Field Operations SOP — LiFePO₄ Batteries in Cold Environments
Purpose: provide clear, practical steps so LiFePO₄ packs survive and perform in cold environments while minimizing long-term damage.
Scope: applies to LiFePO₄ battery packs and systems used outdoors or in unheated enclosures where ambient temperatures may fall below freezing. Use this SOP for deployment, operation (discharge-only scenarios), return-for-charge, and long-term storage.
Key Temperature & SOC Rules (Use these as baseline)
Minimum recommended charging temperature: 0°C (32°F) — do not charge below this.
Example minimum discharge temperature (short-term): −20°C (−4°F) — some cells may be rated to this for short-term discharge; confirm with the datasheet.
Example minimum storage temperature (long-term): −10°C (14°F) — manufacturer limits for extended storage.
Recommended storage SOC for long-term: 40–60% (confirm vendor spec).
Cold-operation SOC guard: avoid leaving packs at SOC < 20% during prolonged cold exposure.
Reduce currents in cold: limit continuous discharge current below the normal rating when temperature < 0°C; recommend 0.2–0.5 C as a conservative range depending on cell specs.
These numbers are conservative templates. Always follow the specific battery datasheet where it differs.
Pre-deployment Checklist (Before placing packs in cold field)
Verify battery datasheet: minimum discharge temp, minimum storage temp, minimum charging temp, recommended storage SOC.
Program BMS: set charging cutoff at ≤0°C, set temperature-based discharge current limits, enable temperature alarms and logging.
Charge to recommended storage SOC (40–60%) if units will be stored. For immediate field operation, set SOC per mission needs but avoid < 20% if cold exposure will be long.
Confirm thermal mitigation: insulation, passive enclosure, or active heaters present and functional. Verify heating power source (battery, solar, fuel) is adequate.
Test telemetry and alarms: temperature, pack voltage, SOC, and BMS fault reporting must reach the monitoring station.
Label each pack with its safe-temperature limits and SOP reference.
Cold Operation (Discharge-only in the field)
Treat low-temperature discharge as short-term operation. Expect reduced usable capacity and higher internal resistance.
Limit depth-of-discharge and avoid repeated deep cold cycles. Shallower cycles cause less cumulative wear.
Enforce reduced discharge currents when pack temp < 0°C. If BMS supports it, use temperature-dependent current limiting.
Monitor pack temperature and SOC remotely. If pack temp falls below the manufacturer’s storage limit or SOC drops below the cold-operation guard (e.g., 20%), initiate retrieval or apply warming measures.
Use warnings: if the BMS signals a low-temperature fault or abnormal voltage sag, stop discharge and warm the pack.
Return-for-charge Procedure (Critical steps)
Never charge until pack temperature ≥ 0°C (32°F). Preferably get the pack to ≥10°C (50°F) for full-rate charging and reduced stress.
If pack was deeply discharged in the cold, allow a stabilization period after warming before charging; monitor internal resistance/voltage behavior during the first charge cycle.
Charging in stages is safer when returning from cold: begin at reduced charge current, monitor for signs of lithium plating (abnormal voltages or unexpected heating), then restore normal charge rate only if behavior is normal.
Record each cold-return event: ambient temp, SOC at retrieval, pre-charge pack temp, and post-charge health metrics.
Long-term Storage Procedure
Store at manufacturer-recommended temperature; if that is −10°C (14°F), avoid storing colder. Warmer is better for longevity within safe bounds.
Set SOC to mid-level (40–60%) for long storage. Verify a maintenance/float plan if storage lasts months.
If storage in cold is unavoidable, use insulated containers or controlled-temperature storage. Check packs periodically (monthly or per vendor guidance).
Before bringing stored packs into service, inspect physically and measure open-circuit voltage and internal resistance after they return to room temperature.
BMS & System Recommendations
Temperature thresholds: enforce a charging lockout at ≤0°C and a configurable discharge limit at the pack’s rated low-temperature discharge.
Current derating: implement temperature-based current derating curves; example: full current above 10°C, 50% current at 0°C, 20–50% below 0°C depending on cell capability.
Alarms & trip actions: high/low temp alarm, low SOC alarm, voltage sag alarm; automatic protective actions should be conservative (disconnect charge/limit discharge).
Logging: keep a log of temperature, SOC, current, and faults. Telemetry helps spot trends before irreversible degradation occurs.
Thermal Mitigation Options (Choose per site constraints)
Passive insulation: foam enclosures, insulated boxes, or buried cabinets reduce thermal swings.
Active heating: resistive heaters or self-heating packs. Remember heaters consume energy; budget supply accordingly.
Combined approach: insulation to reduce heater duty cycle. For solar-powered sites, verify worst-case energy budget (snow or multi-day cloud cover).
Self-heating cells/packs: convenient but still require energy and proper control logic.
Monitoring & Maintenance Schedule
Field monitoring: continuous telemetry preferred; at minimum, daily status checks for critical sites.
Short-term deployments (<7 days):check SOC and temperature at deployment and recovery.
Medium-term (weeks–months): weekly remote checks; physical inspection monthly where feasible.
After cold exposure: run a health check after the first warm charge—measure capacity, internal resistance, and record anomalies.
Troubleshooting Quick Guide
Severe voltage sag during cold discharge: stop discharge, apply warming (insulation/heater), retrieve if persistent.
BMS low-temp charge lock active: do not override; warm the pack first.
Unexpectedly low capacity after cold storage: run capacity test at ambient temperature; if capacity loss is > expected aging, tag pack for detailed diagnostic.
Heater failure / insufficient power: switch to insulated-only mode, retrieve packs if temps approach storage limit.
Short Datasheet Paragraph (for manuals)
Cold-environment guidance: Packs may be discharged short-term to −20°C (−4°F) where specified, but long-term storage must not fall below −10°C (14°F). Charging is prohibited below 0°C (32°F). For extended storage, maintain 40–60% state-of-charge. Implement temperature-based current derating and ensure a BMS enforces charge lockout below 0°C. Use insulation or active heating for deployments that require long-term reliability in freezing conditions.
One-page Decision Flowchart (Text version)
Is pack temperature ≥ 0°C?
Yes → Normal charge/discharge per datasheet.
No → Go to step 2.
Is operation discharge-only and short-term?
Yes → Allow discharge within rated cold-discharge limit; reduce current and monitor.
No → Retrieve or warm pack; do not charge.
Did SOC drop below cold-SOC guard (e.g., 20%) or did temperature reach storage limit?
Yes → Retrieve and warm; move to return-for-charge procedure.
No → Continue monitoring.
After warming for charge, is pack ≥ 0°C and behavior nominal?
Yes → Charge at reduced current, monitor health; restore full-rate if OK.
No → Continue warming and diagnostics.