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Old 29-06-2012, 03:19   #1
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House bank at 12.2v ok?

My house bank is 8 Trojan t105s, 4 are brand new and 4 are from the previous owner and apparently only a year old.

But after using only 160 amp hours the bank is down to 12.2v. I was reading Nigel Calder saying this meant the batteries are 75% spent. In which case the older batteries must be on the way out.

Is this true?

Thanks geniuses!
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Old 29-06-2012, 03:55   #2
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Re: House bank at 12.2v ok?

To covert battery voltage to SOC you need to take into account the previous discharge or charge current, use an accurate voltmeter and then compensate for temperature. In practice this is difficult to do with any acuracy.

A useful measurement (but still a rough guide only) can be obtained by eliminating all charge discharge currents for 24 hours, or preferably more.
If you did this and the voltmeter is accurate and the temperature was not too low 12.2v does indicate a lowish SOC, or possibly some or even a single battery performing poorly. New and older batteries are not great mixed.

If the voltage reading was after a rest period I would do a capacity test on each battery separately.

When you have sorted out the problem ( check the charging as well) get yourself a battery monitor it will pay for itself.
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Old 29-06-2012, 04:48   #3
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Re: House bank at 12.2v ok?

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Originally Posted by noelex 77 View Post
To covert battery voltage to SOC you need to take into account the previous discharge or charge current, use an accurate voltmeter and then compensate for temperature. In practice this is difficult to do with any acuracy.

A useful measurement (but still a rough guide only) can be obtained by eliminating all charge discharge currents for 24 hours, or preferably more.
If you did this and the voltmeter is accurate and the temperature was not too low 12.2v does indicate a lowish SOC, or possibly some or even a single battery performing poorly. New and older batteries are not great mixed.

If the voltage reading was after a rest period I would do a capacity test on each battery separately.

When you have sorted out the problem ( check the charging as well) get yourself a battery monitor it will pay for itself.
....load/capacity testing is the only good test I knpw of in this case..
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Old 29-06-2012, 05:20   #4
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Re: House bank at 12.2v ok?

Trojan's Battery Users Guide- http://www.trojanbattery.com/pdf/TRJ...UsersGuide.pdf (Table 7, page 17) says that 12.1 V is 50% discharged. Their T105 golf cart batteries have 220 amphours each. Since half of your batteries are in series (and therefore add nothing to amphours) the total amphour capacity you have is 880.

160/880 means you have used 20% of the capacity, so something doesn't jive. It could be bad batteries but it could just as easily be meter error or not waiting at least 6 hours before measuring resting voltage.

Get a battery hygrometer and check the cell specific gravity. The Trojan guide also gives capacity vs gravity. That is much, much more accurate than voltage.

David
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Old 29-06-2012, 09:22   #5
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Re: House bank at 12.2v ok?

If you expect good life from your batteries you should limit discharge to 50% of capacity. Below that you rapidly start to reduce life. So if 12.1 is 50% discharged and you are at 12.2 you should "consider" them 90% discharged to achieve good life.

You don't say what the charging state was prior to this discharge cycle. Making sure your charging process is working correctly should be checked before you agonize over bad batteries.
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Old 29-06-2012, 09:33   #6
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Re: House bank at 12.2v ok?

Try to get an English translation of Calder.

At 12.2 volts a conventional wet battery would be down 0.4 volts, which is 40% down. It would have 60% of capacoity remaining.

Now, if you only want to cycle your batteries to the 50% point, then being down 0.4 volts mens you have already used 80% of the capacity that you want to use. But I have never heard of someone referencing capacity that way.

If you want to be more conservative and only cycle your batteries down to the 70% point (which increases the number of charge cycles radically) then you've already gone too far.

But do check again, something has gotten lost in translation.
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Old 29-06-2012, 09:46   #7
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Re: House bank at 12.2v ok?

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Originally Posted by djmarchand View Post
Trojan's Battery Users Guide- http://www.trojanbattery.com/pdf/TRJ...UsersGuide.pdf (Table 7, page 17) says that 12.1 V is 50% discharged. Their T105 golf cart batteries have 220 amphours each. Since half of your batteries are in series (and therefore add nothing to amphours) the total amphour capacity you have is 880.

160/880 means you have used 20% of the capacity, so something doesn't jive. It could be bad batteries but it could just as easily be meter error or not waiting at least 6 hours before measuring resting voltage.

Get a battery hygrometer and check the cell specific gravity. The Trojan guide also gives capacity vs gravity. That is much, much more accurate than voltage.

David
The issue I have with using resting voltage for state of charge is that you have to remove all current draw and charging from the battery bank for at least six hours before taking a reading. Also, the voltmeter or multimeter has to be accurate as one tenth of a volt makes a difference. If you only have a sealed no maintenance battery, then maybe this is the way, but in this case I would rather discharge the batteries over a given amount if time and rate of discharge. See graphs here to choose your discharge rate and point of discharge http://www.scubaengineer.com/documen...ing_graphs.pdf However, I really prefer using a hydrometer because little time is needed to determine state of charge and it is also possible to detect cells that are going bad. If you have all cells in the green and one at red part of the hydrometer, you need a new battery. Temperature compensation is easier for the hydrometer vs taking a volt reading. Also, you can add water to the battery after you do the hydrometer readings. The only other thing I can think of to watch out for with a hydrometer is that if the battery is being charged, small bubbles will form in the electrolyte that attach to the float in the hydrometer making the battery seem more discharged than it really is, and watch out for the acid as it will eat holes in your clothes. If the water level has fallen below the plates, my experience is that you cannot get a full charge after that has happened and you need a new battery.
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Old 29-06-2012, 20:39   #8
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Re: House bank at 12.2v ok?

Recently I tried a new trick. With two separate battery banks of three 4D batteries each, I used a digital infrared temperature meter that I bought at an auto supply. I found two bad batteries quickly before doing all the fancy testing. The weaker batteries seem to take in more charge, causing a higher temperature on the top of the cases. I am hoping to hear if anyone else has used this method of testing or monitoring their batteries. After pulling out the warmer batteries I checked them with a Midtronics battery tester, and they showed very low amp hours compared to the others. The digital IR thermometer has saved me lots of time, usually following the engine cooling system or shaft packing type concerns. The batteries stay charged automatically with battery chargers, so the voltage never gets very low.
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Old 30-06-2012, 00:27   #9
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Re: House bank at 12.2v ok?

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. The digital IR thermometer has saved me lots of time, usually following the engine cooling system or shaft packing type concerns..
I agree with your comments about the IR thermometer. It it one of those pieces of equipment that surprisingly is not often carried yet is reasonably inexpensive and very usuful.
( I would also put a clamp on multimeter and GOOD torch in this category)

Heat is very usuful in diagnosing electrical problems, not just batteries, but troubleshooting any circuit that carries reasonable current. Often just using your hand you can feel the heat given out by a high resistance connection.
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Old 30-06-2012, 04:07   #10
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Re: House bank at 12.2v ok?

Quote:
The weaker batteries seem to take in more charge, causing a higher temperature on the top of the cases. I am hoping to hear if anyone else has used this method of testing or monitoring their batteries.
As those batteries go bad the tops start to bulge upward from the heat. When they go bad you can see it too. Your test is pretty good but is a bit on the too late side. The problem is that the cells as they go bad slightly drop voltage and so they fake out the charge controller into adding a higher acceptance thus overcharging the good batteries and the bad ones really never come fully up to charge heat up and get worse. Thus the higher heat you are reading. Not sure how well it would detect an early sign or not. Good batteries do warm up during charging too but they would be more equal and the charge controller should have a temp sensor to lower the acceptance.

You can also examine water levels in the cells and notice cells that they take proportionally more water than the others as a sign of a bad cell. Removing a bad cell is the idea to keeping the rest of then bank healthy. Golf carts having only three cells means one bad cell costs you two more. The whole battery has to be removed.
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Old 30-06-2012, 07:37   #11
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Re: House bank at 12.2v ok?

A hydrometer (not a hygrometer which measures relative humidity) is your best friend for something like this. As many have pointed out, voltage measurements can only be used as a rough indicator since there are so many things that influence it and result in miss-readings. A Hydrometer measures the state of the chemical reaction involved in charge/discharge, and is the only definitive measure of state of charge. It's a pain in the butt to take the measurements and correct them for temperature, but it's really the only way to assess SOC, and you can do it on a cell by cell basis which will also surface problem cells. The good news is that once you get to know your batteries and what voltages are typical, and once you have confirmed via specific gravity measurements that you are reaching full recharge on your batteries, you only need to take SG measurements on occasion to check in on things.

Oh, and taking hydrometer measurements can only be done with flooded batteries. Those of us with AGMs, Gel Cells, or other sealed batteries only have voltage and current measurements to work with, and the thermal measurements which is pretty cool (no pun intended).
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Old 02-07-2012, 20:43   #12
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Thanks a lot for the comments!

I should say that when I saw the 12.2v it was not resting with everything disconnected for 6 hours. It was the reading off my battery monitor and I confirmed it with my multimeter.

I'll get a hydrometer!
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