They're used in low-priced tools with fixed batteries, which are stored on their chargers. At their low charging rates, excess electricity during overcharge is dissipated as heat at non-damaging levels. Quick chargers recharge in three to five hours. These higher-quality battery cells are designed to handle the increased overcharge rate. Again, excess charge is dissipated as heat, but leaving the battery in the charger for a long time after it's full will shorten its life.
Fast chargers are the best option for professional-quality power tools.
They recharge the battery in one hour or less. At these high charge rates, overcharge would damage or even explode the battery pack. So, as the battery nears full charge, the units turn off, or switch to a safer trickle charge to top off the battery. What differentiates fast chargers is the way they determine when the battery nears maximum charge. Budget chargers measure the total electricity applied to the battery or time the charging process.