One of the decisions you need to make before getting a new charger is how much power you need. Electrical power is measured in watts, and this is the key to figuring out how many amps your charger can provide for your charging needs. The formula for this is: watts/voltage = amps.
To decide if a charger is right for you, divide the charger’s power rating by the voltage of the largest battery pack you’ll need to charge. Since LiPo cells have 3.7V per cell, you multiply 3.7 by the number of cells (e.g., 7.4V for two cells, 11.1V for a three cells, etc.). As an example, let’s take a 4S (4-cell) pack with 14.8V and a charger rated at 100 watts. Using the formula, 100 watts divided by 14.8V equals 6.75 amps, which is the maximum current the charger will be able to deliver for a 4S battery. For a 6S LiPo (22.2V), the same 100-watt charger will deliver 4.5 amps. This means that a 6S 4500mAh pack would have a 1C charging rate. Any battery larger in capacity would be charged at less than the 1C rate, which means it might take a little longer to charge depending on the discharge condition of your battery pack. For larger cell-count packs, use higher-power chargers in the 200W to 500W range so that your charge current can be higher.