A normal lead acid battery which is in good condition and fully charged should read 12.8 volts in static mode. When being charged on a car it will probably go up to 13.8 volts and sometimes a little more depending upon the alternator and the accuracy of the control circuitry. The readings are also temperature sensitive.
The following is a way to check the system
Heres a couple or three checks you can do with a volt meter, but the real test is to have it checked by an autofactor with a proper amp tester.
Read the voltage across the battery and then start the engine and read it again with the engine running.
If both readings are more or less the same then swap the meter to read ohms, switch off the engine, and take an ohm reading between the battery negative terminal and a good earth point on the engine and also a good earth point on the car body. If there is any significant resistance at either test then check the main earth strap(s) for damage and security. If there is no significant resistance then read on:-
Change the meter back to volts and connect the positive probe to the battery positive terminal and the negative probe to the other end of the battery positive cable at the back of the alternator. If the reading is anything other than a 1, -1, or zero, then the battery positive cable needs to be changed.
If all of the above checks out and the voltage with the engine running is around 13.5 volts to 13.9 volts, or maybe a little more, then switch on the headlights, the heater blower onto full, then raise the revs to 2500 and check the volts again. If the reading remains in the 13.5 to 13.9 volt range then all is good in the charging system, otherwise the alternator is at fault.
If all of the above checks out and the battery is not holding a charge or not taking a charge, then the battery is very highly likely at fault.
Last edited by NeilM; 11/07/13 07:11 PM.
