C rating

Batteries are often labelled with a C rating, in the format 20C or 30C. This indicates the current that the battery is able to deliver, as a coefficient of the capacity of the battery(C). It is also used for charging rates as well as discharging rates. If just a C rating is stated, it is usually safe to assume that this is the discharge rate. Sometimes a range is stated, eg 25-30, which are usually claims for continuous and peak currents. Charge C-ratings are usually stated clearly, such as "5C charge".

Discharging

1C discharge rating would mean that the battery is able to deliver its full capacity only at a rate that would discharge it in one hour. For example, a 1000mAh battery with a 1C rating could theoretically deliver 1000mA (ie 1 Amp) for one hour. It cannot deliver a current higher than 1A without significantly reducing the total capacity delivered and/or damaging the battery.

So, a 1000mAh battery with a 25C rating can deliver up to 25A (1Ah x 25C = 25A). At this maximum current the pack would be totally discharged in 2.4 minutes (60 minutes/25).

Similarly, a 2000mAh battery with a 25C rating can delivery up to 50A (also for 2.4 minutes.)

In practice many batteries cannot deliver the stated C rating without producing excess heat, shortening the life of the pack, so a safety margin is advisable.

Charging

Many lipo batteries can only be charged at up to 1C. This means that a 2200mAh pack can be charged with a maximum current of 2.2A. However, as lipo batteries are charged at constant current up to the maximum cell voltage, and then at reducing current on a constant voltage basis, it takes longer than 1 hour to fully recharge a fully discharged pack.

Some newer batteries can be charged at 2 or 5C. However, as this does not accellerate the constant voltage phase of charging as much, this does not reduce charging times by 1/2 or 1/5. It does, however, permit a battery to be perhaps 90% charged in 1/2 or 1/5 of the time that it would take to 90% charge a 1C charge-rated pack, if charging is discontinued immediately the maximum voltage is reached.

For fliers willing to accept an incomplete charge, it can be convenient to 90% (approximately) charge a 5C charge-rated pack in 12 minutes.