Battery capacity in Amp-hours is written as C. 100Ah, for example. It can also be expressed in Wh (watt-hours) or other units.

C notation can also be used to describe the relative amount of current charging/discharging battery bank. For example a 50A load on a 100Ah bank would be C/2 (100/2 == 50).¹⁾. It may also be written as 0.5C.²⁾

Note: most deep cycle manufacturer's rate capacity at C/20, due to Peukert effect on lead chemistries (see below).³⁾. In other words, a 100Ah battery will deliver 100A with a 5A load over 20 hours.

If you have a 200AH battery bank then C = 200AH.

If the manufacturer recommends max charging at C/5 the rate should be at or below 40A (200/5).

If the manufacturer recommends minimum charging at C/10 the rate should be at or above 20A (200/10).

If the manufacturer recommends max discharge (load) be 3C the rate should be at or below 600A (3×200).

If the manufacturer says that Absorption stage is complete when charging current has dropped to C/100 or C/200 then the current to watch for would be 2A (200/100) or 1A (200/200).

Because of the Peukert effect, lead bank measured capacities are strongly affected by dicharge rate. Sample data below is for flooded lead acid.⁴⁾ 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 = 1.30⁵⁾