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Author Topic: 12 volt battery reading 17 volts  (Read 5217 times)

Peterm

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12 volt battery reading 17 volts
« on: October 18, 2007, 06:42:16 PM »

Happened today.   Sailing `Peadar Elaine` crabber.   Went well for about 15 mins, then came to a halt.   Got it in to land, checked fuse, OK, removed 12 volt lead gel battery and it was reading on my DMM as 17 volts!!  Checked DMM on two other batteries and DMM is fine.   I have ditched the battery, but any explanations, please?   Pete M
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malcolmfrary

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Re: 12 volt battery reading 17 volts
« Reply #1 on: October 18, 2007, 10:01:27 PM »

Either a miracle or accidentally reading the main battery and the RX battery in series.
I remember once seeing a nominal 50 volt battery being run by a float power unit with a dud regulator build up to 64.5 volts before it luckily blew a voltage regulator in something fairly harmless (all the other transistors in the building were rated at 65 volts max).  A panicy switch-off of both the standby and main power had the voltage drop to sensible levels after a few minutes due to the load.
I would check the voltage of the next battery to be charged on that particular charger very carefully.  The volts had to come from somewhere, and that is the likeliest culprit.
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Peterm

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Re: 12 volt battery reading 17 volts
« Reply #2 on: October 19, 2007, 10:59:46 AM »

Malcolm, just done some more testing.   Not a miracle, just stupidity on my part.   Checked the DMM battery, (PP3), should have been 9v, was actually 6.5v.   The other batteries that I checked were 6v so presumably the meter could cope with those, but not with a 12v.  I`m supposed to be an expert, but you know the rude definition!   Pete M
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malcolmfrary

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Re: 12 volt battery reading 17 volts
« Reply #3 on: October 22, 2007, 11:20:22 AM »

Having made lots of mistakes and either remembered how to avoid them or blame somebody else, I have occasionally been called an "expert".  Those who call me that usually go on to remind me that "ex" is Latin for "has been" and "spurt" is English for "drip under pressure". O0
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sweeper

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Re: 12 volt battery reading 17 volts
« Reply #4 on: October 22, 2007, 11:27:47 AM »

A variation on the "expert" definition.
An expert is someone who knows more and more about less and less, until they know B****r all about anything.

(Tongue very firmly in cheek, of course!)
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Peterm

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Re: 12 volt battery reading 17 volts
« Reply #5 on: October 22, 2007, 11:51:24 AM »

Malcolm and Sweeper, you are both correct!   Pete M
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andygh

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Re: 12 volt battery reading 17 volts
« Reply #6 on: October 22, 2007, 12:52:40 PM »

But did you find out why the boat stopped? :'(
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Peterm

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Re: 12 volt battery reading 17 volts
« Reply #7 on: October 22, 2007, 03:07:04 PM »

Not yet.   Wife fractured her ankle on Saturday, and she has different priorities!!   Pete M
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cos918

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Re: 12 volt battery reading 17 volts
« Reply #8 on: October 26, 2007, 11:45:29 PM »

hi there reading about your battery. little tip a 12v 7ah bat can read up 13.5v on a dvm but best way to check them is to put them under load few amps ,car headlight. I get 4 year fire alarm battery from work ,   there still OK , any one was reading about 12v no load and it dropped to 5v under load, battery had run it life so i would recommend all when using a dvm check the battery under load to get a true reading works for me.
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Peterm

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Re: 12 volt battery reading 17 volts
« Reply #9 on: October 27, 2007, 09:32:10 AM »

Cos, despite my silly mistake, I too also check batteries under load.   Pete M
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dpbarry

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Re: 12 volt battery reading 17 volts
« Reply #10 on: December 27, 2007, 09:43:17 PM »

hi there reading about your battery. little tip a 12v 7ah bat can read up 13.5v on a dvm but best way to check them is to put them under load few amps ,car headlight. I get 4 year fire alarm battery from work ,   there still OK , any one was reading about 12v no load and it dropped to 5v under load, battery had run it life so i would recommend all when using a dvm check the battery under load to get a true reading works for me.


Hi cos918

I bought a new 12v 18AH battery from maplin and when I connect a DVM onto it, it reads up to 13.5v after I have charged it and left it to settle. 

I gather a fully charged battery should read approx 12.65v so why am I getting a reading of approx 13.5v? Is that because I have no load on it?

Regards

Declan
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Stavros

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Re: 12 volt battery reading 17 volts
« Reply #11 on: December 27, 2007, 10:09:37 PM »

Dont worry at all mine are showing a similar reading after being on charge and I am not concerned at all,lets put it another way a car batt will give a similar reding when fully charged

Stavros
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Colin Bishop

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Re: 12 volt battery reading 17 volts
« Reply #12 on: December 27, 2007, 10:14:39 PM »

As Stavros says, it's quite normal. Once you put a load on the voltage will drop to a bit over 12v. It's just a characteristic of lead acid cells.
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dpbarry

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Re: 12 volt battery reading 17 volts
« Reply #13 on: December 29, 2007, 04:43:28 PM »

As Stavros says, it's quite normal. Once you put a load on the voltage will drop to a bit over 12v. It's just a characteristic of lead acid cells.

Cheers Folks.  So what is the best way to measure the state of the battery?  With a load??

Declan
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Colin Bishop

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Re: 12 volt battery reading 17 volts
« Reply #14 on: December 29, 2007, 05:50:22 PM »

Yes, put a car sidelight bulb or something similar across the terminals and watch the voltage. If it holds steady at around 12v or just under for 5-10  minutes or so then the battery is probably OK. If putting a load on rapidly drops the voltage to under 11 then the battery is not holding its charge properly.
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