Nominal capacity is a time-based rating, not a universal fixed capacity. A battery’s C5, C10, or C20 rating states how much capacity it can deliver when discharged over 5, 10, or 20 hours under the applicable test conditions. Battery testing systems use that rating to calculate the reference current, then derive charging or discharging currents from the selected multiplier.
The rating determines the reference current: (I_n = \text{capacity} \div n), where n is 5, 10, or 20 hours. For a 100 Ah C5 battery, the reference current is 20 A; a charging instruction of 2 × I5 would therefore mean 40 A.
Why Nominal Capacity Depends on Discharge Time
Capacity changes with discharge current
A battery’s measured ampere-hour capacity depends on how quickly it is discharged. Higher discharge currents generally produce less measured capacity than lower currents because of internal resistance, electrochemical limitations, and voltage cut-off behavior.
For that reason, a capacity value is incomplete unless its reference discharge time is also stated.
What the C-number means
The subscript indicates the number of hours over which the battery is expected to deliver its rated capacity under the relevant test conditions.
The basic relationship is:
[ I_n = \frac{C_n}{n} ]
Where:
- (C_n) is the rated capacity in ampere-hours.
- (n) is the reference discharge time in hours.
- (I_n) is the corresponding reference discharge current in amperes.
How C5, C10, and C20 Apply Across Battery Uses
C5 for vehicle, traction, and NiCd batteries
C5 means the battery is rated for a five-hour discharge. A 100 Ah battery rated C5 is therefore associated with a reference current of:
[ I_5 = \frac{100\text{ Ah}}{5\text{ h}} = 20\text{ A} ]
This rating is used for vehicle and traction applications and for NiCd batteries in the reference framework.
C10 for stationary batteries
C10 means the rated capacity is delivered over a ten-hour discharge period. For example, a 100 Ah C10 battery has a reference current of:
[ I_{10} = \frac{100\text{ Ah}}{10\text{ h}} = 10\text{ A} ]
A C10 rating should not be treated as directly equivalent to a C5 rating without considering the different test current and discharge duration.
C20 for starter and small lead-acid batteries
C20 means the battery is rated over a twenty-hour discharge period. A 100 Ah C20 battery therefore has a reference current of:
[ I_{20} = \frac{100\text{ Ah}}{20\text{ h}} = 5\text{ A} ]
This longer test duration is commonly associated with starter batteries and small lead-acid batteries in the stated application categories.
How Testing Systems Convert Ratings Into Currents
Step 1: Identify the capacity rating
The operator or test procedure provides both the nominal capacity and its time basis, such as 100 Ah C5, 100 Ah C10, or 100 Ah C20.
The system must retain both values because “100 Ah” alone does not identify the reference current.
Step 2: Calculate the reference current
The system divides the rated capacity by the rating period:
[ I_5 = \frac{C_5}{5}, \qquad I_{10} = \frac{C_{10}}{10}, \qquad I_{20} = \frac{C_{20}}{20} ]
For a 100 Ah battery, the resulting reference currents are 20 A for C5, 10 A for C10, and 5 A for C20.
Step 3: Apply the requested multiplier
A test instruction may specify a current as a multiple or fraction of the reference current, such as 0.5 × I5, 1 × I10, or 2 × I20.
The system calculates:
[ I_{\text{test}} = k \times I_n ]
Where (k) is the specified multiplier.
Worked example: 100 Ah C5 battery
For a 100 Ah C5 battery:
[ I_5 = 20\text{ A} ]
Therefore:
- (0.5 \times I_5 = 10\text{ A})
- (1 \times I_5 = 20\text{ A})
- (2 \times I_5 = 40\text{ A})
This distinction matters because 2 × I5 is 40 A, not 10 A. A 10 A current would be 0.5 × I5.
Charging Current Versus Discharge Reference Current
The reference current is a calculation basis
The C5, C10, or C20 value primarily defines the battery’s capacity test basis. A laboratory system can use the resulting current as a convenient reference for programming charge and discharge profiles.
This does not mean that the battery must always be charged at its nominal discharge current.
Charging instructions must be interpreted separately
A charging current expressed as a multiple of (I_n) is calculated from the applicable capacity rating, but the selected current must still comply with the battery manufacturer’s charging limits and the test procedure.
For example, if a test specifies charging at (2 \times I_5), a 100 Ah C5 battery would be charged at 40 A mathematically. Whether 40 A is safe or appropriate depends on the battery design, chemistry, thermal conditions, and charging protocol.
Understanding the Trade-offs
Do not compare C-ratings as if they were identical
A 100 Ah C5 battery and a 100 Ah C20 battery have the same stated ampere-hour value but are characterized at different currents. Their real-world performance may differ when both are used at the same load.
The rating period must therefore accompany the capacity whenever test results or operating requirements are compared.
Do not confuse capacity with current
Ampere-hours measure stored charge over a specified test period; amperes measure instantaneous current. The C-number provides the link between them through the reference-duration calculation.
Treating “100 Ah” as though it directly means 100 A is incorrect unless a one-hour rating is explicitly specified.
Do not assume a calculated current is automatically permissible
A testing system can calculate a mathematically correct current while the resulting charge profile is unsuitable for the battery. Current limits, voltage limits, temperature monitoring, and manufacturer instructions remain separate requirements.
Check the notation used by the procedure
The notation C5 can describe a capacity rating, while I5 describes the corresponding five-hour reference current. A procedure that says “2C” may use C-rate terminology differently from one that says “2 × I5,” so the test documentation should define its notation explicitly.
How to Apply This to Your Testing Program
The practical method is to preserve the rating basis throughout test setup, calculation, and reporting.
- If your primary focus is capacity testing: Record the nominal capacity together with its reference rating—such as 100 Ah C5 or 100 Ah C20—and calculate the discharge current as capacity divided by the rating hours.
- If your primary focus is charge-current programming: Calculate (I_n) first, then multiply it by the specified charge-current factor; for example, 2 × I5 for a 100 Ah C5 battery equals 40 A.
- If your primary focus is comparing batteries: Compare batteries using the same rating basis, or explicitly account for the different C5, C10, or C20 test conditions.
- If your primary focus is safe operation: Treat the calculated current as a test-setpoint basis, not as a substitute for the battery manufacturer’s permitted charging limits.
Using the correct time basis turns a nominal capacity rating into a clear, reproducible current for battery testing.
Summary Table:
| Rating | Discharge Time | Reference Current (for 100 Ah) | Typical Application |
|---|---|---|---|
| C5 | 5 hours | 20 A | Vehicle, traction, NiCd |
| C10 | 10 hours | 10 A | Stationary batteries |
| C20 | 20 hours | 5 A | Starter, small lead-acid |
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