I have several different LiPo batteries which I use with my drones, many of them are different voltages and different capacities. How do I know at what current I should charge my drone LiPo batteries?
Charging Lithium batteries is a far more delicate process than discharging them because of the complex chemical processes involved. Batteries that are rated for tens of "c"s of discharge rate are usually only rated for 1-2 "c"s of charging rate. Standard charging rates are 1c for regular speed and 2c for fast charging, with 2c damaging the battery more.
C-ratings are an artificial marketing term that doesn't really help very much without being converted using this formula:
Charging Current (amps) = C-rating * Battery Capacity (amp-hours) where one amp-hour (Ah) is equal to 1000 milli-amp-hours (mAh).
So, for example:
- 1500 mAh battery charging @ 1c = 1.5 A charging current
- 2000 mAh battery charging @ 1c = 2.0 A charging current
- 2000 mAh battery charging @ 2c = 4.0 A charging current
- 2000 mAh battery charging @ 0.5c = 1.0 A charging current
Charging at higher currents (higher c-ratings) is more damaging to the battery's cells and is more likely to cause complications like fires and explosions while charging. The opposite is true for charging at lower currents. It is hardly ever recommended to charge at more than 2c, and staying as close as possible to 1c is always recommended for safety and battery longevity.
Generally, when charging LiPo batteries, you should charge them at a 1c charge rate for best longevity. This means that you charge them at 1 amp per amp-hour of capacity. so, for example, you charge a 1500mAh LiPo at 1.5 amps. (1000mAh → 1 Ah)
The cell count of the LiPo (voltage) doesn't really impact the charging current, but it is important to charge a LiPo at the correct voltage. Most LiPo chargers will let you set the voltage, current, and sometimes other things.
For small batteries that have an unusual capacity, charge them at the closest possible current. For example, you could charge a 450mah LiPo at either 0.4 or 0.5 amps.
Also, there's nothing stopping you from charging a LiPo at more or less than 1c. So, if you need a battery to charge quickly and you don't care if the battery gets damaged or explodes, you could charge it at a higher current. And there's not really any reason to do this, but you can always charge a LiPo at a lower current although it will take a really long time to charge.
A LiPo battery should be charged at a maximum rate of 1C, where 'C' is the capacity of the battery in amp hours divided by hours - so a 2200mAh battery can be safely charged at 2200mA (i.e. 2.2A.) For some high performance batteries the manufacturer may also state a charge C rating above 1C (such as some Turnigy Graphene batteries), but if this is not given it is safest to assume 1C. It is important to note that headline C rating of a battery pack shows the safe continuous discharge rate of the battery - the safe charge rate is significantly lower.
The reason for this limit is that when a battery is charging heat is created inside the battery. The faster you put charge in, the quicker the heat is generated - and if the battery is unable to dissipate the heat, it may catch fire. It is worth noting that while charging in a LiPo bag will contain a fire, it also further insulates the battery thermally which paradoxically increases the overheating risk.
It is safe to charge a battery at less than the recommended charge rate; this can be a good idea if you have a suspect (potentially damaged or over-discharged) battery that you wish to recharge for testing.