These devices are mainly for sulphate prevention. They can only remove minimal sulphate once it has built up. All it does is charge up to some voltage (in my case 13.5) and then let the battery float down .1V and then repeat. I have one on some batteries I have for standby power. I have not had it long enough to know (check back in 5 or 10 years...). However, it is only for batteries kept on long term storage. A starting battery in normal use in a car doesn't need it. And if you had a battery sitting around for a long time it probably wont help it.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra