Engineering Blog
10 Common Battery Runtime and Pack Sizing Mistakes
Battery sizing fails when a clean nominal calculation is mistaken for real delivered runtime. A reliable design treats energy, usable capacity, load profile, voltage droop, temperature, protection, and aging as one system.
- Reading Time
- 16 min
- Difficulty
- Intermediate
- Last Updated
- September 29, 2026
Start With the Right Model
The ideal equations remain useful, but only after the load convention is clear. Constant-current runtime uses charge capacity; constant-power runtime uses energy. Neither equation includes every real pack limitation.
Formula reference
Constant-current model
t_ideal = Q / I_batteryVariable definitions
- t
- ideal runtime (h)
- Q
- battery capacity (Ah)
- I_battery
- average battery-side current (A)
Formula reference
Constant-power model
t_ideal = E_battery / P_batteryP_battery = P_load / efficiencyVariable definitions
- E_battery
- nominal energy (Wh)
- P_battery
- battery-side power (W)
- efficiency
- conversion efficiency as a decimal
For the full formula workflow, use the Battery Capacity, Runtime, C-Rate, Energy, and Pack Sizing Guide. This article focuses on what goes wrong after the first calculation.
Ten Battery Runtime and Pack Sizing Mistakes
Mistake #1: Treating Rated Capacity as Fully Usable
A 10 Ah nameplate does not mean every application can extract 10 Ah. Cutoff voltage, state-of-charge window, discharge rate, temperature, age, and the BMS determine usable capacity.
Better approach: Apply a justified usable-capacity factor and verify the manufacturer discharge curves at the expected current and temperature.
Mistake #2: Using Ah When the Load Is Specified in Watts
Amp-hours describe charge, not energy. A 12 V 10 Ah pack and a 24 V 10 Ah pack do not contain the same nominal energy.
Better approach: Convert to watt-hours with E = Vnom × Q before estimating runtime for a constant-power load.
Mistake #3: Ignoring Conversion Efficiency
A regulated load receives less energy than the battery supplies. Converter loss, wiring loss, protection devices, and standby current all shorten runtime.
Better approach: Use battery-side energy or current and evaluate efficiency across the operating voltage and load range.
Mistake #4: Averaging Current Without Modeling Duty Cycle
Wireless bursts, motor starts, display backlights, heaters, and sleep states can produce a load profile that one typical-current number does not represent.
Better approach: Calculate time-weighted average current, then check peak current separately for voltage droop and protection limits.
Mistake #5: Ignoring Pulse and Startup Current
A pack may have enough stored energy but still reset the system when internal resistance and interconnect resistance cause a transient voltage drop.
Better approach: Check peak current, pulse duration, cell impedance, BMS limit, connector resistance, wiring, and minimum input voltage.
Mistake #6: Assuming Capacity Is Independent of Temperature
Cold cells usually deliver less usable capacity and exhibit higher impedance. High temperature can improve short-term output while accelerating aging and creating safety concerns.
Better approach: Use temperature-specific manufacturer data and test at the actual environmental limits.
Mistake #7: Ignoring Aging and End-of-Life Requirements
A prototype sized with fresh-cell capacity can fail its runtime target after calendar aging and cycling reduce capacity or increase resistance.
Better approach: Size against the required end-of-life state of health, not only beginning-of-life typical capacity.
Mistake #8: Adding Ah in Series
Series cells increase voltage. In an ideal matched string, amp-hour capacity remains that of one cell or one parallel group.
Better approach: Calculate pack voltage from series count and pack Ah from parallel count; then multiply them for nominal energy.
Mistake #9: Ignoring Cell Matching and Current Sharing
Parallel strings do not necessarily share current equally when cells, fuses, conductors, temperatures, or connection resistances differ.
Better approach: Use matched cells, symmetric interconnects, appropriate protection, and pack-level validation.
Mistake #10: Treating Runtime as a Guaranteed Number
Runtime calculations are engineering estimates. Load variation, converter behavior, self-discharge, BMS quiescent current, cutoff hysteresis, and measurement uncertainty remain.
Better approach: State assumptions, calculate a range, and validate the complete system under representative load and environment.
Practical Examples
Constant-current estimate
A 5 Ah pack supplying 1 A has an ideal runtime of 5 h. At 80% usable capacity and 92% delivery efficiency, a first-order adjusted estimate is 5 × 0.80 × 0.92 = 3.68 h.
Constant-power estimate
A 12 V, 10 Ah pack stores about 120 Wh nominally. A 30 W load through an 88% efficient converter requires about 34.1 W from the pack, giving 120 / 34.1 ≈ 3.52 h before capacity and cutoff corrections.
Pulse-load voltage droop
A 4 A radio burst through 120 mΩ total source and path resistance causes ΔV = I × R = 0.48 V. A system can brown out even though average current and nominal Wh appear acceptable.
Series-parallel pack
Eight 3.6 V, 2.5 Ah cells arranged 4S2P form an ideal 14.4 V, 5 Ah, 72 Wh pack. The series count changes voltage; the parallel count changes Ah.
Battery Design Review Checklist
| Review Area | Inputs | Engineering Check |
|---|---|---|
| Energy | Nominal voltage, Ah, Wh | Use a representative voltage for the load model; do not use charge voltage as nominal energy voltage. |
| Usable capacity | SOC window, cutoff, reserve | Base the factor on chemistry, protection, and required lifetime. |
| Load profile | Active, idle, sleep, burst | Use time weighting for runtime and peak values for transient checks. |
| Power path | Converter, BMS, fuse, wire | Include load-dependent efficiency and I²R losses. |
| Environment | Cold, heat, enclosure | Capacity, impedance, charging permission, and aging all depend on temperature. |
| End of life | Capacity fade, resistance rise | Validate the required runtime and peak voltage at the specified service life. |
The Battery Capacity, Energy, and C-Rate Reference provides definitions and condition-aware lookup notes. Chemistry-specific voltage limits are summarized in the Battery Chemistry and Nominal Voltage Reference.
A Better Verification Workflow
- 1. Define minimum runtime, peak load, minimum input voltage, and environmental limits.
- 2. Build active, idle, sleep, startup, and burst load states.
- 3. Convert the requirement into both average charge and energy where practical.
- 4. Apply usable capacity, conversion loss, reserve, and end-of-life assumptions separately.
- 5. Check peak droop from cell, BMS, fuse, connector, and wiring resistance.
- 6. Verify charge and discharge current against cell and BMS ratings.
- 7. Review series/parallel voltage, capacity, energy, matching, and balancing.
- 8. Test fresh and aged representative packs at hot and cold limits.
Summary
A useful runtime estimate separates nominal capacity from usable capacity, average demand from peak demand, and beginning-of-life performance from end-of-life requirements. Pack voltage, Ah, Wh, C-rate, efficiency, cutoff, temperature, resistance, protection, matching, and aging must agree. Calculate a range, document assumptions, and validate the complete power path.
Support reference
FAQ
Why is actual battery runtime shorter than Q divided by I?
Q/I assumes constant current and fully usable rated capacity. Real systems also have cutoff voltage, efficiency loss, temperature effects, aging, varying load, self-discharge, and protection-circuit consumption.
Should battery runtime use Ah or Wh?
Use Ah for a reasonably constant battery-side current. Use Wh when load power is the better-known quantity or when voltage conversion is involved. In both cases, apply consistent assumptions.
How should converter efficiency be included?
For an energy model, divide required load energy by efficiency to estimate battery energy. For a current model, use battery-side current rather than load-side current. Efficiency may vary with voltage and load.
What is usable battery capacity?
Usable capacity is the portion available between the permitted charge and discharge limits under specified current, temperature, age, and cutoff conditions.
Why do pulse loads cause resets?
Peak current flowing through cell and path resistance causes voltage droop. The BMS may also trip its current limit even when average current and stored energy are acceptable.
Does connecting batteries in series increase Ah?
No. In the ideal matched-cell model, series connection increases voltage while Ah remains equal to one cell or one parallel group.
Does connecting batteries in parallel always divide current equally?
No. Sharing depends on cell impedance, state of charge, temperature, fusing, wiring resistance, and connection symmetry.
How much battery sizing margin should I use?
There is no universal percentage. Margin should follow load uncertainty, temperature, required end-of-life capacity, cutoff behavior, manufacturing variation, reliability goals, and manufacturer data.