Battery Voltage

Submitted: Sunday, Apr 20, 2008 at 10:57
ThreadID: 56798 Views:1771 Replies:7 FollowUps:2
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My starter battery is over 3 years old and is working OK. However due to its age I thought I would test its voltage. I got the following readings after being stood overnight and without the motor running.

Initial test - 12.54V
Put on headlights and tested it - 12.05V
Tested it after the headlights had been turned off - 12.48V

How do I interpret these results, and should I be looking for a new battery soon.


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Reply By: Sand Man (SA) - Sunday, Apr 20, 2008 at 11:44

Sunday, Apr 20, 2008 at 11:44
The battery voltage sounds normal to me.

When you switch on your headlights, you are drawing a considerable amount of current and the voltage drops a little as a result.

Consider that when you normally use you headlights you are traveling and therefore have the engine (and alternator) running. This more than adequately puts back what you are taking out.

The problem that occurs when a battery no longer stores the energy being supplied from the alternator and cannot cope with current draw from other devices such as headlights.

Generally, the first signs of a drop in battery performance is when you experience prolonged starting, usually on a cold winter morning. The battery is not storing sufficient energy output from the alternator the previous time the vehicle was driven, especially at night when the lights are on and it would take longer to "top up" the battery.

If you are in any doubt as to your battery's condition, drop in to any battery retailer and ask for a "load test" which is a free service.
This will give a true indication of whether the battery is still functioning successfully.



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AnswerID: 299414

Reply By: Dunaruna - Sunday, Apr 20, 2008 at 12:10

Sunday, Apr 20, 2008 at 12:10
What type of battery? If it's a wet lead acid then your readings are in the ball park of being O/K. (a little low, but consistent with a 3 year old battery).

Turning on the high beam and watching the voltmeter is an effective load test, couple that with a hydrometer test to read each cell and your good to go.
AnswerID: 299415

Reply By: Thermoguard Instruments - Sunday, Apr 20, 2008 at 14:28

Sunday, Apr 20, 2008 at 14:28
Your figures sound pretty fair.

A better check is to measure it about an hour after a decent drive (15 min minimum). A fully-charged lead acid battery should be sitting around 12.8 V after some time to 'settle' following charging (i.e. after the engine has been engine switched off).

With the engine running it should read 14.0 to 14.4V with not much electrical load on.

As mentioned by another, a hydrometer measurement is the best way to establish the state of charge. keep the de-min water up to it and you night get another few years.

AnswerID: 299433

Reply By: Member - Phil G (SA) - Sunday, Apr 20, 2008 at 15:49

Sunday, Apr 20, 2008 at 15:49
Just did the same with my 18mth old N70 620CCA Extreme. (3 voltmeters gave almost identical readings)

Initially: 12.78V
Headlights on high beam for 2 minutes: settled on 12.27V
After headlights turned off came back up to 12.73V

Sounds to me like the battery is not quite charged or your voltmeter is underreading a bit. If it cranks fine, then I'd keep using it, unless about to embark on a remote trip.
AnswerID: 299443

Reply By: Member - Davoe (Yalgoo) - Sunday, Apr 20, 2008 at 16:51

Sunday, Apr 20, 2008 at 16:51
take it to an auto sparky and get it load tested (should be free). you will get no useull info about your batteries health with a voltage check
AnswerID: 299455

Follow Up By: Dunaruna - Sunday, Apr 20, 2008 at 18:07

Sunday, Apr 20, 2008 at 18:07
I don't wish to start an argument but a load test machine that sparkys use IS a voltage drop load test.
FollowupID: 565636

Follow Up By: Member - Davoe (Yalgoo) - Monday, Apr 21, 2008 at 02:37

Monday, Apr 21, 2008 at 02:37
Then let me re phrase that
you will get no usefull info about your batteries health with a standard multi metre
FollowupID: 565748

Reply By: Paul Grabonski. Vic - Sunday, Apr 20, 2008 at 19:11

Sunday, Apr 20, 2008 at 19:11
If it is a diesel vehicle I would replace the battery. Going into winter with a battery with the readings you have given is not smart. Diesel starters are expensive to repair and anything less than full battery voltage of 12.7v is not good. 12.05v with the headlights is a sure indication the battery is well on the way out.
AnswerID: 299479

Reply By: Member - Mike DID - Monday, Apr 21, 2008 at 07:35

Monday, Apr 21, 2008 at 07:35

To estimate a battery's state-of-charge (how full it is compared with what it could hold) using a voltmeter you need to leave its standing without no charging or discharging (not even the interior roof light) for 4 hours - 12 hours if its a deep cycle wetcell battery.
There are tables that show 100% to % and the corresponding voltage, but these voltages vary with battery type - standard wetcell, calcium wetcell, AGM etc. The voltage varies temperature too.


Do not confuse state-of-charge with capacity - your N70 may have had 80 amphours capacity when new, but after being badly treated it may now have a capacity of 20 amphours. An open circuit voltage test after 4 hours rest may show that it’s at 50% capacity - that meant 40 amphours when new, but now it’s only 10 amphours.

Voltage during discharge
Stopping discharge from your fridge when the voltage reaches 12.3 volts does NOT mean you have 50% remaining - the X% capacity versus battery voltage ONLY apply for testing many hours after stopping discharge. You will actually have more than 50% - it depends on your battery type an dhow much the fridge draws.

Voltage for full charging
The voltage at the end of a charge depends on the battery temperature.
AnswerID: 299562

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