C-rate is a way of writing current as a fraction of a battery's nameplate capacity in amp-hours. At 1C, the current is the current that would empty that nameplate capacity in one hour. At 0.5C the current is half of that, so the same arithmetic runs for two hours. At 0.2C it runs for five. The letter C is a ratio. It becomes amperes only after you insert the amp-hour figure printed on the battery you actually have.
The arithmetic, labeled as arithmetic
Use 100 Ah as a teaching figure. It is not a product on a shelf, not a quote, and not a size this page recommends. On that figure, 1C is 100 A, because 100 amp-hours divided by one hour is 100 amperes. Half of that rate, 0.5C, is 50 A. A tenth, 0.1C, is 10 A. Those three currents exist only because the definition says so. They are not evidence that any 100 Ah battery may be charged or discharged at 50 A. Swap in the amp-hour number from your label and the currents change. A 200 Ah nameplate at 0.5C is 100 A by the same definition, and it is still only a definition.
Time and rate are the same statement read backwards. A discharge at 0.5C of the nameplate would take two hours to move the whole nameplate capacity, if the battery actually delivered that capacity at that rate, and if you were allowed to take all of it. Real batteries are limited to a window of state of charge, and capacity itself moves with temperature and with the rate. The definition ignores those limits on purpose, so that two datasheets can be compared in one language. The limits reappear in the next table on the page.
How to find the amp-hour figure, the test conditions behind it, and the rates the maker allows is the work of how to read a battery datasheet. If the label says amp-hours at a stated hour rate, such as a 20-hour rate, that condition is part of the nameplate. Using the number and forgetting the condition is how a C-rate gets built on a capacity the battery only shows in a gentle lab test.
The datasheet still sets the allowed current
A serious sheet separates charge current from discharge current, and it may separate a continuous rate from a short peak. The peak has a duration. A surge the inverter draws for ten seconds is not permission to run that current until the battery is empty. Charge current is often the tighter of the two on a solar bank, because the array and the charger can sit at a high current for hours at midday. The charger setting has to respect the battery's charge-current line, not only the inverter's maximum charge setting.
Depth of discharge answers a different question: how much of the capacity you use, not how fast. A modest C-rate held for many hours is a deep cycle. A high C-rate for a few minutes can be a shallow one. Both numbers belong in the design, and the depth discussion is in depth of discharge and charge stages. How many such cycles the warranty will stand, on the calendar as well as on the counter, is in cycle life and calendar life.
Frequently asked questions
Does a higher C-rate mean a better battery?
A higher allowed C-rate means the sheet permits more current. Life at that current is a separate line, usually shorter than life at a gentler rate, and only the warranty at a stated depth and temperature says how much shorter. More current is more capability. Years stay on the warranty line.
Is C-rate the same thing as depth of discharge?
C-rate is current relative to nameplate amp-hours. Depth of discharge is the fraction of capacity removed. You can cycle deeply at a low C-rate, or briefly at a high one. A datasheet that prints only one of them has left the other question unanswered.
