Lithium Polymer or LiPo batteries have very specific charging requirements and MUST only be changed by specific chargers designed to charge lithium polymer batteries.
A 1s or 1 cell LiPo battery has a nominal voltage of 3.7v. When fully charged it has a maximum voltage of 4.2v and when fully discharged, it should never go below 3.0v without risking cell damage.
A 5s4p battery pack means that the pack contains 5 cells in a series circuit and 4 cells in a parallel circuit.
Since each cell is 3.7v (nominal) a 5s LiPo battery has a nominal voltage of 18.5v, a fully charged voltage of 21.0v and a maximum discharged voltage of 15.0v before damage occurs.
When charging LiPo batteries, they must be charged at the voltage of the number of cells in series, therefore a 5s4p pack must be charged as a 5 cell pack.
The LiPo charger you’re using must be able to handle the cell count of the battery you are charging.
Most of the more expensive LiPo chargers will automatically detect the cell count of the battery being charged while the cheaper ones will require a manual setting. Wile some of the really good ones will allow you to manually select the cell count and then will double check it automatically for you.
Please also note that some chargers (such as those used for toys or cell phones) are made to charge a specific cell count and are not configurable for other cell counts. It is very important that these chargers only be used to charge the batteries they are designed for.
Also, chargers that auto detect the cell count of a LiPo battery can sometimes be wrong. They use the current voltage of the battery to determine the cell count and if the battery is fully charged or at a lower voltage than it should be, it may read the cell count incorrectly. This is why it is very important to double check that it reads he right cell count which is typically displayed on the LCD display.
For example a 5 cell 18.5 volt LiPo battery that’s been depleted to less than 15 volts may be confused with a 4 cell, 14.8v battery and thus charged as such. Also, a fully charged 5 cell battery at 21.0v may be confused as a 6 cell 22.0v battery and charged as such.
Charging a lithium polymer battery at a higher voltage than it’s rated for, or overcharging it, can lead to a fire or an explosion (see video below).
If you notice your LiPo battery pack is swelling, stop the charging process immediately, put the battery in a safe container and observe it for 15 minutes.