What voltage does the device require? How much energy does it need? And how long must it operate between charges?
Choosing the right battery pack starts with these three questions. The calculations below provide a useful first estimate for evaluating a standard pack or developing a custom battery solution.
Voltage, capacity, energy and power
Voltage (V)
Electrical potential supplied by the pack. It must remain compatible with the equipment’s input range from full charge to cutoff.
Capacity (Ah)
Electrical charge available. In theory, a 10Ah battery can provide 1A for 10 hours or 5A for 2 hours.
Energy (Wh)
Total stored energy. Watt-hours are especially useful for comparing batteries at different voltages.
Power (W)
The rate at which energy is used. A pack needs enough energy and enough current capability for continuous and peak loads.
Remember: nominal voltage is only part of the picture. Maximum charge voltage, minimum operating voltage, chemistry, series cell count and the device’s complete input-voltage range must also align.
Calculate required capacity
If average current and desired runtime are known, start with:
A 3A system operating for 5 hours has a theoretical minimum requirement of 15Ah.
Add a realistic margin
The theoretical minimum leaves little room for temperature, aging, conversion losses, BMS operation, cell variation or reserve capacity.
The right margin is application-specific; 20% is an example, not a universal rule.
Estimate runtime more accurately
For a constant-current load, the basic estimate is Ah ÷ A. When loads are specified in watts or the system includes voltage conversion, watt-hours are more useful.
Start with nominal energy
500Wh battery energy
Account for usable energy and losses
500Wh × 90% usable × 92% efficiency = 414Wh delivered
Divide by the average load
414Wh ÷ 80W = 5.18 hours
Runtime calculator
Change the inputs to create a preliminary estimate.
Estimate only. Confirm the full electrical, environmental and safety requirements with a qualified battery engineer.
How series and parallel connections shape a pack
Cells in series increase voltage while amp-hour capacity stays approximately the same. Cells in parallel increase capacity while voltage stays approximately the same.
Series/parallel pack calculator
Worked example: an 8-hour industrial application
| Requirement | Value |
|---|---|
| Nominal system voltage | 24V |
| Average power consumption | 60W |
| Peak power | 180W |
| Required runtime | 8 hours |
| Estimated system efficiency | 90% |
Calculate load energy
60W × 8h = 480Wh
Account for efficiency
480Wh ÷ 0.90 = 533Wh
Add an illustrative reserve
533Wh × 1.20 ≈ 640Wh
The calculation is not finished: the pack must also support the 180W peak, stay within the device’s voltage range, and satisfy mechanical, environmental, charging and safety requirements.
Use a duty cycle for changing loads
Maximum power alone may oversize a battery, while idle power may underestimate it. Calculate the energy used in each operating mode.
| Mode | Power | Time | Energy |
|---|---|---|---|
| Idle | 20W | 2h | 40Wh |
| Normal | 80W | 4h | 320Wh |
| High load | 150W | 1h | 150Wh |
| Total | 510Wh | ||
Capacity is not current capability
Two 20Ah batteries can have very different discharge ratings: one may safely support 10A continuously, while another supports 40A.
Cells, interconnections, wiring, connectors and the BMS must all safely support the application’s current requirements.
What changes real-world runtime?
A calculated runtime is an engineering estimate, not a guarantee. Key influences include:
Environment and aging
Low temperature can reduce available capacity and increase resistance. Excessive heat can accelerate degradation, while normal cycling gradually reduces capacity over time.
System behavior
High discharge rates, conversion losses, intermittent loads and protective BMS cutoff can all reduce the usable energy delivered to the application.
Quick battery calculation reference
| Calculate | Formula |
|---|---|
| Battery energy | Wh = V × Ah |
| Power | W = V × A |
| Current | A = W ÷ V |
| Required capacity | Ah = A × hours |
| Basic runtime | hours = Ah ÷ A |
| Energy-based runtime | hours = usable Wh ÷ W |
| Series pack voltage | cell V × series cells |
| Parallel pack capacity | cell Ah × parallel cells |
| Total cells | S × P |
Battery sizing is more than a formula
Commercial and industrial designs may also need to account for chemistry, cell selection, charge method, temperature, cycle life, dimensions, weight, connectors, communications, BMS protection, ingress protection, transportation and applicable certifications. Two packs with the same voltage and capacity are not necessarily interchangeable.
Standard battery pack
A practical path when voltage, capacity, current and mechanical requirements align with an existing platform.
Custom battery pack
Often appropriate for unusual dimensions, high current, special connectors, communications, environmental protection or application-specific certifications.
Frequently asked questions
How do I calculate how many Ah my battery needs?
Multiply average current in amps by runtime in hours. A 3A load for 6 hours theoretically requires 18Ah. Additional capacity may be needed for losses, temperature, aging and reserve.
How do I calculate battery runtime?
For a constant-current load, divide battery capacity (Ah) by average current (A). For systems specified in watts, divide usable battery energy (Wh) by average power (W).
What is the difference between Ah and Wh?
Ah measures electrical charge capacity; Wh measures energy and accounts for voltage as well as capacity: Wh = V × Ah.
Does a higher Ah battery last longer?
With the same voltage, load and operating conditions, higher usable Ah generally provides longer runtime. Compatibility, discharge current, size, weight and charging must still be checked.
Does connecting cells in series increase capacity?
Series connections increase voltage. Amp-hour capacity remains approximately the same.
Does connecting cells in parallel increase capacity?
Parallel connections increase amp-hour capacity while voltage remains approximately the same. They also affect current capability.
Why is actual runtime shorter than calculated runtime?
Basic calculations assume ideal conditions. Conversion losses, discharge rate, low temperature, aging, BMS limits, reserve capacity and changing loads can all shorten runtime.
How much extra battery capacity should I add?
There is no universal percentage. The appropriate margin depends on temperature, aging, discharge behavior, reliability targets and the complete application requirements.
