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    "Charge at 2C"

    Charge and discharge currents are often specified as so-many "C".

 
 1C is the current which will fully-discharge a full cell in one hour
 1C is the current which will fully-charge an empty cell in one hour [approximately]

 

 

Each of the tables below gives three examples, at 1C, 2C, and10C.

Note that, in all cases, the capacity is equal to the current multiplied by the time, and the units are multiplied, not just the numbers.

The three tables present precisely the same information, but use different units. All are correct, but I hope you will agree with me that the units in table 3 are the most-convenient for our purposes.

 
 Table 1
 capacity [approx]  current  time     "C"
 2 amp hours  2 amps  1 hour     1C
 2 amp hours  4 amps  0.5 hours     2C
 2 amp hours  20 amps  0.1 hours     10 C

 

 Table 2
 capacity [approx]  current  time     "C"
 2000 milliamp hours  2000 milliamps  1 hour     1C
 2000 milliamp hours  4000 milliamps  0.5 hours     2 C
 2000 milliamp hours  20,000 milliamps  0.1 hours     10 C

 

 Table 3
 capacity [approx]  current  time      "C"
 120 amp minutes  2 amps  60 minutes     1C
 120 amp minutes  4 amps  30 minutes     2C
 120 amp minutes  20 amps  6 minutes     10C

 

There are two reasons for saying "approximately".

For a full charge, current times time is always a bit more than capacity.

A slow discharge [say 60 minutes] may deliver nominal capacity, but a fast discharge [say 6 minutes] will probably deliver only about 80% of nominal capacity.

When a "C" number is included in the description of a cell, this indicates the highest current which can be safely drawn, without overheating the cell.

Be careful ! Vendors often provide optimistic numbers.