C notation

C notation is used to express the relationship of a bank's capacity to charge/discharge current. Example: a 20A current into or out of a 100Ah bank is 0.2C (20A / 100Ah). The same calculation could be made with W/Wh since it's about the relationship between the two and not any specific unit of measurement.

This relationship can also be described as a fraction; the above would be C/5.

Note: most deep cycle manufacturer's rate capacity at C/20 (0.05C), due to Peukert effect on lead chemistries (see below).¹⁾. In other words, a 100Ah battery will deliver 100A with a 5A load over 20 hours.

Assuming a 200Ah bank:

• If the manufacturer recommends max charging at 0.2C the rate should be ≤40A (200 x 0.2)
• If the manufacturer recommends minimum charging at C110 the rate should be ≤20A (200 x 0.1)
• If the manufacturer recommends max discharge (load) be 3C the rate should be ≤600A (200 x 3)
• If the manufacturer says that Absorption stage is complete when charging current current has dropped to ≤0.05C then the current to watch for would be ≤10A (200 x 0.05)

Because of the Peukert effect, lead bank measured capacities are strongly affected by dicharge rate. Sample data below is for flooded lead acid.²⁾ C/20 capacity is multiplied by the factor below to find projected capacity at given discharge rates.

Example: our battery is rated at 100Ah when drained at 5A over 20 hours. If it is drained at 20A in 5 hours it will have an observed capacity of 70Ah (100Ah x .7).

1 Hour = .3
2 Hour = .5
3 hour = .6
4 hour = .65
5 hour = .7
6 hour = .75
8 hour = .8
10 hour = .85
12 hour = .9
16 hour = .95
20 hour = 1
24 hour = 1.05
36 hour = 1.1
72 hour = 1.25
100 hour = 1.30³⁾