Hydrometer v Volt meter

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richardabeattie

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Which should I believe? The hydrometer says 1275 - exactly on the boundary of White and Green on the float which I think indicates well charged. The multimeter says 12.55 Volts which is a bit low isn't it? Both readings taken several days after the last charge. Battery is disconnected and at home so no load.
 
Try a new multmeter.

1275 is 95% charged, and equal 12.67 V at 15.6 deg C but varies with temp.

What was the temp when checking ?

Brian
 
So what do you want to know. Is it fully charged? I think the best way to ensure it is fully charged is to put a known amount of amp hours into it at a fairly slow rate so you don't boil it. Then you can be sure it is as charged as it can be.

If you want to know if the battery is any good? ie how much life left or will it do the job I want of it then I suggest you try it. A discharge test will give some indication of how good it is if you want to use it as a service battery. Even if you only get half of the rated amp hours it is probably still good. You might also need to do a high current test on t if you need it for starting. The best way being to start your engine with it.

Batteries can drop dead with sudden failure or die gracefully. Either way you probably have or should have a second battery system for emergency starts.

A hydrometer test can indicate a particular battery or cells state of charge if you know the hydrometer reading when you know it is fully charged. The problem is that some acid may have been lost in its life by boiling or spilling and been replaced with water. This reduces the SG even when fully charged. The battery will function on a lower SG acid (ie weaker) but the readings will not give an indication of charge. The indications on the hydrometer are based on the assumption that correct SG acid is in the battery to start with.

Voltage is often touted as the best indication of charge state but I am a little sceptical of the accuracy. I would say at 12.55v after being idle is probably charged OK.
So quit worrying its probably OK
olewill
 
Readings taken at normal room temperature
Yes the SG did vary from cell to cell but only very slightly.
Where do I get 12V supply at a level of accuracy greater than my multimeter?
Your helpful answers seem to indicate that the hydrometer is likely to be more accurate than any off the shelf pocket multimeter.
This is battery No 3 in a 2 battery installation so, yes, I have a spare!
 
The voltage check is only based on a approx conversion from S.G. just that a volt check simpler.

S.G. varies by 7 points per 10 Deg C, voltage by 0.01 volt per 10 points on S.G.

So not a lot to worry you.

Brian
 
I am not so sure about assuming that a hydrometer is more accurate than a digital multimeter, at least not the cheap ones from car parts shops.

I think if that battery was mine I would put it on charge, watching the volts, until it has risen to about 14.4 and been there for about an hour. Then check it after resting for 12hours. If all is well it will then be around 12.7 to 12.8. Then check it periodically over a period of days and weeks to make sure it retains its charge.

My leisure battery, which is 18 months old, was last charged about 7 or 8 weeks ago and still reads 12.64 volts. The hydrometer reading is about 1275 or just a little less. Current temp in the garage is 15C

Cheap Maplin multimeter, really cheap and nasty Halfords hydrometer. Origin of thermometer unknown.
 
Digital multimeters (even cheap ones) are usually accurate. I'd tend to believe it. If you don't then buy another cheap one, perhaps a different brand, and compare readings.
 
An on-load test is the best way to check the state of a battery. Specific gravity tests only measure the ability of a cell or battery to achieve a full charge. After time the actual available capacity of the battery may be well below its original "when new" capacity.

When measuring the voltage remeber that its the value at least an hour after all load has been removed that's important. Otherwise you may think your batteries are more discharged than they really are.
 
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