In the context of a most 12v batteries, "C20" refers to a standard rating used to specify the capacity of the battery over a 20-hour discharge period. It's a measure of how much energy the battery can deliver over a 20-hour period while maintaining a relatively consistent voltage.
Here's what C20 means in more detail:
- C20 Capacity: The C20 capacity indicates the amount of charge a battery can deliver over a 20-hour period before its voltage drops below a specified level. It is usually expressed in ampere-hours (Ah) or amp-hours (Ah). For example, if a car battery has a C20 capacity of 50Ah, it means that it can theoretically deliver a current of 2.5 amps (50Ah / 20h) continuously for 20 hours while maintaining a stable voltage.
- Application in Car Batteries: Car batteries are typically designed to provide high currents for short durations (e.g., starting the engine) rather than delivering a constant current over a long period (Deep cycle) . As a result, the C20 rating is not the most critical specification for car batteries. However, it still provides a standardised way to compare the capacities of different batteries.
- C-Rate Variations: The C20 rating assumes a discharge over a 20-hour period, but batteries can be tested and rated at different C-rates as well. For example, a battery tested at C5 (5-hour discharge rate) or C1 (1-hour discharge rate) might have a different capacity because its internal resistance and efficiency can affect how quickly it can deliver energy.