Why Battery Capacity Ratings Can Be Confusing
If you have ever compared two battery specifications and found that the same battery has two different capacity numbers, you are not alone. The same 100Ah battery might be rated at 80Ah at C20 and 110Ah at C100. Which number is correct? The answer: both – and understanding why is key to choosing the right battery and sizing your system correctly.
What Do C20 and C100 Mean?
The “C” rating in battery specifications refers to the discharge rate, specifically the time period over which the battery is discharged to full exhaustion. C20 means the battery is fully discharged in 20 hours. C100 means the battery is fully discharged in 100 hours.
C20 Capacity: The total energy the battery can deliver when discharged at a constant rate over exactly 20 hours, down to a specified cutoff voltage (typically 10.5V for a 12V battery).
C100 Capacity: The total energy the battery can deliver when discharged at a constant rate over 100 hours, down to the same cutoff voltage.
Because of Peukert’s Law (the phenomenon where batteries deliver less total energy when discharged at higher rates), C100 capacity is always higher than C20 capacity for the same physical battery.
Why C100 Is Higher Than C20
When a battery is discharged slowly (low current over long time), the chemical reactions inside have more time to proceed evenly across the plate surfaces. When discharged quickly (high current), the chemical reactions concentrate at the plate surfaces, creating resistance and preventing the deeper layers from fully reacting.
A 100Ah battery rated at C20 delivers: 100Ah / 20h = 5A continuous for 20 hours
A 100Ah battery rated at C100 delivers: 100Ah / 100h = 1A continuous for 100 hours
The C100 battery will deliver significantly more total energy because the lower discharge current allows more complete utilization of the active material.
Understanding Your Actual Use Case
For Solar Energy Storage Systems
Solar battery banks typically discharge over 4-12 hours (overnight, from sunset to sunrise). This means C10 to C20 rates are most relevant. Always use the C20 (or C10) rating for sizing solar battery banks – using the C100 rating will result in a significantly undersized battery that runs out of energy faster than expected.
For Backup/UPS Applications
UPS systems typically need to provide full power for 15 minutes to several hours. C5 to C10 ratings are most relevant here. UPS batteries are often rated at C10 (10-hour discharge) or C8 (8-hour discharge).
For Telecom Applications
Tower backup systems may need to run for 2-24 hours. The telecom industry standardized on C8 and C10 ratings for battery sizing calculations. For long backup durations (24+ hours), C24 or C100 ratings may be more relevant.
For Recreational/Marine Applications
Trolling motors and marine house banks typically run at moderate loads over 4-8 hours. C20 or C8 ratings are most appropriate for these applications.
How to Convert Between C-Ratings
While exact conversion requires manufacturer data, a reasonable approximation for lead acid batteries:
- C20 to C100: Multiply C20 by approximately 1.15-1.25
- C100 to C20: Divide C100 by approximately 1.15-1.25
- C20 to C10: Multiply C20 by approximately 0.90-0.95
- C5 to C20: Multiply C5 by approximately 0.80-0.85
Note: These are rough estimates. For accurate sizing, always use manufacturer-provided discharge curves or tables.
What About Lithium Batteries?
Lithium batteries (LiFePO4) are much less affected by Peukert’s Law than lead acid batteries. A quality LiFePO4 battery will deliver nearly the same capacity whether discharged in 20 hours or 5 hours, making the C-rating much less critical for sizing. Most LiFePO4 manufacturers quote a single capacity rating (e.g., 100Ah) that applies across a wide range of discharge rates.
CHISEN Battery Capacity Specifications
CHISEN Battery provides comprehensive capacity ratings for all our batteries, including C3, C5, C8, C10, C20, C24, C50, C72, and C100 values where applicable. Our product datasheets include full discharge curves showing capacity at different rates, allowing precise system sizing for any application. Contact our technical team for detailed specifications and sizing support.
Contact CHISEN Battery
Jack Chen | General Manager | CHISEN Battery
Tel: +86 131 2666 8999
Email: jack@chisen.cn | www.chisen.cn