Get the car started a take a Volt reading across the battery and slowly rev up the engine. It should go up to 14.5 Volts and no more. This shows charging system OK and Voltage Regulator OK.
Drive the car or let it run with a little revs for 10 minutes.
Switch off and take another Volt reading, Should be 12 Volts
Leave for 20 minutes then take another Volt reading
Still 12 Volts battery is OK
Less and you need a new battery
Batteries are 6 X 2 Volt cells but will only give you 6 X lowest cell so 6 X 1.5 Volts = 9 Volts
Andy C
Battery should put out about 12V, even under load. If it has a shorted cell, it will only put out 10V. The car probably wont start, and you need a new battery
If a cell is open, it wont put out any volts under load, and you also need a new battery.
You also get a situaton where there is no dead cells, just the battery no longer has enough capacity to start the car reliably. In that case you have 12v at first, but it drops away fast as you you try and start the car. You guessed it, new battery.
With the engine running at idle, & the car in PARK, check the voltage across the batter terminals. IF the voltage is like 14.7 volts, then the alternator is working, so the battery is suspect. IF the voltage is 12, then the alternator is NOT working.
a full battery should have 12.6 volts form a 6 cell battery. each cell is 2.1 volts, roughly. if you get 9-10 volts aft er a full charge, a cell can be assumed as shorted .
If it's 100% dead, you'll get 10V instead of 12V (or less if it's not passing current).
If it's just a half-dead cell, it won't store so much charge. At full charge, you get 12V OK, but after discharging for a bit it drops to 10V, then maybe 8V etc.
yes
