LiFePO4 Battery Runtime Guide
LiFePO4 (LFP) has become the dominant chemistry for stationary storage, RV, marine, and solar applications. Its flat discharge curve, deep DoD tolerance, and long cycle life make it the most practical choice for applications where runtime matters more than weight.
Why LFP Excels for Runtime
LFP cells maintain a remarkably flat voltage between 20% and 90% SOC, holding 3.2–3.3V per cell throughout most of the discharge. This means the power delivered to your load remains consistent from nearly full to nearly empty, unlike NMC or lead-acid where voltage drops progressively.
The practical benefit: runtime calculations are more predictable because the battery voltage does not sag significantly until the last 10–15% of capacity. Combined with 80–90% usable DoD and minimal Peukert effect, LFP delivers the most usable energy per Ah of any common battery chemistry.
LFP Runtime Formula
For a 4S (12.8V) pack: multiply Ah × 12.8V × DoD. For 8S (25.6V): Ah × 25.6V × DoD. For 16S (51.2V): Ah × 51.2V × DoD.
Worked Example
Given: 200Ah 48V LFP server rack battery, 300W continuous load, 92% efficiency
Step 1: Usable energy at 80% DoD:
Step 2: Effective load:
Step 3: Runtime:
LFP vs. Other Chemistries
| Feature | LFP | NMC | Lead-Acid |
|---|---|---|---|
| Usable DoD | 80–90% | 80% | 50% |
| Cycle Life | 3,000–5,000 | 2,000–3,000 | 500–1,000 |
| Thermal Stability | Excellent | Moderate | Good |
| Voltage Flatness | Very flat | Moderate slope | Steep slope |
| Weight | Heavier | Lightest | Heaviest |
Temperature Effects
Discharge in Cold
At -10°C, LFP delivers roughly 85% of rated capacity. At -20°C, capacity drops to 70–75%. Discharge is safe but reduced.
Charging Limits
Never charge LFP below 0°C. Lithium plating on the anode causes permanent capacity loss and safety risks. Use a BMS with temperature-gated charge cutoff.
Related Articles
Frequently Asked Questions
How long does LiFePO4 last compared to NMC?
LiFePO4 (LFP) typically delivers 3,000–5,000 cycles at 80% DoD, while NMC delivers 2,000–3,000 cycles. LFP also has better thermal stability and does not require active cooling. The trade-off is lower energy density — LFP batteries are heavier for the same capacity.
What is the runtime advantage of LFP over lead-acid?
LFP batteries provide 80–90% usable DoD versus 50% for lead-acid. A 100Ah LFP battery delivers 1,024 Wh usable at 12.8V, while a 100Ah AGM battery delivers only 640 Wh. LFP runtime is roughly 60% longer for the same Ah rating.
Can I use LFP batteries in cold weather?
LFP batteries should not be charged below 0°C due to lithium plating risk. Discharge is possible down to -20°C but with reduced capacity (10–30% loss). For cold-climate applications, consider battery enclosures with heating elements.
Why is LFP voltage so flat during discharge?
LFP chemistry has a very flat discharge curve between 20% and 90% SOC, maintaining 3.2–3.3V per cell. This makes SOC estimation from voltage difficult but provides consistent power delivery throughout the discharge cycle, improving runtime predictability.