Battery Chemistry
Fox ESS utilise cobalt free LiFePO4 cells across its range of battery storage products (the most stable of all battery chemistries), providing high chemical stability and thermoelasticity. The phosphate cathode material in LiFePO4 cells is notably less susceptible to thermal runaway than other lithium-ion battery chemistries.
Operating Temperature Range
Charge: 0°C – +55°C
Discharge: -10°C – +55°C
These ranges represent the absolute maximum temperature range; however certain derating and protection measures will deploy within this range as detailed below.
While batteries can operate within the defined range detailed above, protection measures to derate the charge & discharge rates will be applied. See Performance Management. In addition, the battery may enter Power Reserve Mode (detailed below) which will temporarily raise the minimum Soc during periods of extreme cold.
How is temperature monitored?
The battery BMS collects temperature readings from internal sensors and this is communicated to the inverter.
What happens is the temperature falls below the defined minimum?
Should the temperature fall slightly outside of the defined range, the battery will enter a dormant (hibernation state) and remain idle until the temperature falls within the operating range. It is important that batteries are located in an environment where there is no prolonged exposure to extreme cold outside of the operating ranges as defined. This would not apply to batteries with internal heating where internal temperatures are regulated.
Are there any self-correcting measures?
For batteries that don’t include internal heating functionality, there will some measures deployed in periods of prolonged exposure to low temperatures. The battery BMS will monitor the battery SoC (state of charge) and will use small amounts of PV or grid energy at temperatures below 3°C to maintain a healthy state of charge – this process will, in turn, help raise the internal battery temperature.
Power Reserve Mode
Power Reserve Mode (PRM) activates when the minimum cell temperature is <3°C and the SOC is <60%, the system deploys protection measures (low-temperature power reserve mode), and the current limit value of discharge is temporarily reduced to 0, and the discharge is prohibited; when the SOC is >=65% or the minimum cell temperature increases to 5°C and above, the system exits the low-temperature power reserve mode, and the discharge is restored to normal. This is a protective mechanism for battery health and will clear when the temperature rises.
Please note, for EP5 and EP11 batteries, it is possible to alter the logic outlined above to reduce the SOC threshold deployed in PRM in certain instances. This would require special firmware which can be applied on a case by case basis.
Performance Management
Exposure to temperatures at the upper and lower limits of the operating ranges will result in reduced rates of charge and discharge and Power Reserve Mode will activate to temporarily raise the minimum SoC.
Why do charge/discharge rates reduce in low temperatures?
Reducing the charge rate at low temperatures is a deliberate protective measure by the charger/inverter to:
- Avoid lithium plating.
- Compensate for increased resistance and reduced electrolyte performance.
- Prevent long-term damage to the battery.
- Ensure compliance with the BMS and maintain safe charging conditions.
This behaviour extends the battery’s lifespan and ensures reliable performance in challenging environments.
Derating at low/high SOC
At very low or high states of charge, charge and discharge rates are automatically reduced to protect battery health and ensure long-term performance. When the SOC is extremely low, the battery management system (BMS) limits charge rates to prevent excessive stress on the cells, which could lead to accelerated degradation. Similarly, at very high SOC, charge rates are throttled to avoid overvoltage conditions and excessive heat buildup. This intelligent control mechanism helps maintain battery efficiency, prolong lifespan, and enhance overall system safety.
It is important to note, such measures are not specific to Fox. They are driven by the battery chemistry and therefore deployed across the industry.
Efficiency impact at low temperature
LFP (Lithium Iron Phosphate) batteries maintain their rated usable capacity across a wide range of temperatures. However, at low temperatures, chemical reactions inside the battery slow down, and internal resistance increases. This leads to a temporary reduction in system efficiency — meaning not all of the stored energy reaches the connected loads.
In practical terms, the battery still charges to full capacity and discharges down to its minimum state of charge, but more energy is consumed internally (as heat or voltage drop), so users may see less delivered output.
This is a normal characteristic of all lithium-ion battery systems and will return to normal performance as the temperature rises. For full details, please refer to this article.
