Battery Capacity Calculator

• Provides a robust 7.4 V output, suitable for devices requiring higher voltage specifications for efficient performance.
• Offers substantial power capacity, ensuring prolonged usage durations ideal for devices demanding sustained power.

Part 1. What is battery capacity (Amp Hours)?

Battery capacity is a measure of the total electric charge a battery can store and deliver under specified conditions, expressed in ampere-hours (Ah) or milliampere-hours (mAh). It represents the product of current (in amperes) and time (in hours) that the battery can sustain a discharge before reaching its cut-off voltage.

Mathematically:

1Ah=1A×1hour

Key points:

• mAh is used for smaller batteries (e.g., smartphone batteries rated 4,000 mAh).
• Ah is used for larger batteries (e.g., deep-cycle lead-acid battery rated 100 Ah).
• Nominal capacity is determined under standard test conditions — typically at a constant discharge rate, specified temperature, and down to a defined end voltage.
• Real-world usable capacity is usually lower due to inefficiencies and environmental effects.

Example:

A battery rated 5 Ah can theoretically deliver:

• 5 A for 1 hour
• 2.5 A for 2 hours
• 0.5 A for 10 hours
• In practice, the actual runtime may differ due to the Peukert effect and other losses.

Part 2. How to calculate battery capacity (Ah) – Battery capacity formula

The correct calculation method depends on the available parameters. The three main formulas correspond to the calculators provided:

A From Current and Time

If you know current draw (in mA) and discharge time (in hours):

Capacity(mAh)=Current(mA)×Time(h)

Example:

Device current draw = 800 mA

Runtime = 3.5 hours

Capacity = 800 × 3.5 = 2800 mAh (2.8 Ah)

This formula assumes constant current discharge and ignores temperature or internal resistance effects.

B From Voltage and Energy

If you know the battery’s voltage (V) and stored energy (Wh):

Capacity (Ah)=Energy (Wh)/Voltage (V)

Example:

Battery energy = 96 Wh

Nominal voltage = 12.8 V (LiFePO₄ chemistry)

Capacity = 96 ÷ 12.8 = 7.5 Ah

Important: This calculation is only valid at the battery’s nominal voltage. Lithium-ion cells, for example, vary between 4.2 V (full) and ~3.0 V (empty), so capacity ratings are based on nominal voltage (usually 3.6–3.7 V per cell).

C From Power, Voltage, and Time

If you know power draw (W), voltage (V), and runtime (h):

Capacity (Ah)=Power (W)×Time (h)/Voltage (V)

Example:

Power draw = 60 W

Voltage = 24 V

Runtime = 2.5 h

Capacity = (60 × 2.5) ÷ 24 = 6.25 Ah

This is useful for systems where you measure load power instead of current directly, such as AC inverters or DC appliances.

Real-world adjustment:

For accurate sizing, you may need to account for:

• Discharge efficiency (85–98% for Li-ion, 70–85% for lead-acid)
• Temperature derating (capacity may drop 20–50% in cold conditions)
• Peukert’s law for lead-acid batteries at high discharge rates.

Practical note: If you require batteries with specific voltage, capacity, or form factor for your project, Ufine Battery can design and manufacture custom lithium batteries — from LiPo and LiFePO₄ packs to cylindrical 18650, ultra-thin, and high-rate cells — ensuring precise performance for your calculated requirements.

Part 3. Converting between amp hours and amps

Amp-hours (Ah) represent capacity, while amps (A) represent instantaneous current. Conversion is straightforward when you know the operating time.

Amps=Ah/Time (h)

Ah=Amps×Time(h)

Example 1:

A 100 Ah battery running for 5 hours → Current = 100 ÷ 5 = 20 A

Example 2:

A device draws 4 A for 3 hours → Capacity needed = 4 × 3 = 12 Ah

When dealing with mAh:

1 Ah=1000 mAh

Always match units in your calculations — mixing mA with hours in Ah calculations will cause errors unless converted.

If your application requires high current delivery, Ufine can provide custom high-discharge lithium batteries optimized for your operating current and runtime targets.

Part 4. The Relationship between battery capacity and battery life

Battery life depends on capacity and load current, but the relationship is not perfectly linear.

Factors influencing runtime:

• Peukert effect: Higher discharge currents reduce effective capacity in lead-acid and NiMH batteries. Li-ion is less affected.
• Temperature: Low temperatures slow chemical reactions, reducing available capacity. High temperatures accelerate aging.
• Battery health: Aging, cycle count, and deep discharges permanently lower capacity.
• Discharge cut-off voltage: Some devices stop drawing power before the battery is fully depleted to protect the cells.

Example:

A 12 V, 100 Ah lead-acid battery may provide close to 100 Ah at a 20-hour discharge rate (5 A) but only ~60 Ah at a 2-hour discharge rate (50 A).

Part 5. How to check battery capacity? Beware of fake capacity indications

1 Testing Methods:

1.Constant Current Discharge Test

• Fully charge the battery.
• Discharge at a known constant current until reaching the manufacturer’s cut-off voltage.
• Measure elapsed time. Multiply by current to get capacity.

2.Coulomb Counting

Use a battery analyzer that integrates current over time to calculate delivered charge.

3.Smart Battery Readout

Some lithium batteries have built-in BMS with digital capacity reporting.

2 Identifying Fake Ratings:

• Unrealistic numbers (e.g., a standard 18650 cell claiming 10,000 mAh). Genuine high-end 18650 cells are ~3,000–3,600 mAh.
• Excessively low weight for the claimed capacity (higher capacity means more active material, so the cell is heavier).
• No brand name or datasheet available.

Part 6. What other parameters should you consider?

When selecting a battery, capacity is just one factor. Other essential specifications include:

• Nominal Voltage (V) – Must match the system. Series or parallel configurations may be required to achieve the target voltage or capacity.
• C-rate (Continuous & Peak Discharge) – The maximum current a battery can supply without damage. Example: A 5 Ah battery with a 2C continuous rating can deliver 10 A continuously.
• Cycle Life – Number of charge/discharge cycles before capacity falls to ~80% of original.
• Internal Resistance – Affects voltage drop under load and heat generation.
• Self-discharge rate – Some chemistries, like NiMH, lose charge faster when unused.
• Operating Temperature Range – Important for outdoor and automotive applications.
• Form factor and weight – Crucial for portable devices and EVs.

Ufine can customize size, voltage, capacity, and discharge rate to match your exact project requirements.

Part 7. Is a higher battery capacity always better?

While a higher capacity battery offers longer runtime, it is not always the optimal choice.

Trade-offs:

• Increased size and weight.
• Higher cost.
• Potential compatibility issues with chargers or devices not designed for large capacities.

Charging considerations:

Larger capacity requires more energy and may result in longer charging times unless the charger supports higher current.

System design impact:

Oversizing beyond actual need may reduce efficiency in weight-sensitive or space-constrained applications.

Example:

In drones, a heavier, higher-capacity battery may increase flight time marginally but can reduce maneuverability and put more stress on motors.