Battery Pack Calculator
Calculate battery pack capacity, voltage, current, runtime, and cost for lithium-ion batteries. Essential tool for electric vehicle conversion, solar energy storage, DIY power banks, e-bike batteries, and custom battery pack design. Get accurate specifications for 18650, 21700 cells with series parallel configuration calculator.
Complete Battery Pack Design Guide
Everything you need to know about designing, building, and maintaining lithium-ion battery packs
Key Terminology
Capacity (Ah/mAh)
Amount of charge a battery can store. Higher capacity = longer runtime.
Voltage (V)
Electrical potential difference. Nominal voltage for Li-ion is 3.6-3.7V per cell.
C-Rate
Charge/discharge rate relative to capacity. 1C = full capacity in 1 hour.
Energy Density (Wh/kg)
Energy stored per unit weight. Critical for portable applications.
Pack Configuration
Series (S)
Increases voltage. 4S = 4 × 3.7V = 14.8V nominal
Parallel (P)
Increases capacity and current capability. 2P doubles capacity.
Series-Parallel (SxP)
Combines both. 4S2P = 14.8V with double capacity.
Essential Calculations
Pack Voltage
V_pack = V_cell × SSeries cells add voltages
Pack Capacity
Ah_pack = Ah_cell × PParallel cells add capacity
Pack Energy
Wh = V × AhTotal energy stored
How to Calculate Battery Pack Specifications
Master the essential formulas and calculations for designing lithium-ion battery packs. Learn step-by-step methods used by professionals.
Total Capacity (Ah) = Single Cell Capacity × Parallel Groups
Example: 10 cells of 3000mAh in 2P5S = 3Ah × 2 = 6Ah total
Step-by-Step Process:
- 1Determine single cell capacity in Ah
- 2Count parallel groups (P value)
- 3Multiply: Cell Capacity × Parallel Groups
- 4Result is your pack's total capacity
Pack Voltage = Nominal Cell Voltage × Series Count
Example: 5S pack with 3.7V cells = 3.7V × 5 = 18.5V nominal
Step-by-Step Process:
- 1Find nominal voltage per cell (usually 3.6-3.7V for Li-ion)
- 2Count series connections (S value)
- 3Multiply: Cell Voltage × Series Count
- 4Consider voltage range: 3.0V-4.2V per cell
Max Current = Cell Max Current × Parallel Groups
Example: 20A cells in 3P configuration = 20A × 3 = 60A max
Step-by-Step Process:
- 1Check cell datasheet for maximum discharge current
- 2Count parallel groups
- 3Multiply: Cell Max Current × Parallel Groups
- 4Apply safety factor (typically 80% of max)
Runtime (hours) = Pack Capacity (Ah) ÷ Load Current (A)
Example: 10Ah pack with 2A load = 10Ah ÷ 2A = 5 hours
Step-by-Step Process:
- 1Calculate total pack capacity in Ah
- 2Determine average load current in Amperes
- 3Divide capacity by current
- 4Apply efficiency factor (85-95% typical)
Pro Tips for Accurate Calculations
Safety Considerations
- • Always apply 20% safety margin to current calculations
- • Use 85-90% of theoretical capacity for real-world runtime
- • Consider temperature derating (10-20% capacity loss in cold)
- • Account for cell aging (20% capacity loss over 2-3 years)
Optimization Tips
- • Balance series/parallel for optimal voltage and capacity
- • Choose cells with similar internal resistance
- • Consider C-rate requirements for your application
- • Factor in BMS voltage drop (0.1-0.3V typical)
Community Questions & Expert Answers
Real questions from battery enthusiasts across forums and communities, answered by experts. Learn from common challenges and solutions.
21700 cells offer higher capacity (4000-5000mAh vs 2500-3500mAh) and better power density. However, 18650s are more widely available and cheaper. For high-capacity applications like EVs, 21700s are preferred. For smaller projects, 18650s are often sufficient and more cost-effective.
Your BMS should match: 1) Cell count (S rating), 2) Maximum current (continuous + peak), 3) Cell chemistry. For a 7S pack with 30A continuous draw, choose a 7S BMS rated for 40-50A continuous. Always add 20-30% headroom for safety and account for inrush currents.
Voltage sag under load is normal due to internal resistance. Excessive sag indicates: 1) Undersized cells for the load, 2) Poor connections, 3) Aging cells, or 4) Imbalanced pack. Check C-rate requirements - most Li-ion cells perform best under 1C discharge rate.
Never mix different capacities in parallel groups - this causes current imbalance and reduces pack life. You can use different capacity cells in series if they have similar discharge characteristics, but the pack capacity will be limited by the smallest cell. Best practice: use identical cells throughout.
Quality Li-ion cells typically provide 500-1000 cycles to 80% capacity when discharged to 20% (80% DOD). Factors affecting cycle life: discharge depth, temperature, charge rate, and cell quality. Limiting discharge to 90-10% can double cycle life. Samsung, LG, and Panasonic cells generally offer the best longevity.
Always use a proper BMS with balancing, charge at 0.5-1C rate maximum, monitor temperature during charging, and use a quality charger with CC/CV profile. Never charge unattended initially. Set charge voltage to 4.1V per cell for longer life (vs 4.2V max). Consider fireproof charging location for large packs.
Have a Question?
Use our calculator above to get instant answers, or join the community discussions on popular forums like Reddit r/batteries, Endless Sphere, and DIY Electric Car forums.
Frequently Asked Questions
Everything you need to know about battery pack calculations, design, and safety. Get expert answers to the most common questions.
Important Disclaimer
Calculation Accuracy
The calculations provided by this tool are estimates based on theoretical values and manufacturer specifications. Real-world performance may vary due to temperature, age, manufacturing tolerances, and usage patterns. Always verify calculations with professional tools and testing before implementation.
Safety Warning
Lithium-ion batteries can be dangerous if mishandled. Improper assembly, charging, or usage can result in fire, explosion, toxic gas release, or personal injury. Always follow proper safety procedures, use appropriate protective equipment, and consult with qualified professionals before building battery packs.
Professional Consultation
For commercial applications, high-power systems, or safety-critical applications, always consult with qualified electrical engineers and battery specialists. This tool is intended for educational and preliminary design purposes only.
Legal Disclaimer
BatteryPackCalculator.com and its operators assume no responsibility for any damages, injuries, or losses resulting from the use of this calculator or the information provided. Users assume all risks and responsibilities for their battery pack designs and implementations. By using this tool, you acknowledge that you understand these risks and agree to hold harmless the website operators.
Recommended Safety Practices
- • Always use a proper Battery Management System (BMS)
- • Never exceed manufacturer specifications for current, voltage, or temperature
- • Use appropriate fuses, circuit breakers, and safety disconnects
- • Ensure proper ventilation and thermal management
- • Follow local electrical codes and regulations
- • Keep fire extinguishing equipment nearby during assembly and testing
- • Never leave charging batteries unattended