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Old 01-19-2008, 10:21 AM   #1
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Measuring specific gravity of battery electrolyte

Measuring specific gravity of battery electrolyte

Recently I purchased a hydrometer to check the cells in my batteries. My main interest was whether any cell or cells deviated substantially from the others.

I checked all 24 cells in my 4 batteries and found them to be very close and concluded that I didn't have any bad cells. BUT, the value of specific gravity I got suggested that the batteries were quite discharged. I charged the batteries again, let them 'settle' for 45 minutes and checked them all again. This time I got a slightly higher reading but still not 'in the green' on the scale. Beginning to suspect that my newly purchased hydrometer might be mis-calibrated I borrowed a friends hydrometer and checked all the cells. On his device all the cells read higher but still not 'in the green'.

I'm wondering if the low readings might be due to the temperature of the batteries (in the 50's)?

I also wonder if there is a way to check the accuracy of an hydrometer, maybe using distilled water?

Both hydrometers I used are like turkey basters with a float inside. I wondered if the other sort that seems to have a curl in it and claims to be temperature compensated are any better.

Last, I poked around looking for an electronic device to measure battery electrolyte specific gravity. I didn't find any. Is such a thing made? If so, how expensive are they?

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Old 01-19-2008, 11:21 AM   #2
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Don't think electronic gravity checker exist.
As you experienced all gauges have margin for error. (I own 5 tire pressure gauges and none of them repeats reading of the other one).
I think you worry too much. Make sure the batteries are charged and go from there. Gravity checker is necessary only when you are checking potential cell failure. The battery in our SUV is turning 10 years in the matter of days. Have never touched it. In my motorhome I have automatic battery charger. With that advanced gizmo took me 3 seasons to notice electrolyte level drop big enough to add some distillate water. Other members using converters have to add water on monthly basis.

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Old 01-19-2008, 11:26 AM   #3
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hi guy...

yep i'd speculate it's the temperature, which can affect charging and stored potential...

take one battery inside and warm to 70 or so...

then re test it.

as i recall almost all battery references/specs are at established 70-75 degrees...

and the graphs show clearly the drops offs with progressively cooler or really hot temps....

the main value of the turkey baster is comparing cells and looking for a bad one in the mix.

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Old 01-19-2008, 12:15 PM   #4
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Wet storage batteries should have a fully charged specific gravity of about 1.265 at 80 degrees F.
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Old 01-20-2008, 05:55 AM   #5
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My cold batteries read ok

FWIW, I store my 2 batteries in the garage during the winter and use a slow automatic trickle charger once per month to maintain them. The garage is about 40 and after a charge both batteries were in the green on my $5 hydrometer.

So, I don't think the temperature has as much effect as some would think on the hydrometer reading.
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Old 01-20-2008, 07:28 AM   #6
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Originally Posted by guy99
Last, I poked around looking for an electronic device to measure battery electrolyte specific gravity. I didn't find any. Is such a thing made? If so, how expensive are they?
Here's your electronic hydrometer. Look at the fifth item from the top. I don't know the price, but it probably costs as much as your four batteries.

You can check the calibration of your hydrometer if you have or can borrow an accurate graduated cylinder and gram scale. If you have a pharmacist in the family this would be easy. At 77F, a 100 ml sample of battery juice should weigh exactly the same as the reading on the hydrometer divided by 10. If the hydrometer reads 1220, the sample should weigh 122.0 grams.

A distilled water sample should read 1000 on your hydrometer, but they usually aren't marked that low.

Temperature compensation charts are available. If your battery is at 50F, the correction factor is -12 points, or -.012 if reading specific gravity directly.
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Old 01-20-2008, 07:31 AM   #7
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Temperature correction

Here is a formula for temperature compensation. The 0.03 pts per 10F uses 70F as a starting point.

Correction factor = (0.331 x T/F - 23) / 100 or 0.03 pts per 10 F

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