I used to be a telephone technician back in the day. All the exchanges had lead\acid batteries as backup in case of power failure. Part of my duties was to " cycle" the batteries, which meant taking a specific gravity check of all the cells using a hydrometer and then discharging the batteries until they dropped to a specific voltage. Batteries were then put on charge until they reached a specific voltage and the s\g stayed constant over 3 consecutive readings an hour apart. That indicated the battery was fully charged. The point of discharging was to remove the buildup of crud on the plates, prolonging the life of the battery. These things were a metre high, by 1\2 metre square , were open to the atmosphere and weighed heaps, so you didn't want to be replacing them too often. The good battery " conditioners" do the same task, "cycling the battery to minimize sulphation of the plates. As mentioned by others, will pay for itself in battery life in no time, especially as bike batteries tend to lead a hard life, often not being used for months at a time. Our bikes, in standard form at least, don't have a residual drain on a battery when idle, but modern ones with anti theft, clocks, computers etc do, and will often discharge a battery within a few weeks of idleness. Back in the day this was not an issue as usually our bike was our sole (and soul) means of transport and they didn't sit round long enough to go flat!