I have always checked various batteries for state of charge by measuring their voltage (when rested for at least 8 hours) using a good digital voltage multimeter. I have several graphs of voltage v. state of charge and have assumed that a battery in good fettle would have a voltage of approx. 12.6 volts. I was therefore surprised when measuring my new car's battery to find that I got a reading of 12.3v measured in almost identical conditions. Has battery type and manufacture got something to do with this discrepancy? The new battery is made by Varta.
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Unfortunately battery voltage, with no load on it, doesn't mean much. A difference of only 0.3 volts could be just manufacturing tolerance on the battery.
About the only meaningful test you can do with a voltmeter across the battery is to check the alternator charging voltage, preferably with a good load such as headlights and heated rear screen switched on. It should then read somewhere about 14,5 volts.
An abnormally low voltage, below say 10 volts, with a similar load could indicate either a faulty cell or just a flat battery!
The easiest way is to get a discharge test done at a local tyre/battery factors.
It sounds as if you haven't a problem, and are just interested, am I correct?
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Apart from what mjm said. Best test of all is turn all electrical circuits on.Engine running. Do a voltage test after a few minutes.If their are any problems with alternator it will show on voltage drop from battery..ie Alternator should be able to run all circuits-plus recharge battery.if it only supplys the circuits-not recharging battery its alternator fault. though from what you said I doubt a problem?
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Steve
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I thought voltage, after a wait if recently on charge, was a very good indication of the state of charge of the battery - with correction for temperature. Both my bikes and the car workshop manuals go into great detail with voltage/time/temp graphs showing how to check battery condition. 12.3 does seem down a touch if its had a good charge.
Martin
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I thought voltage, after a wait if recently on charge, was a very good indication of the state of charge of the battery - with correction for temperature. Both my bikes and the car workshop manuals go into great detail with voltage/time/temp graphs showing how to check battery condition. 12.3 does seem down a touch if its had a good charge. Martin
My sentiments entirely! Has always worked for me. I was surprised to see my fully charged new car's battery with this sort of reading.However it has started beautifully, instantaneously and vigorously since I have driven it from new since taking delivery last Christmas, despite frosts and snow and being parked outside.
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MJM is perfectly correct.You can get an acceptable voltage reading from a duff battery.That is why battery testers have a wavy piece of metal between the prongs,it is a resistance to put a load accross the battery.Another good test is to check the specific gravity of the electrolyte with a hydrometer.
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Another good test is to leave the car outside on a cold night and see if it starts easily the next morning!
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Some recent (new) Ford Transit batteries I've come aross have been difficult to diagnose problem wise. After an overnight stand they have a good voltage (12.6 or so), stand up to a voltage drop test, and a hydrometer shows at least a 3/4 charge. They won't start the engine though. A similar spec. new battery sorts it out, and Ford have replaced a couple under warranty. Cliff Pope has got it about right !
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The battery needs to be tested when under a suitable load.
Measurement of the cranking voltage is the one that I use myself, which is measuring the voltage across the battery terminals whilst the engine is being turned over on the starter motor. Since the starter motor typically draws about 300 Amps initially, which drops to around 100 Amps once the engine is turning over, this is a pretty severe test. It's far more severe than just turning the headlights on!
It will be necessary to prevent the car from starting whilst carrying out this test because that would rather defeat the object. Suggest removing the fuel pump fuse on a modern car, or, on an older model, just pull the LT terminal off the distributor.
The cranking voltage will vary depending upon several things, including size of battery, state of battery and state of engine, but I wouldn't expect it to be much less than 11.0 volts for a battery in good condition.
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The test equipment with the prongs is called a drop tester. The voltage when running is important on modern cars with engine management because the ECU needs a specific voltage to work properly. Recently my Range Rover was running roughly, wouldn't idle correctly but started ok. A test on the battery showed it to not be charging properly thus the engine management went into 'limp' mode.
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\"Nothing less than 8 cylinders will do\"
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