The "SmartGuage" looks like a clever attempt to improve upon pure voltage measurements, by monitoring system voltages over time and comparing them to a number of stored profiles. It attempts to represent remaining energy capacity
, which is not the same thing as amp hours.
Though the SmartGuage folks have tried to simplify things and, in the process, have included some technical inaccuracies, much of the discussion re: the problems with shunt-type battery monitors is revealing, and should be read by those who "swear by their battery monitors", as well as those who've noticed that the readings they're getting don't necessarily correspond to reality.
I don't particularly like battery monitors and, like Barnakiel and Calder, believe that the intelligent use of an accurate digital voltmeter -- coupled with a knowledge of your particular boat -- will give you all the insight you need into battery condition and state of charge.
However, the shunt-type battery monitors have come a long ways. Some, like the Victron, include compensation for Peukert's equation, taking account of the rate of discharge and monitoring the rate of charge over time. This, coupled with other information, allows a fairly accurate calculation of remaining capacity of the battery bank. However, it is necessary to calibrate the meter when installed and, at least once a month to re-calibrate it (set it to zero) when the batteries are fully charged. Failure to do this will result in the types of inaccuracies we often hear about from boat owners.
I like the Victron stuff. On my boat I have a MultiPlus inverter/charger and have installed and programmed several others on customer's boats. The build quality of these machines is very very high, though some of the mechanical design "features" could stand a re-work as has been noted elsewhere. The new Victron BMV600 battery monitor
looks very good indeed. Maybe good enough to put one on my boat this year and get rid of my very RFI-noisy Blue Sea Systems digital voltmeter :-)