This extra battery is exclusively for the SPEED FEED trimmer model kit only. One-speed [off and high]. It has "Nickel Metel Hydryde Battery System" - meaning the batteries do not have a memory so you can groom at ease however short or long you wish - no need to let the battery run down. Each battery will last about an hour of actual clip time. The charger has a FAST charge [charges batteries in 20 min.] and a SLOW thourough charge [charges batteries in 40 min.]. Either battery can be put in either SLOW or FAST charge mode. It doesn't matter. You can leave your batteries overnight... all week... whatever. They won't melt down or overcharge. The battery will never fall out like the other arco brand. The SPEED FEED has an E-Z lock battery... finally a trimmer with a brain!