[alanw]

Unless you have NiCad you should not discharge your batteries down that low. I think 50% is the recommended max on lead acid, unless it's a deep cycle which can go as low as 20% but for longest life should stick with 50% still.

You can check your battery charge level with a voltmeter. 12.6 volts is fully charged; 12.4 volts is 75% charged; 12.2 volts is 50% charged; and 12.0 volts is 25% charged.