Sealed Lead-Acid Batteries.

This technology is pretty old but is very reliable. In terms of watt-hours/kilogram, it is one of the worst performers due to the lead in the battery. But in terms of charging circuits, this is pretty trivial. It is also quite resilient as a power source, but besides the low power density, it also suffers from a relatively shallow charge cycle. The battery cannot usually be discharged below about 50% or it will lose capacity. Also, the charging curve is such that it is quick to get to 85% charge, but the final 15% can take a while. Most new designs would use LiPo or LiFePO4 batteries.