Battery Voltage
Submitted: Sunday, Apr 20, 2008 at 10:57
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pauls
Hi
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.
Thanks
Paul
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.
Bill.
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.
Ian
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 - Mike DID - Monday, Apr 21, 2008 at 07:35
Monday, Apr 21, 2008 at 07:35
State-of-charge
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.
Capacity
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