TheUndertaker replied:

A very difficult question to answer ,far to many variables involved to give a "correct " answer, motor size, compression ,state of tune ,spark resistance ect ect ect,,,however populist theory dictates that you should not discharge any battery lower than 50% of its capacity to increase its longevity and servicability ,,50% of charge is generaly 12.06 volts [rested batt].
Try www.windsun.com

Member - Sand Man (SA) replied:

Craig,

It's not the voltage you need to concern yourself with, but the Cold Cranking Amps available from the battery that will determine if the donk can be turned over.

The only way I know of to determine cranking capacity is to perform a load test on the battery. Any Battery Reseller outlet can do this for you. (free of charge)

To check the health of the battery cells, you can use a hydrometer, if the battery is not a sealed type. This will soon tell you if one or more cells are caputsky.

B0XER replied:

SandMAn is on the money,

Power is the result of current and voltage, therefore to try and give a definitive answer on what voltage is minimum for a starter motor to operate is difficult. It will also depend on the batteries ability to provide the required current.

However it is not only current either - as a 6volt battery supplying 800CCA will still not turn over your engine - it is the interaction of both.

A good rule of thumb is to leave at least 11.5 - 11.7v in the battery under load (in good condition) to start your engine. This is provided you have the correct capacity battery installed in your vehicle

old-plodder replied:

Above has covered it pretty well using the starter motor.

You can get a digital voltmeter combined into the battery terminal from Jaycar.

Absolute minimum voltage is about 10.5 volts for a clutch start.

Anything less and there may not be enough voltage for the coil and to allow the alternator to charge.
Yes, I did get caught once where I couldn't even clutch start it. :-)
I had the police there to verify it. :-(

Member - Craig M (NSW) replied:

Thanks for the replys guys. I guess I had better just keep an eye on what we run off the 12v.

Cheers

pixiemops replied:

State of Charge 12 Volt battery Volts per Cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75

this is for no load