Lithium Battery Runtime Guide
Lithium batteries — including LFP, NMC, and NCA chemistries — offer superior runtime characteristics compared to lead-acid. Their flat discharge curves, minimal Peukert effect, and deep DoD tolerance make them the preferred choice for applications requiring predictable, sustained power delivery.
Discharge Curve Characteristics
Unlike lead-acid batteries where voltage drops progressively throughout discharge, lithium batteries maintain a relatively flat voltage until the last 10–15% of capacity. This flat curve means the power delivered to your load remains consistent throughout most of the discharge cycle.
For LFP, the cell voltage stays between 3.2V and 3.3V from 90% down to 10% SOC. For NMC, the curve has a moderate slope from 4.2V to 3.5V. Both are significantly flatter than lead-acid, which drops from 2.1V to 1.75V per cell over the discharge cycle.
Lithium Runtime Formula
LFP nominal: 3.2V/cell. NMC nominal: 3.6V/cell. Multiply by series cell count for pack voltage.
Worked Example
Given: 100Ah 12.8V NMC battery, 200W load, 90% efficiency, 80% DoD
Step 1: Usable energy:
Step 2: Effective load:
Step 3: Runtime:
Lithium Sub-Chemistry Comparison
| Feature | LFP | NMC | NCA |
|---|---|---|---|
| Nominal Voltage | 3.2V | 3.6V | 3.6V |
| Usable DoD | 80–90% | 80% | 80% |
| Cycle Life | 3,000–5,000 | 2,000–3,000 | 1,000–2,000 |
| Energy Density | Moderate | High | Highest |
| Thermal Stability | Excellent | Good | Moderate |
| Best For | Stationary, RV, Marine | EV, Portable | Power tools, EV |
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Frequently Asked Questions
What is the difference between lithium-ion and LiFePO4 runtime?
For the same capacity and voltage, runtime is similar. The key difference is cycle life and DoD tolerance. LFP supports 80–90% DoD for 3,000–5,000 cycles, while NMC supports 80% DoD for 2,000–3,000 cycles. LFP also has a flatter voltage curve, providing more consistent power delivery.
How does the Peukert effect affect lithium batteries?
Lithium batteries have minimal Peukert effect — they maintain nearly full capacity even at high discharge rates. This is a major advantage over lead-acid, which loses 20–40% of usable capacity at high C-rates. Lithium runtime is more predictable across different load levels.
Can lithium batteries deliver their full rated capacity?
Yes, under normal conditions. Lithium batteries maintain 95–100% of rated capacity down to 20% SOC. Below 10% SOC, voltage drops rapidly and the BMS disconnects to prevent damage. Usable capacity is typically 80–90% of the rated Ah rating.
How does lithium battery age affect runtime?
Lithium batteries degrade gradually — typically reaching 80% SOH after 2,000–5,000 cycles depending on chemistry and conditions. At 80% SOH, runtime is 20% shorter than when new. Calendar aging also contributes roughly 2–3% capacity loss per year.