| Metric | Value |
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This battery storage calculator gives a compact engineering estimate of how much charge a battery will lose during long term storage. It accounts for calendar self discharge, parasitic current from BMS and accessories, initial state of charge, storage temperature and pack capacity. Supported chemistries include lead acid and large traction lithium packs. For EV packs you may enter energy in kWh and convert to amp hours using pack voltage.
Table of Contents
Input parameters for the calculator
- Battery type: selects sensible defaults and temperature sensitivity for lead acid, small Li ion and traction Li ion.
- Voltage in volts: used to convert kWh to Ah for large packs.
- Capacity in amp hours: nominal Ah rating of the pack.
- Capacity in kilowatt hours: enter for EV packs. Calculator converts to Ah via V.
- Initial SoC in percent: starting charge level before storage. Typical recommendations vary by chemistry.
- Calendar self discharge percent per month at 25 °C: default values provided but editable.
- Parasitic current in mA: total standby draw from BMS, alarms and telematics.
- Storage temperature in °C: modifies effective self discharge.
- Storage duration in months: projection period.
- Cutoff SoC in percent: minimum allowable state of charge for safe storage.
Key formulas used in battery storage calc
Convert kWh to Ah
C Ah = E kWh × 1000 ÷ V
Parasitic loss per month in Ah
A par = p mA × 24 hours × 30 days ÷ 1000
Temperature multiplier
f T = clamp 2^((T − 25) ÷ 10 , f min type , f max type
Effective monthly self discharge
s eff = s × f T
Calendar loss in Ah per month
A cal = C Ah × s eff ÷ 100
Total monthly loss
A tot = A cal + A par
Initial stored amp hours
A0 = C Ah × SoC start ÷ 100
Remaining after n months
A n = max 0 , A0 − A tot × n
State of charge after n months
SoC n = 100 × A n ÷ C Ah
Months until cutoff
n cut = (A0 − C Ah × SoC cutoff ÷ 100) ÷ A tot when A tot > 0
📝 Self discharge accelerates with temperature. The calculator uses a practical model that approximates doubling of rate every 10 degrees above 25 °C. To avoid unrealistic results the multiplier is clamped by chemistry dependent bounds. Typical boundaries are lower sensitivity for LTO and higher sensitivity for lead acid.
Suggested clamp ranges
- Lead acid multiplier range 0.25 to 4.0
- Li ion and Li polymer multiplier range 0.35 to 3.0
- LTO multiplier range 0.20 to 2.5
Worked examples
Lead acid example
Input data
- Capacity 85 Ah
- Start SoC 70 percent
- Self discharge s = 2.5 percent per month
- Parasitic p = 12 mA
- Storage temperature 10 °C
- Duration n = 4 months
- Cutoff SoC 30 percent
Compute
- A par = 12 × 24 × 30 ÷ 1000 = 8.64 Ah per month
- Temperature multiplier at 10 °C approx 2^((10 − 25) ÷ 10) = 0.53 clamp to 0.25..4 gives 0.53
- s eff = 2.5 × 0.53 = 1.33 percent per month
- A cal = 85 × 0.0133 = 1.13 Ah per month
- A tot = 1.13 + 8.64 = 9.77 Ah per month
- A0 = 85 × 0.70 = 59.5 Ah
- A after 4 months = max 0 , 59.5 − 9.77 × 4 = 20.42 Ah
- SoC after 4 months = 20.42 ÷ 85 × 100 = 24.0 percent
- Months to cutoff = (59.5 − 85 × 0.30) ÷ 9.77 = (59.5 − 25.5) ÷ 9.77 ≈ 3.47 months
EV pack example
Input
- Energy 72 kWh at 420 V
- Start SoC 55 percent
- Self discharge s = 1.0 percent per month
- Parasitic p = 25 mA
- Temperature 30 °C
- Duration 6 months
- Cutoff 20 percent
Compute
- C Ah = 72 × 1000 ÷ 420 ≈ 171.43 Ah
- A par = 25 × 24 × 30 ÷ 1000 = 18.00 Ah per month
- Temperature multiplier at 30 °C approx 2^((30 − 25) ÷ 10) = 2^0.5 = 1.414 clamp to 0.35..3 gives 1.414
- s eff = 1.0 × 1.414 = 1.414 percent per month
- A cal = 171.43 × 0.01414 = 2.42 Ah per month
- A tot = 2.42 + 18.00 = 20.42 Ah per month
- A0 = 171.43 × 0.55 = 94.29 Ah
- A after 6 months = max 0 , 94.29 − 20.42 × 6 = −27.03 Ah then clipped to 0 Ah
- SoC after 6 months = 0 percent
- Months to cutoff = (94.29 − 171.43 × 0.20) ÷ 20.42 = (94.29 − 34.29) ÷ 20.42 ≈ 2.94 months
Practical guidance and additional checks
- Measure parasitic current with a precise meter. Even tens of milliamps accumulate to many amp hours over months.
- Prefer storage SoC 30 to 50 percent for lithium chemistries and 60 to 80 percent for lead acid to reduce freezing risk.
- If monthly loss exceeds 5 percent of capacity take action. Options include disabling non essential systems, using a maintenance charger or periodic top ups.
- At subzero temperatures avoid charging lithium cells. For lead acid avoid low SoC near freezing.
- For fleet or remote assets log pack voltage and standby current to refine model inputs and detect rising parasitic drains early.
Typical reference values by chemistry
| Parameter | Typical | Note |
|---|---|---|
| Self discharge lead acid | 2 to 4 % per month | Depends on age and temperature |
| Self discharge Li ion | 0.5 to 1.5 % per month | Depends on cell chemistry and SoC |
| Self discharge LTO | 0.2 to 0.8 % per month | Very low calendar loss |
| Parasitic current | 5 to 50 mA | Telemetry and alarms often dominate |
| Recommended SoC for storage Li ion | 30 to 50 % | Balance between ageing and reserve |
Use this tool to estimate percent capacity loss over time and to determine when a pack will reach a safety cutoff. The main levers are lowering parasitic drains, selecting an appropriate start SoC and storing at moderate temperatures. For EV packs monitor parasitic amp hours closely because they can quickly consume available charge at low starting SoC.
Further reading
- “Battery Management Systems for Large Lithium Ion Battery Packs” by Davide Andrea
- “Battery Technology Handbook” by H.A.C. Kiehne
- “Lithium-Ion Batteries: Fundamentals and Applications” by Yoshio, Brodd and Kozawa
- “Lead-Acid Batteries: Science and Technology” edited by D.R. Macdonald
David Parry — Senior Engineering Analyst
Specializing in electronics and physics-based simulations with 20+ years of engineering experience. David ensures the mathematical and physical accuracy of the tools at ProCalcLab.
