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Old 06-04-2019, 02:57   #1
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Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Hoping someone can help me to understand. I bought a boat last year that has 4 145ah Calcium Lead-Acid Sealed batteries. Their charging is controlled by a smart battery management system (bulk/absorption/float) both on the solar side and the shore power side. We are liveaboards and are relatively energy intensive because of our laptops. We rarely go below 70% of capacity though, but we also rarely charge them fully to 100% because it takes forever and is only really achievable on shore power.

At the same time, the batteries are somewhere between 3 and 4 years old (it's unclear), and on 2 of them, the diodes have turned white, indicating that they theoretically should be replaced. So I charged them to 100%, unplugged three of them (including both of the worse off ones), leaving one on the trickle charger as a control, let them sit for 14 hours, then used a voltmeter on each.

Results are: 13.1 and 13.21 on the two "bad" batteries and 13.23 on the good battery.

My understanding is that the resting voltage should fall somewhere between 12.9 and 11.5 volts (I know it can vary on either end), and I had planned to roughly estimate the remaining capacity based on that range, but um, my voltage is too high? What am I doing wrong?

Thank you!
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Old 06-04-2019, 05:47   #2
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

First off, you should try to upgrade your system so you do very regularly get to 100% Full, as per mfg trailing amps spec, or use .005C as endAmps.

When a batt is "fully" charged and isolated from loads, a certain degree of the resting voltage may be "surface charge".

Pull even a fraction of an Ah and you'll get a more accurate resting voltage.

None of which has to do with the health (capacity) of that battery.

Voltage doesn't even give an accurate reflection of SoC either.
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Old 06-04-2019, 07:03   #3
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Thanks for your reply, John! Any advice as to how I can determine the current health/capacity of the batteries?


Quote:
Originally Posted by john61ct View Post
First off, you should try to upgrade your system so you do very regularly get to 100% Full, as per mfg trailing amps spec, or use .005C as endAmps.

When a batt is "fully" charged and isolated from loads, a certain degree of the resting voltage may be "surface charge".

Pull even a fraction of an Ah and you'll get a more accurate resting voltage.

None of which has to do with the health (capacity) of that battery.

Voltage doesn't even give an accurate reflection of SoC either.
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Old 06-04-2019, 09:04   #4
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Quote:
Originally Posted by cstan22 View Post
Hoping someone can help me to understand. I bought a boat last year that has 4 145ah Calcium Lead-Acid Sealed batteries. Their charging is controlled by a smart battery management system (bulk/absorption/float) both on the solar side and the shore power side. We are liveaboards and are relatively energy intensive because of our laptops. We rarely go below 70% of capacity though, but we also rarely charge them fully to 100% because it takes forever and is only really achievable on shore power.

At the same time, the batteries are somewhere between 3 and 4 years old (it's unclear), and on 2 of them, the diodes have turned white, indicating that they theoretically should be replaced. So I charged them to 100%, unplugged three of them (including both of the worse off ones), leaving one on the trickle charger as a control, let them sit for 14 hours, then used a voltmeter on each.

Results are: 13.1 and 13.21 on the two "bad" batteries and 13.23 on the good battery.

My understanding is that the resting voltage should fall somewhere between 12.9 and 11.5 volts (I know it can vary on either end), and I had planned to roughly estimate the remaining capacity based on that range, but um, my voltage is too high? What am I doing wrong?

Thank you!
12.7-12.8V is a reasonable resting voltage for normal L-A batteries (straight lead or w antimony). Two reasons for your higher value:

- Calcium adds 0.5 to 1V to the resting voltage. Read the manufacturer's literature for details.

- Surface charge as pointed out above.

Batteryuniversity.com has pages that explain both points in detail.

By now you have almost learned that trailing amps into the battery is the only way to know SOC is 100%...
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Old 06-04-2019, 11:11   #5
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

To determine capacity you need to do a 20h discharge test. Google it.
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Old 06-04-2019, 15:12   #6
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

cstan-
"the diodes" ? There are many kinds of diodes, white ones mean nothing. I'm guessing that's some kind of arbitrary voltage monitor system you have.
A brand new full battery will typically show 12.6-12.8 volts, possibly a point more for rare types, if you are measuring TRUE resting voltage. Anything higher is just float voltage. Put a load on the batteries for 3 minutes (inverter and hair drier, whatever) and let them rest another half hour, and NOW measure them. That will be resting voltage with any float charge knocked off. Also, it can take 24 hours for the electrolyte and charge to equal out in a wet cell battery, after all charging has ended. "Resting" voltage can be a moving target.
Your volt meter can also be the problem. If it is not calibrated, it can be off by 0.2-0.4 volts, and that's a 20% to 40% charge loss or gain.
For each 1/10th of a volt that your rested batteries are below their maximum voltage (~12.8) there's a 10% capacity loss. If you have been cycling them to 70% discharge, they'll get used up fast. Better not to take them below 50% down, or even better 30% down, to get the longest life out of them.
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Old 06-04-2019, 18:33   #7
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Quote:
Originally Posted by cstan22 View Post
Any advice as to how I can determine the current health/capacity of the batteries?
http://www.cruisersforum.com/forums/....php?p=2732929
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Old 06-04-2019, 20:12   #8
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

As others have posted forget about resting voltage. It has little meaning as a determination of battery health.

Second, and as others have posted run your batteries up to full charge.

IF... If you had a good SOC meter you could get an aproximation of battery health. Thats the only real way to do it given that you live on the boat and are constantly using your batteries.

The easiest real life test of your batteries with you living aboard and no instruments is if they provide a reasonable amount of time of discharge with your real life loads, and if the voltage tends to be pretty even on each battery with a heavy load.
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Old 07-04-2019, 07:05   #9
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Thanks for the note about calcium adding .5 - 1v more to the resting voltage. Good to know! And thanks for recommending Battery University. I couldn't find anything about "trailing amps" though?

Quote:
Originally Posted by svlamorocha View Post
12.7-12.8V is a reasonable resting voltage for normal L-A batteries (straight lead or w antimony). Two reasons for your higher value:

- Calcium adds 0.5 to 1V to the resting voltage. Read the manufacturer's literature for details.

- Surface charge as pointed out above.

Batteryuniversity.com has pages that explain both points in detail.

By now you have almost learned that trailing amps into the battery is the only way to know SOC is 100%...
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Old 07-04-2019, 07:12   #10
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

These particular batteries contain on each a magic-eye diode that, if I'm understanding the sticker guide, show "white" (as opposed to "green") to indicate that the battery should be replaced. Since I didn't buy these batteries nor have I owned a boat before this, I'm trying to get a sense of whether that diode should be heeded and ultimately what the state of health of the batteries are. This is part of my larger scheme to go from net-output to a net-input of energy (mostly solar vs. us running the inverter to power our laptops). Replacing the batteries seems like an obvious first step, but if it's not necessary yet, maybe the better step is to increase solar input or add a wind generator. Not sure yet.

Clarification: we rarely discharge more than 30%.

We're not really ever able to get to 100% capacity unless we hook up to shore power because the Voltronic has to float the input for days to knock out that final few percent, assuming we turned off our refrigerator and all.

Quote:
Originally Posted by hellosailor View Post
cstan-
"the diodes" ? There are many kinds of diodes, white ones mean nothing. I'm guessing that's some kind of arbitrary voltage monitor system you have.
A brand new full battery will typically show 12.6-12.8 volts, possibly a point more for rare types, if you are measuring TRUE resting voltage. Anything higher is just float voltage. Put a load on the batteries for 3 minutes (inverter and hair drier, whatever) and let them rest another half hour, and NOW measure them. That will be resting voltage with any float charge knocked off. Also, it can take 24 hours for the electrolyte and charge to equal out in a wet cell battery, after all charging has ended. "Resting" voltage can be a moving target.
Your volt meter can also be the problem. If it is not calibrated, it can be off by 0.2-0.4 volts, and that's a 20% to 40% charge loss or gain.
For each 1/10th of a volt that your rested batteries are below their maximum voltage (~12.8) there's a 10% capacity loss. If you have been cycling them to 70% discharge, they'll get used up fast. Better not to take them below 50% down, or even better 30% down, to get the longest life out of them.
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Old 07-04-2019, 07:19   #11
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Thanks again! Definitely will save this guide for the future. Looks like I'd need to have the boat essentially offline for up to 44 hours? Not going to happen anytime soon haha.

One thing that catches my attention about what ksanders says below - is that "a good SOC meter" should do the trick. I guess that's what our Magnetronic Battery Control System functions as in this case? And in this case, it reads 580 ah at full charge, which is the manufactured capacity of the batteries. It takes a few days to eek out the final 10%, even on 220v because of the regulated charging, but it gets there. Does this mean that the batteries still have 100%, and if so, how is that possible after 3-4 years? Or does this mean that the amp hour meter doesn't calibrate along with the batteries as they age?

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Originally Posted by john61ct View Post
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Old 07-04-2019, 07:21   #12
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Quote:
Originally Posted by cstan22 View Post
Thanks for the note about calcium adding .5 - 1v more to the resting voltage. Good to know! And thanks for recommending Battery University. I couldn't find anything about "trailing amps" though?
Regarding amps, read your battery's tech info from its manufacturer. You can find the explanation in the documents for flooded Trojan batteries (sth like battery is full when current INTO BATT TERMINALS at voltage AT BATTERY TERMINALS equal to manufacturer's specified ABSORPTION voltage is equal to Ah capacity divided by 200 hours, same as times 0.05%)

Capitals don't mean I am rude, they mean thousands of batteries have been murdered by letting an impatient timer say that the battery is full. Read Mainesail for gort detail.

And yes, you can fin info on trailing amps (when is my battery full) in batteryuniversity.com

If Ibwas you I would post the technical info of your batteries. Otherwise you are asking people to give you irrelevant advice.
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Old 07-04-2019, 07:24   #13
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Hiya, thanks for your reply! See my other response to John. I have a gauge (Magnetronic) that tells me that, in fact, the batteries are at 100% (580ah). The batteries are at least 3 years old. So, I don't know if this means that the batteries still have full capacity or if the gauge doesn't calibrate along with the degraded state of the batteries. Thoughts? Also, is their ability to reach 100% the only indicator of health? For example, would an older battery dispense its energy faster or take longer to charge?

Quote:
Originally Posted by ksanders View Post
As others have posted forget about resting voltage. It has little meaning as a determination of battery health.

Second, and as others have posted run your batteries up to full charge.

IF... If you had a good SOC meter you could get an aproximation of battery health. Thats the only real way to do it given that you live on the boat and are constantly using your batteries.

The easiest real life test of your batteries with you living aboard and no instruments is if they provide a reasonable amount of time of discharge with your real life loads, and if the voltage tends to be pretty even on each battery with a heavy load.
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Old 07-04-2019, 07:32   #14
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

Sure thing. Good idea!

4x Hankook Power Control Sealed Maintenance Free Battery
Calcium+
MF64589
12V
145AH (20HR)
260MIN (RC)
800 A (EN)

That's all I got. The closest I could find to these was on Battery Megastore, where they're indexed as Truck batteries (not sure if that matters?), and I haven't been able to find any manufacturing documentation tucked away on the boat, although the previous owner carefully preserved the manuals for the hatch arms, induction hot plate, small 12v fan, electric hot water kettle, etc. haha

Quote:
Originally Posted by svlamorocha View Post
Regarding amps, read your battery's tech info from its manufacturer. You can find the explanation in the documents for flooded Trojan batteries (sth like battery is full when current INTO BATT TERMINALS at voltage AT BATTERY TERMINALS equal to manufacturer's specified ABSORPTION voltage is equal to Ah capacity divided by 200 hours, same as times 0.05%)

Capitals don't mean I am rude, they mean thousands of batteries have been murdered by letting an impatient timer say that the battery is full. Read Mainesail for gort detail.

And yes, you can fin info on trailing amps (when is my battery full) in batteryuniversity.com

If Ibwas you I would post the technical info of your batteries. Otherwise you are asking people to give you irrelevant advice.
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Old 07-04-2019, 08:39   #15
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Re: Resting 12v Battery Voltage Reading Is 13.2v; What Am I Missing?

You can pull them out and take to your local auto parts store and have them load tested for free. That will answer your concerns.
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