There was a time when batteries not in use lost power at a rather fast rate. Some people will also say that once the battery is charged, you should disconnect it from the charging station or risk battery cell failures. Neither of these is the case with lithium ion batteries, which are the norm in cordless vacs. However, one on our list has a NiMH battery that requires special treatment. A battery will slowly leak the charge, but it’s so slow you wouldn’t notice it unless you didn’t use the machine for a month or more. Even then, it would still work for a time before stopping. These days, charging stations are made for vacuums to be plugged into them all the time, and that way you’re always ready to clean when you have the time to do so.