Charger Wattage vs Amperage - max charging rates explained
Many chargers are rated with a max wattage and amperage but these maximums cannot be realized in all situations with batteries of all sizes. How can you decipher the specifications to find the maximum amperage capability of a charger for various battery sizes?
AMPS = WATTS / VOLTS
Using this formula allows us to figure out a charger’s maximum amperage capability for batteries of different sizes.
First, recall the voltages of various size LiPo batteries:
- 2 cell LiPo = 8.4V max
- 3 cell LiPo = 12.6V max
- 4 cell LiPo = 16.8V max
- 6 cell LiPo = 25.2V max
Now we’ll plug the above voltages into our AMPS = WATTS / VOLTS formula and figure out the max charge rates of a 50W charger:
- 50W / 8.4V (2 cell) = 6A max charge rate
- 50W / 12.6V (3 cell) = 4A max charge rate
- 50W / 16.8V (4 cell) = 3A max charge rate
- 50W / 25.2V (6 cell) = 2A max charge rate
As you can see the maximum charge rate of a 50W charger changes depending on the number of cells in the LiPo battery pack.
Want to charge a 6S battery at a higher amp rate? You need a charger with more wattage. Here is what to look for using a 6S 5000 pack as an example:
Calculate a 1C charge rate
We need to divide LiPo pack capacity (in mAh) by 1000 to convert from mA to A. This yields 5000mAh/1000 = 5Ah. Hence a 1C charge rate for a 5000mAh battery is 5A. A 2C charge rate would be double this or 10A.
Figure out charger wattage needed to charge at 1C (5A in this case)
Recall from above 25.2V is the amount of voltage needed to charge a 6 cell battery. 5A is our 1C charge rate for a 5000mAh battery pack. We’ll use the basic formula WATTS = AMPS x VOLTS.
- AMPS = 5A
- VOLTS = 25.2V
- WATTS = 5A x 25.2V = 126
- This means we need a charger capable of 126 Watts in order to charge a 6S 5000 at 1C (5A).