Using voltage to measure your battery
Submitted: Monday, Dec 20, 2004 at 23:37
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Member - Jimbo (VIC)
As a follow up to my Solar Test posts there was comment that voltage readings are not an accurate measure of charge as you need to let the batteryseetle for 24 hours to get an accurate reading.
This is correct. My battery was showing 12.83 after 30 mins of settling. I have just checked it another 30 hours later and it is showing 12.75. When I tested it yesterday the ambient temp was 25 C and at present it is 18 C and we know that batteries read higher when warmer.
So although the voltage reading will be different after 24 hours, the difference is so miniscule, in my opinion, to be of almost no significance.
As a rule, 12.0 to 12.1 indicates around 50% charge which is a level you should keep to in the interests of battery life; higher is better. Therefore I believe taking a reading after as little as 15 min settling will give you a fairly accurate indication of where your battery is at.
Cheers,
Jim
Reply By: Rosco - Bris. - Tuesday, Dec 21, 2004 at 08:08
Tuesday, Dec 21, 2004 at 08:08
Interesting point Jim, but it's only a guide. I've been having probs with a battery and the Waeco. Could charge the battery up to 13V and it would run the compressor no worries, plus a 100W hand held.
Hook up the fridge and the bloody error light blinks. Off I went to the battery
shop where they did a load test. Hooked it up to some gizmo and cranked the load up to about 300A on one dial and on the other the voltage did a Gold Medal dive.
End result, the battery is on the way out and so is my Chrissy play money. So unfortunately voltage only tells part of the story when it comes to battery condition.
Have a good one
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Reply By: drivesafe - Tuesday, Dec 21, 2004 at 12:18
Tuesday, Dec 21, 2004 at 12:18
Jimbo, you have only put one situation forward where you have conducted a voltage test to try and determine the stored capacity of a battery. You have made your measurements on a battery coming off a full charge.
Try charging the battery to full charge again and then put a decent load on the battery. Let the battery drop to around 11 volts with the load on it, remove the load at 11 volts and I’ll bet over the next few hours, the battery will gain at least a volt. Reading the voltage of a battery to get an idea of it’s stored capacity is not nearly close enough to get any where near an accurate idea.
Each individual operation of the battery, whether it is charging or powering a load or even a combination of both, you can regularly have situations where the load is constantly changing.
All these factor will have an effect on the recovery rate the battery will go through before it gets to a true stable voltage.
As for leaving the battery for 24 hours, this is just a way to get a reading that is closer to the true capacity of the battery, but waiting for 24 hours is quite obviously unrealistic and still not guaranteed to be correct, especially if the battery is not in good nick.
The test you were doing with the solar panel was a far more viable means of determining how long the battery is going to last under given circumstances but to get a full idea, you would have been battery to leave the set-up running until the battery started to show that it was not up to full charge after being on the solar panel all day.
As you put it, the test satisfied your needs, but the test did not reach any form of conclusion which puts the voltage readings in the same boat.
Cheers
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Follow Up By: Member - Jimbo (VIC) - Tuesday, Dec 21, 2004 at 18:48
Tuesday, Dec 21, 2004 at 18:48
D/S,
I do understand the voltage under load/reading thing. But is impractical to do anything else in a real life situation with a sealed battery. When i take a reading of the battery after it has been taking charge i make an assumption that the reading is high. Conversely when I take a reading in the morning when the battery has not been charged and has been drained by the fridge I estimate it to be low.
The test did form a conclusion; that is the panel will adequately supply me with enough power for two or three days without dmamging my battery. As I said from the start, that was all I was trying to establish.
It was a practical, not scientific test. Will a 64 watt Uni panel keep you going in the bush for two or three days? The answer is yes in the conditions I tested it.
Cheers,
Jim.
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Follow Up By: Mike Harding - Tuesday, Dec 21, 2004 at 19:32
Tuesday, Dec 21, 2004 at 19:32
What drivesafe says is correct.
On my workbench (just behind me) I have five SLA batteries, 4 x 12V at 7Ah and 1 x 6V at 4Ah. Using my (recently calibrated) DMM I have just checked the unloaded terminal voltage of them, the readings were:
12.99 - 12.21 - 6.22 - 13.04 - 12.73. Now I know three of those batteries are totally stuffed and will not supply even a small load for more than 2 or 3 minutes - can you tell from those voltages which the bad ones are?
Having said that - I agree it is difficult to measure state of charge of SLA (without discharging the battery - which sort of defeats the point :) and I use a DMM myself when out bush for that purpose _but_ it is only a rough indication and if the battery is wearing out is probably quite misleading.
Mike Harding
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Follow Up By: TheUndertaker - Tuesday, Dec 21, 2004 at 21:55
Tuesday, Dec 21, 2004 at 21:55
Mike i,ll bite and guess ... your 6v = stuffed ..12vs = 12.21 + 13.04 are both us. tell me Im wrong ?? and best of all ,why.
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Follow Up By: drivesafe - Wednesday, Dec 22, 2004 at 00:23
Wednesday, Dec 22, 2004 at 00:23
Hi Mike, I will try a guess and say the three 12 volt batteries with the highest voltage reading.
Over the years i have found that batteries that are stuffed and have been on some sort of charge, can show a high voltage while there was no load on the battery.
Place the smallest of loads on the battery and the voltage went south in a very short time. Remove the load and the voltage would rise again.
cheers
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348635
Follow Up By: Mike Harding - Wednesday, Dec 22, 2004 at 06:23
Wednesday, Dec 22, 2004 at 06:23
Brave tries guys :)
The first three are US - the 13.04 and 12.73 are good.
>and best of all ,why.
I don't really know - it's a chemical process and "I know nothing" :) about chemistry. The dead batteries have sulphated I presume so whilst each cell still generates a potential difference of ~2V it is only capable of holding a very small amount of charge and, of course, a DMM only takes a tiny amount of energy from the battery to measure it's voltage.
Mike Harding
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Follow Up By: Member - Jimbo (VIC) - Wednesday, Dec 22, 2004 at 08:32
Wednesday, Dec 22, 2004 at 08:32
OK,
Here's something I forgot to mention. I also take readings when the fridge is cycling and generally find a drop of about (and I'm working from memory) less than .2 volts. This I assume will vary from fridge to fridge according to how much they draw.
When I start to see the drop between standing and working increase I will assume the batt is on the way out, Fair assumption? I will then head to my friend at battery World and get him to load test it.
Cheers,
Jim.
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Follow Up By: Mike Harding - Wednesday, Dec 22, 2004 at 13:59
Wednesday, Dec 22, 2004 at 13:59
A much better way to do things Jimbo. The difference between the two readings will increase as the charge level of the battery drops and as the battery ages and looses it's ability to hold charge the time period associated with the above will also reduce. But certainly measuring terminal voltage under load is going to give you a better idea of battery state than a no-load measurement.
Mike Harding
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