Battery Engineering
Calculators

Calculator results are useful for early engineering estimation, but they should not be treated as the final production specification. A production-ready battery design must also confirm real load profile, peak current, cutoff voltage, PCM/BMS limits, connector resistance, thermal behavior, mechanical space and sample test results.

For example, a calculated 5-hour runtime may become shorter if the device has high startup current, frequent peak load, low-temperature operation, early BMS cutoff or voltage drop across wires and connectors.

The same voltage and capacity do not guarantee the same real performance. Differences in cell chemistry, internal resistance, discharge rate, electrode design, cell brand, PCM/BMS current limit, wire gauge, connector quality and pack structure can change runtime, voltage stability, heat generation and safety behavior.

This is why a custom battery project should not be selected only by “V + mAh”. The battery must be matched with the actual device current, peak load, available space, charging method and protection requirements.

Runtime formulas usually use average current and rated capacity, but real devices rarely operate at a perfectly stable load. Actual runtime can be affected by:

• Peak current during startup, motor load, heating or wireless transmission
• BMS or PCM cutoff voltage before all rated capacity is used
• Temperature, battery aging and discharge rate
• Voltage drop caused by wires, terminals and connectors
• Device power management efficiency and shutdown voltage

Required capacity should be estimated from average current and target runtime, but peak current must be reviewed separately. Capacity mainly affects runtime, while peak current affects cell discharge capability, PCM/BMS current limit, connector selection, wire gauge and heat rise.

A device with low average current but high short-duration peak current may still require a higher-rate cell, stronger protection circuit or lower-resistance connector design, even if the calculated capacity looks small.

For batteries with the same voltage, mAh can be useful. For battery packs with different voltage platforms, Wh is usually more meaningful because it represents energy.

For example, a 3.7V 5000mAh battery is about 18.5Wh, while a 7.4V 5000mAh pack is about 37Wh. Both are 5000mAh, but the 7.4V pack stores about twice the energy. OEM projects should compare voltage, capacity and Wh together.

Series connection increases voltage, while parallel connection increases capacity and current capability. For example, a 5S2P pack using 3.7V 5000mAh cells has a nominal voltage of about 18.5V and a capacity of 10000mAh.

However, series-parallel design must also match BMS series count, balancing design, full charge voltage, discharge current, welding process, housing space, heat dissipation and charging method. A mathematically correct configuration may still fail if the BMS, structure or load condition is not suitable.

Charging time is not simply capacity divided by charging current. Lithium battery charging usually includes a constant-current stage and a constant-voltage stage. The final part of charging becomes slower as the battery approaches full voltage.

Real charging time also depends on charger voltage, charging current limit, cell chemistry, series count, PCM/BMS protection, temperature and charger control strategy. Using the wrong charger voltage or charging current can create safety risk or reduce battery life.

C-rate should be calculated from required current and battery capacity, but the final selection must be confirmed by the real load profile. Power tools, RC models, drones, heated devices and motor-driven products often have high peak current that is much higher than average current.

High-current projects should review cell discharge rating, internal resistance, heat generation, connector contact resistance, wire gauge, PCM/BMS current limit and enclosure ventilation. A battery that works in a short test may still overheat or shut down during continuous load.

Yes. You can send your calculated voltage, capacity, Wh, runtime target, average current, peak current, series-parallel idea, available battery space, connector type, wire length, charging method and application. THOR Power can review whether the calculated direction is practical for sample development.

Our team can also help confirm whether your project is more suitable for a custom LiPo battery, lithium-ion battery pack, 18650 / 21700 pack, LiFePO4 pack or custom special battery pack.