What is the Peukert Effect
Peukert effect describes how a battery’s available capacity drops as discharge current increases. At low current you get close to the rated amp-hours; at high current the battery delivers fewer amp-hours and the runtime shortens. This relationship is formalized by Peukert’s law and its Peukert exponent (also called the Peukert number).
Peukert Exponent and Peukert Number
The Peukert exponent k is a chemistry- and construction-dependent number that quantifies how strongly capacity shrinks under higher discharge.
- Lower k (~1.03–1.08) → small loss at high loads (LiFePO4, most lithium-ion).
- Higher k (~1.20–1.35) → large loss at high loads (flooded lead-acid, AGM, some GEL).
You will also see Peukert number used interchangeably with exponent.
Peukert Equation and Simple Example
A practical form (assuming a 20-hour rating) for runtime t in hours at discharge current I is:
t=H(C/IH)k
Where:
- CCC = rated capacity (Ah) at H hours (commonly 20 h)
- III = discharge current (A)
- kkk = Peukert exponent (Peukert number)
- ttt = expected runtime (h)
Example A — LiFePO4 (k = 1.05): 12 V 100 Ah pack rated at 20 h.
- At 5 A: t=20(1005⋅20)1.05≈20.9t = 20 \left(\frac{100}{5 \cdot 20}\right)^{1.05} \approx 20.9t=20(5⋅20100
)1.05≈20.9 h - At 50 A: t≈2.01t \approx 2.01t≈2.01 h
Example B — AGM (k = 1.25): same 100 Ah.
- At 5 A: t≈21.7t \approx 21.7t≈21.7 h
- At 50 A: t≈1.26t \approx 1.26t≈1.26 h
Takeaway: at high current the AGM loses far more runtime than LiFePO4 because its k is larger.
Battery Capacity vs Discharge Rate
Battery labels are typically based on C/20 (discharge in ~20 h). If you draw faster than C/20, voltage sag and internal resistance raise losses, and Peukert’s law predicts less usable amp-hours. Slower than C/20, usable capacity can be slightly higher than the label.
Peukert Exponent for LiFePO4
Real-world LiFePO4 packs usually fall around k = 1.03–1.08. Many field tools and monitors start LiFePO4 at 1.05. That’s why LiFePO4 holds runtime well even with inverter loads. For best accuracy, confirm Peukert exponent for LiFePO4 with your BMS/monitor data or a timed discharge test.
Peukert Exponent for Lead Acid and AGM
Typical values: flooded 1.25–1.35, AGM 1.20–1.30, GEL 1.15–1.25. These chemistries show a strong Peukert effect and can deliver much less runtime than the 20-hour label when running large loads like power tools or air-conditioners.
Typical Peukert Number by Chemistry
| Chemistry | Typical Peukert exponent k | Effect at high current | Notes |
|---|---|---|---|
| LiFePO4 | 1.03–1.08 | Low | Holds voltage well, low internal resistance |
| Lithium-ion NMC/NCA | 1.05–1.12 | Low–Moderate | Pack design dependent |
| GEL | 1.15–1.25 | Moderate | Better than flooded under deep cycles |
| AGM | 1.20–1.30 | High | Common in RV/boats; sensitive to high loads |
| Flooded Lead-Acid | 1.25–1.35 | High | Large loss at inverter loads |
Values are typical ranges seen in labs and manufacturer tools; exact k depends on brand, temperature, age, and state of health.
Battery Runtime Calculator Peukert
You can estimate runtime without a web calculator:
- Find C (Ah) and H (usually 20 h) on the label.
- Choose k from the table above or your monitor.
- Measure planned I (A).
- Compute: t=H(C/IH)k.
- Multiply ttt by battery nominal voltage if you need Wh.
Worked scenarios at 12 V 100 Ah
| Scenario | Chemistry (k) | Load I (A) | Runtime t (h) | Energy (Wh ≈ 12 V × I × t) |
|---|---|---|---|---|
| Small DC loads | LiFePO4 (1.05) | 10 | 4.09 | ~491 Wh |
| Same load | AGM (1.25) | 10 | 3.45 | ~414 Wh |
| 1 kW inverter (≈83 A DC) | LiFePO4 (1.05) | 83 | 1.23 | ~1,226 Wh |
| Same 1 kW inverter | AGM (1.25) | 83 | 0.67 | ~667 Wh |
Illustrative only; inverter efficiency, cable loss, temperature, and cut-off voltage will change results.
C Rate DoD SoC and Temperature
- C-rate = current normalized to capacity (e.g., 1C on 100 Ah = 100 A). Higher C-rate magnifies the Peukert effect.
- DoD and SoC shift voltage; shallow cycling at moderate current yields more stable runtime.
- Temperature reduces available capacity and increases internal resistance; Peukert’s law does not directly correct for temperature.
Victron Battery Monitor Settings
For LiFePO4 systems, many integrators start with:
- Peukert exponent 1.05–1.07 for LiFePO4
- Charge efficiency factor 99–100% (LiFePO4)
- Tail current ~2–4% of capacity
- Charged detection time 3–10 min
- Current threshold 0.0–0.1 A (to ignore noise)
These align with common Victron SmartShunt and battery monitor settings used in the field; always verify with your battery’s datasheet and installation notes.
When the Peukert Effect Matters
- High surge or continuous heavy loads on lead-acid/AGM → plan for a much larger bank or shift to LiFePO4.
- Steady DC loads on LiFePO4 → Peukert penalty is small; sizing focuses on daily Wh, DoD targets, and charge rate.
- Voltage-sensitive devices → Peukert plus internal resistance can cause early low-voltage cutoffs on lead-acid.
Quick Start Settings for New Builds
- Off-grid RV or marine with inverter: prefer LiFePO4 and start k = 1.05; validate with a 0.5C runtime test.
- Legacy AGM bank: assume k = 1.25 for calculations; keep continuous discharge ≤ 0.3C for reasonable runtime.
FAQ
What Peukert exponent should I use for LiFePO4 with inverter loads
For new LiFePO4 packs, a field-proven starting point is k = 1.05. If your inverter draws ≥ 0.5C, some integrators tighten to 1.06–1.08 to account for cable loss and low-temperature operation. Confirm by logging a 0.5C discharge and adjusting until your monitor runtime and calculated runtime match within ~5%. Source: Victron battery monitor guidance and industry practice.
How large is the runtime error if I ignore Peukert on AGM at 1C
On a 100 Ah AGM with k ≈ 1.25, discharging at 1C = 100 A yields ~0.8–0.9 h instead of the naïve 1.0 h, a 10–20% runtime loss—often worse in cold conditions. Calculations with the table and equation above show similar results. Source: lab-typical AGM k-ranges and Peukert runtime estimates.
How can I measure my own Peukert exponent at home
Fully charge, rest, then perform two timed constant-current discharges (e.g., 0.1C and 0.5C) to the same cut-off voltage. Record the two (I, t) pairs. Solve for k using the Peukert equation rearranged for two points, or fit the log-log slope. Repeat at room temperature and healthy SoC. Source: standard test practice derived from Peukert’s law.
Does Peukert change with temperature or battery age
The exponent k reflects the cell design but the observed effect grows in cold and with higher internal resistance as the pack ages. Many monitors keep k constant and let temperature and internal resistance show up as runtime differences. Source: manufacturer monitors and technical manuals.
What Victron settings are best for LiFePO4 to reduce runtime error
Use Peukert 1.05–1.07, CEF 99–100%, tail current 2–4%, detection time 3–10 min, and confirm with a logged discharge. For mixed loads with frequent surges, some installers set k = 1.06–1.08. Source: Victron SmartShunt documentation and field installer guidelines.
Is Peukert exponent for lithium-ion always 1.05
No. Cylindrical NMC/NCA packs often fall 1.05–1.12 depending on design, BMS limits, and temperature. LiFePO4 tends to be closer to 1.05 due to its lower internal resistance and flatter voltage curve. Source: test reports and integrator experience across lithium chemistries.
Can I use a Peukert calculator to size my inverter battery
Yes—use battery runtime calculator Peukert math with your continuous DC current (AC watts ÷ inverter efficiency ÷ DC voltage). Add 20–30% buffer for lead-acid; 10–15% for LiFePO4 if operating below 10 °C or with long cables. Source: practical design margins common in RV/solar sizing.
How does Peukert exponent for LiFePO4 compare with the Peukert constant for lead-acid
LiFePO4 1.03–1.08 vs lead-acid 1.20–1.35. At 0.8–1.0C, LiFePO4 can deliver ~1.5–2× the runtime of similar-Ah AGM due to the lower exponent and lower voltage sag. Source: comparative calculations using typical k-ranges.
