When you charge your battery(ies) they need to be balanced charged. Most battery packs consist of multiple cells connected in series, these cells need to be charged together to stop over-charging and make them last longer.
Most batteries will have the positive and negative power cables as well as balance cables, which go to each cell. You need to plug both these in when charging to balance charge.
The simplest way to charge your battery(ies) is with a balance charger.
When you get a charger, you need to make sure you are able to connect your batteries via the correct connector (XT-90, bullet, ect.), and that it’s compatible with the number of cells your battery(ies) has/have.
Consider using a BMS (battery management system).
A BMS is a small circuit board that manages and balances the charging and discharging of your batteries. Using a BMS in your board it a little more difficult to set up and can be a bit more expensive, but it becomes much easier to charge thereafter.
See our BMS board parts list for more information.