Battery Ratings and Specifications

Decoding battery specifications is essential for proper selection. A single number like "5000mAh" doesn't tell the complete story—you need voltage, C-rating, and chemistry to make informed decisions.

Voltage Rating

Nominal vs Minimum vs Maximum

Each battery has three voltage specifications:

Example: 3S LiPo (Three cells in series)

Maximum voltage: 4.2V × 3 = 12.6V (just charged)
Nominal voltage: 3.7V × 3 = 11.1V (50% discharged)
Minimum voltage: 3.0V × 3 = 9.0V (depleted)

Why This Matters

Many components require specific voltage ranges:

Motor: Rated for 12V ±10% = 10.8V to 13.2V
Microcontroller 5V supply: ±5% = 4.75V to 5.25V

At minimum voltage (9V on 3S), the motor supply drops to ~4.5V, which is below acceptable range for 5V logic!

Solution: Use voltage regulator (buck converter) or choose battery with higher minimum voltage.

Capacity Rating

mAh vs Ah

Both measure charge storage, just different units:

1 Ah = 1000 mAh

Example:
5000 mAh = 5 Ah
2500 mAh = 2.5 Ah

What Capacity Means

A 5000 mAh battery can provide:

5A for 1 hour, OR
2.5A for 2 hours, OR
10A for 0.5 hours (30 minutes)

Formula:

Runtime (hours) = Capacity (Ah) / Average Current (A)

Example: Mobile Robot

Battery: 5000 mAh (5 Ah)
Robot draws: 10A average

Runtime = 5 Ah / 10 A = 0.5 hours = 30 minutes

Rated vs Usable Capacity

Manufacturers sometimes overstate capacity. Reality:

Rated capacity: 5000 mAh (what they claim)
Usable capacity: 4500 mAh (typically 80-90%)
Lost to: Protection circuits, internal resistance, safety margin

Rule of thumb: Assume 70-80% of rated capacity is actually usable.

C-Rating (Discharge Rate)

What C-Rating Means

C-rating specifies safe maximum discharge current:

C = Battery capacity / 1 hour

Example: 5000 mAh battery
C = 5000 mAh / 1 hour = 5 A

Therefore:
1C = 5A (discharge in 1 hour)
2C = 10A (discharge in 30 minutes)
50C = 250A (discharge in 72 seconds)

Why C-Rating Matters

High discharge current causes:

• Heat generation
• Voltage sag (temporary drop)
• Battery damage if exceeded
• Fire if too high

Safe operation:

Actual current ≤ C-rating × Capacity

Example: Safe Discharge

Battery: 5000 mAh 50C

Maximum safe discharge = 50C × 5Ah = 250A

If you draw 300A, you exceed the rating and risk damage!

Energy Rating (Watt-hours)

Energy vs Capacity

Capacity (mAh) tells you charge, but energy (Wh) tells you total usable power:

Energy (Wh) = Voltage (V) × Capacity (Ah)

Example: Different Voltages, Same Capacity

5V 5000mAh battery = 5V × 5Ah = 25 Wh
12V 5000mAh battery = 12V × 5Ah = 60 Wh

Same capacity, but 12V battery has 2.4× more energy!

Practical Impact

Motor powered by 12V uses less current than 5V:

P = V × I, so I = P / V

1000W motor:
At 12V: I = 1000W / 12V = 83A
At 5V: I = 1000W / 5V = 200A (impossible for small batteries!)

Higher voltage = lighter wiring, smaller batteries, more efficient!

Real Battery Labels Explained

Example LiPo Label: "3S 5000mAh 50C 11.1V"

3S           = 3 cells in series (lithium)
5000mAh      = 5 Amp-hour capacity
50C          = Maximum discharge rate (250A)
11.1V        = Nominal voltage (3 × 3.7V)

Calculated:
Energy = 11.1V × 5Ah = 55.5 Wh
Max current = 50C × 5A = 250A
Min voltage = 3 × 3.0V = 9.0V

Example Li-ion Label: "18650 2600mAh 10A"

18650        = Standard cell size (18mm × 65mm)
2600mAh      = 2.6 Amp-hour capacity
10A          = Maximum discharge current

Calculated:
Voltage = 3.7V nominal
C-rating = 10A / 2.6A = 3.85C (approximately)
Energy = 3.7V × 2.6Ah = 9.6 Wh

Selecting a Battery

Step-by-Step Selection

Motors (4×): 4A each = 16A
Servos (2×): 1A each = 2A
Microcontroller + sensors: 0.3A
Total: 18.3A
With margin: 22A

Capacity needed (Ah) = Peak current (A) × Runtime (h) / Usable capacity
                     = 22A × 1h / 0.8
                     = 27.5 Ah

Required C-rating = Peak current (A) / Capacity (Ah)
                  = 22A / 25Ah
                  = 0.88C

Battery Specifications Comparison Table

Common Battery Mistakes

Summary

Key Takeaways:

✓ Nominal voltage is middle of operating range ✓ Capacity (mAh/Ah) determines runtime ✓ C-rating limits safe discharge current ✓ Energy (Wh) is total usable power ✓ Higher voltage = more efficient, lighter

Quick Selection Guide:

1. Calculate peak current draw
2. Choose voltage matching components
3. Calculate capacity for desired runtime
4. Verify C-rating supports peak current
5. Check weight not exceeding 30% robot weight
6. Add 20-30% safety margin
7. Test with sample batch before large deployment