)
There is only two ways to check batteries and voltage. One is the battery itself... under load (engine starter for example) when cranking, should read anywhere above 9.5 volts. Anything less the battery should be changed. The second way, battery voltage charge, engine running above idle should read 13.7 to 14.2 volts.
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Old 10-06-2018, 11:02   #9
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Re: How to measure the voltage of a battery?

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There is only two ways to check batteries and voltage. One is the battery itself... under load (engine starter for example) when cranking, should read anywhere above 9.5 volts. Anything less the battery should be changed. The second way, battery voltage charge, engine running above idle should read 13.7 to 14.2 volts.

Sorry but that could be really confusing as it depends on the controller. With a 3 step regulator voltage may go down to that level after a preset TIME but that does not mean the battery is at 100% charge. If you are using an on-board regulator it may give you 14.4v at the alternator regardless of state of charge. That often means 14.2 or less at the battery even if it is at 60% charge. The voltage in that case will only reduce if the charge acceptance rate is greater than the alternators max output. You also need to look at the charge acceptance rate at that voltage. Charge acceptance is very voltage critical, 0.5v increase will more than double charge exceptance rate.
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Old 10-06-2018, 11:48   #10
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How to measure the voltage of a battery?

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Originally Posted by gauvins View Post
Fantastic. Thanks for sharing this.



Yes, differences are small and predictable. Really to bad that they start at 1h. Would have been great to start at 5 mins rest...



Well. That could be a project for this Summer.


OK, I wasnít clear. This is a voltage /SOC chart with bank under a LOAD, not resting. The 1 hour is under such a load that it will be dead in 1 hour, 8 hours, 20 hours and of course 120 hours.
Most of us are on the 20 hour scale, however as I said once you get out of the insane load levels like the 1 hour one, the voltages per SOC are close.

Also know this chart is only useful on a Lifeline AGM bank, your bank will have different numbers, most likely.

However if you use the chart for resting voltage under no load for when you are under a load, it will be conservative, but will still give you an idea of SOC. If you use that chart it will say your at 50% SOC for example when your really at 60% or so, but that just gives you a safety margin.
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Old 10-06-2018, 20:33   #11
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Re: How to measure the voltage of a battery?

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Originally Posted by gauvins View Post
Voltage is indicative of the state of charge, but it should be measured at rest. Based on what I read, it is supposed to be 12+ hours. Not practical...

Why not practical?

I have 6 batts making up the house bank, each on a latching relay. Hence it's easy to disconnect one batt and let it sit for 12h, no problem at all. In fact, two batts have been sitting there disconnected for more than a month now, but that's for a different test.
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Old 11-06-2018, 07:05   #12
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Re: How to measure the voltage of a battery?

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I have 6 batts making up the house bank, each on a latching relay.
Most would consider that setup impractical.

Of course once you have it then rotating / resting bank members is easy.

Have you ever quantified the extra resistance you're adding to the intra-bank connections?
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Old 11-06-2018, 07:11   #13
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Re: How to measure the voltage of a battery?

Get a SmartGauge.

Under your usual usage patterns, document the V vs SoC correspondences.

Now you don't need the SmartGauge anymore.

Until the charts needs revising, as the bank ages. Or your patterns change.

Maybe get a group of owners together, each has the SG for a couple months at a time?

Personally, I'll just hang on to one myself, and never mind the charts.

For those that don't want / need precision, just keep ballparking off voltage no worries.

But it is a lot of trouble to try to get precise SoC off voltage.
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Old 11-06-2018, 17:07   #14
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Re: How to measure the voltage of a battery?

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Most would consider that setup impractical.

Of course once you have it then rotating / resting bank members is easy.

Have you ever quantified the extra resistance you're adding to the intra-bank connections?
Even if I had such a meter it would be difficult to reliably measure sub-milliOhms as even a bit of corrosion or flexing of the cable could change that value.

The relays themselves add less than 3 mOhms each, according to the specs. Some of that added resistance is being offset by smaller currents flowing through the bus, as big consumers like windlass and inverter have batts close-by to cover most of the high currents. The charge currents are usually smaller than the peak discharge currents.

Hence I'm reducing the losses and increase the reliability by de-centralising the electrical system.

Most boats have one central breaker panel, which means that (almost) all the power has to go there first and then back to the consumers. That's not my idea of efficiency.

Admittedly, my boat already came with two breaker panels, a smalle one aft with 8 circuits and the main one in the salon with 20+. So it wasn't my idea in the first place, I just extended it. Also, the boat had one batt bank aft which was fed straight from the wind turbine and the second one in the wheel house fed by the solar charge controller. Now I also have two LFPs under the settee feeding straight to the 1.5kW inverter plus one batt in the anchor locker for the windlass.

Simple yet efficient.
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