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Don't forget, the inverter isn't 100% efficient either. Some of the power it draws from the battery is converted to heat in the inverter itself, which is why they have fans. Assuming the inverter is 90% effecient (an educated guess, I don't know for sure), then in order to produce 1200 watts at 120 volts it would be drawing about 1330 watts from the battery. At 13 volts, the current drain would be about 102 amps. By the time it got down to 10 volts, the inverter would be drawing 130+ amps. Ain't no way a "100 amp" alternator is going to keep up with that. As others have pointed out, the only accurate way to assess the state of charge of the battery is to measure the specific gravity in each cell. However, in practice, by the time you drive around a little the battery will be back at full charge. Other than shortening the life of the battery a little, you won't notice much effect. I certainly wouldn't bother to check the specific gravity unless there were other indications of problems, such as hard starting, and even then the only thing you can do about it is replace the battery. A word of caution, lead acid batteries contain sulferic acid, which will do a real number on skin, clothes, metal, etc. Be careful what you do with a sample! I once made the mistake of wiping the top of a battery with a rag - a day later there was nothing left except some dust where it had contacted the battery. Keith
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