What Is RC (Reserve Capacity)?
Reserve Capacity (RC) is the number of minutes a fully charged 12V battery can deliver 25 amps at 80°F (27°C) before voltage drops to 10.5V.
RC is measured in minutes, not amp-hours.
RC to Ah Conversion Formula
Since RC is tested at 25 amps, we can convert it to amp-hours (Ah) using:Ah=60RC×25
Simple Formula:
Ah ≈ (RC × 25) ÷ 60
Step-by-Step Example
Example 1:
Battery RC = 120 minutes
- Multiply by 25 amps
120 × 25 = 3000 - Divide by 60 minutes per hour
3000 ÷ 60 = 50 Ah
✔ 120 RC ≈ 50 Ah
Example 2:
Battery RC = 150 minutes
150 × 25 = 3750
3750 ÷ 60 = 62.5 Ah
✔ 150 RC ≈ 62.5 Ah
Quick RC to Ah Conversion Table
| RC (Minutes) | Approx Ah |
|---|---|
| 90 RC | 37.5 Ah |
| 100 RC | 41.6 Ah |
| 120 RC | 50 Ah |
| 140 RC | 58.3 Ah |
| 160 RC | 66.6 Ah |
| 180 RC | 75 Ah |
Why RC to Ah Conversion Is Approximate
Important:
- RC test uses 25A constant load
- Ah rating often uses 20-hour discharge rate
- Real-world conditions vary
- Temperature affects results
So this conversion gives a close estimate, not exact manufacturer rating.
RC vs Ah – Key Difference
| RC | Ah |
|---|---|
| Measured in minutes | Measured in amp-hours |
| Tested at 25A | Tested at 20-hour rate |
| Shows emergency backup time | Shows total stored capacity |
Car Battery RC to Ah Conversion
Most car batteries list RC instead of Ah.
To estimate Ah:
- Find RC value on label.
- Apply formula.
- Result gives approximate amp-hour capacity.
Example:
Car battery with RC = 140
Ah ≈ (140 × 25) ÷ 60
Ah ≈ 58.3 Ah
That means the battery is roughly a 58Ah battery.
Battery RC Value to Ah Conversion (Technical Insight)
RC measures time under fixed current.
Ah measures current over time.
Since:Amp Hours=Current×Time
And RC uses 25A:Ah=25×(RC÷60)
This is simply applying electrical capacity mathematics.
When Should You Convert RC to Ah?
You may need conversion when:
- Designing a solar backup system
- Comparing automotive vs inverter batteries
- Estimating battery runtime
- Replacing a car battery with a similar capacity
Important Professional Note
For solar, UPS, or inverter systems:
Always check the actual Ah rating from manufacturer instead of relying only on RC conversion.
RC is mainly for automotive emergency backup measurement.
Final Conclusion
✔ RC (Reserve Capacity) is measured in minutes.
✔ Ah (Amp Hours) is total battery capacity.
✔ To convert RC to Ah:
Ah ≈ (RC × 25) ÷ 60
This formula gives a practical and reliable estimate for car battery RC to Ah conversion.
