In everyday language, and as a crude approximation my working guide for typical car systems with ordinary batteries is that you should observe :
Rest, fully charged, no load: Voltage > 12.6V. This is not a sensitive early sign of battery failure. But if the voltage is lower there is a problem.
Cranking: Voltage > 9.5V. Low voltage while cranking can be the first sign of battery failure. The voltage at the starter is sensitive to high resistance from poor connections etc regardless of the condition of the battery.
Charging (say 3,000 rpm to allow for unusual pulleys): Voltage > 13.8V, and usually > 14.2V. This is a test of the charging system, but don't try it without a battery.
Electronic systems eg ignition can't be relied on to work below 9.5V.
I'm happy to be corrected by the experts.
From my experience and the archives:
1 Most problems can be quickly localised with these three tests, which need a multimeter, and sometimes some good jump leads and another car alongside. But in many threads it's hard to extract this information. If you're asking for advice adding the observed voltages is going to save you time.
2 Inspecting, checking and cleaning feeds and earths as early as possible also saves time in the long run.
Jonathan
Edited by - Jonathan Kay on 17 Jul 2014 08:00:45