The problem with Accutrons is that they were designed, in the late 1950s, to be powered by the new 1.3 volt mercury button cells. For health reasons, mercury batteries were phased out in the 1980s and were replaced by 1.5 volt silver oxide batteries.

This increase in voltage from 1.3 to 1.5 volts doesn't affect many of the contemporary watches like the Hamilton Electrics but the tuning fork on an Accutron vibrates at a greater amplitude when run at 1.5 volts and this can lead to double-indexing.

Double indexing means that, occasionally, two teeth of the index wheel are advanced with each vibration rather than the usual one tooth. A very fast (i.e. + hours/day) running Accutron is a sure sign that double indexing is taking place and it's probably due to the higher voltage from a silver oxide batteries.

Various solutions have been developed to enable Accutrons to run in the 21st century. One is to adjust, or re-phase, the indexing system to cope with the higher voltage. In my experience, this is possible with most Accutrons, provided the index wheel is in good condition. However, re-phasing should be carried out by a competent Accutron repairer.

Another solution is to run the watch on modified silver oxide batteries known as Accucells. Accucells have a tiny circuit that lowers the voltage to 1.3 volts. If you have a very fast running Accutron and you do not wish to have it serviced and re-phased, you can always buy an Accucell from ebay and see if this cures the problem (note: this is not a substitute for a proper service!).

For a more in-depth discussion on battery technology and re-phasing etc, I strongly recommend you take a look at Rob Berkavicius's excellent site here.