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C-rates are useful in determining which battery is best for your operation, e.g. Table of battery comparison, especially in conjunction with the battery capacity.

Battery capacity is the Amp-hr (Ah) rating of a battery and describes how long a battery can provide a certain current (A). For example, a 100Ah battery can provide 100 Amps to a system for 1 hour (100A x 1hr = 100Ah), or 50 A for 2 hours (50A * 2hr = 100 Ah), or 10A for 10 hours (10A * 10hr = 100 Ah).

C-Rate is a measure of that battery capacity rating. A battery that is rated to discharge its total capacity (Ah) in 1 hour would be rated at 1C, or in 2 hours would be rated at .5C, or in 4 hours would be rated at .25C, or, in the faster direction, in 30 minutes would be rated at 2C. See Table 1 for some examples.

Because of internal resistance loss and battery chemistry, a battery rarely delivers nameplate rated capacity in only one hour (1C) and typically maximum capacity is found at a lower C-rates. Plus, faster charging and/or discharging (aka at a higher C-rate) typically reduces the total capacity and lifespan of a battery. [1][2]

Table 1: C-Rate with Average Current and Charge Time for a 50 Ah battery
C-Rate Rated Capacity (Ah) Average Current (A) Charge (or discharge) Time
5C 50 Ah 250 A 12 min
2C 50 Ah 100 A 30 min
1C 50 Ah 50 A 1 hour
.5C (aka C/2) 50 Ah 25 A 2 hours
.25C (aka C/4) 50 Ah 12.5 A 4 hours
.1C (aka C/10) 50 Ah 5 A 10 hours

Battery manufacturers often publish datasheets with graphs showing capacity versus C-rate curves. The C-rate is also used as a rating on batteries to indicate the maximum current that a battery can safely deliver in a circuit. So a battery rated at 1C on the battery, indicates that it can safely discharge its full storage in 1 hour. Some batteries, e.g. Lead Acid Absorbent Glass Mat (AGM), typically have a low C-rate of about 0.25C (safe and efficient charging or discharging of the rated storage over a 4 hour span), where as other batteries, e.g. Lithium Iron Phosphate (LiFePO4), can have a high C-rate of over 3C (safe and efficient charging or discharging of the rated storage in just 20 minutes!).

## References

1. https://en.wikipedia.org/wiki/Electric_battery#C_rate
2. Battery University - What is a C-Rate

Page data
Authors Lonny Grafman 2021 CC-BY-SA-4.0 126 Lonny Grafman (2021). "What is a c-rate for batteries". Appropedia. Retrieved June 30, 2022.