How to prepare your solar battery bank for winter
Winter weather can drastically cut battery capacity and lifespan—but it doesn’t have to. Proper storage, depth of discharge and maintenance will help prepare any battery bank for winter and maximize lifespan and capacity.
Storing batteries provides protection from cold temperatures
Most batteries are rated at 77°F, and their ideal operating temperature is between 50°F and 85°F. Batteries lose about 10% of their capacity for every 15°F to 20°F below 80°F. Their internal chemistries slow down, resistance increases and capacity and charge acceptance drop. This reduced capacity is temporary. However, it can present a problem because most renewable energy systems have the shortest days (i.e. lowest solar production) and highest loads during the winter, when capacity is lower.
Common battery storage solutions such as tin shelters, refrigerators or homemade boxes offer little protection from cold winter temperatures. And during the summer, temperatures in such enclosed spaces can exceed 140°F—hot enough to greatly accelerate battery deterioration.
A better approach is storing batteries in a well-insulated space with sufficient thermal mass and protection from direct sunlight. AGM and other no- or low-maintenance batteries can be stored inside a home. Flooded batteries should be stored in a locked, well-vented box in a garage or shed. (See National Electrical Code, Article 490.9(A) for details.) Leave 1/2-in. air gaps to ensure consistent temperatures.
If for any reason batteries cannot be protected from low temperatures, the owner may have to invest in a larger battery bank to compensate for decreased capacity.
If batteries will be unused for the season, such as in a summer home, there are several options to help prepare batteries for winter. Some inverters and chargers feature an automatic generator start function for use if panels are covered in snow. If this isn’t an option, turn off the inverter and all DC loads while leaving the charge controller on.
For extended storage, keep lead-acid batteries at 100% capacity if possible and disconnect them. Discharge lithium-ion batteries to approximately 40% of capacity and store at temperatures between 41°F and 68°F. Refer to the battery manual for specifics.
Using battery monitors and battery management systems
Technology also plays a key role in protecting batteries from harsh temperatures. Sensors can provide early warning if battery temperatures drop below recommended levels. Also, high-quality charge controllers can adjust voltage based on battery temperature to help cells reach 100% state of charge. This is important because cold batteries should be charged to a higher voltage than warm batteries to achieve the same state of charge.
For lithium-ion batteries, a battery management system (BMS) is a necessity. A BMS ensures lower-capacity cells aren’t overcharged and reduces the risk of lithium-ion batteries going into thermal runaway and causing fires and explosions. Other battery chemistries also benefit from the monitoring capabilities of BMS. These may include total and per-cell voltage, temperature, depth of discharge and more. BMS can even help maximize capacity and protect against extreme temperatures.
Playing it safe with depth of discharge (DOD)
Some battery makers tout their technology’s ability to discharge 100% and recommend sizing battery banks based on this unrealistic number. To ensure longevity and to keep the lights on, never discharge batteries below 80% DOD—50% DOD or lower is ideal.
All batteries require extra power reserves for days with low renewable energy production or higher electrical usage. Thus, a healthy safety margin is important regardless of battery chemistry.
If batteries still discharge too deeply, consider adding additional batteries. Depending on electrical consumption, seasonal availability of renewable energy sources and presence of a backup generator, it’s recommended to have three to six days of stored energy for off-grid systems.
Performing regular maintenance
Even low- and zero-maintenance batteries require regular inspection for proper performance. Battery manuals offer guidelines and maintenance schedules.
Depending on the battery technology, periodic maintenance may include cleaning battery terminals, checking fluid levels, adding distilled water, verifying terminal connections are secure and checking cell voltage.
Lead-acid batteries require equalization (intentional overcharging) to remove sulfation from battery plates and to bring all cells to similar levels. Lithium-ion batteries cannot be equalized by intentional overcharging. Instead, charge can be removed from high cells to low cells, or dissipative techniques may be used. Consult your battery manufacturer for best practices.
Whichever battery technology a system features, these strategies will maximize capacity and longevity all winter long.