By John Connell, vice president of Crown Battery Manufacturing’s SLI Products Group
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.
Is voltage still the best way to measure % available during the winter? I mean is 12.3 still 50% in the winter? Or is it just displaying lower power than it really has?
I’ve read batteries will last longer in the winter and vice-a-versa in the summer.
I hope this makes sense.
I need to know if I can go below 12.3 in the winter.
Hello, I have 1525 W of 33v solar panels, and I plan to have an approximately 1600ah AGM battery bank, which gives me 800ah of use each day. I will be living in a Tiny House On Wheels, mostly traveling between Mexico, Costa Rica, and Panama.
What I need to know is this; as I use my appliances, will the sun continue to feed my batteries? I.E., if I use the full 800W of power during the day, isn’t the sun constantly replenishing my batteries so that they will still have nearly a full charge for night use?
2nd, IF my batteries are continuously being charged during the day, do I really need a 1600ah battery pack? I’ve created a spreadsheet that shows the MAXIMUM amount of ah’s I could possibly use during the day AND during the night. which comes to no more than 50% of my 1525 watts of solar (762ah’s). But if my batteries are constantly being charged, would it be feasible for me to go down to a 1200ah or even an 800ah battery pack (assuming I don’t use more than about 400ah’s during hours without sun)?
Thank you
PS: I realize that Winter poses some issues for charging, but I’ve included “cloudy day use” in my spreadsheet, too.
You can have current going into the cell at the same time you output current.
For example whe you charge your laptop and use at same time. Your actually using laptop from wall power. The battery is only being charged not discharged at same time.
It is possibpe to get a high quality switching inverter that could switch when youre using really low power. But if youre pulling continuous output you can not be inputting current at the same time.. if i understand correctly.
I have 4 six volt flooded batteries hooked up to a 235 w panel and an inverter/ charger. I only use this system in the winter at my cabin. It generally works well for a two night stay. I monitor battery voltage and it is usually around 12.8-12.9 when we arrive and will drop down to 11.7-11.8 after two nights. Batteries are stored in a shed inside a box made of 2” styrofoam. I was wondering if it would be beneficial to add a 12v bulb inside the box to heat the batteries in this cold environment. I live in Canada.
HI Guy – I have exact same issue. Did you get an answer?
Any ideas on an insulated solar blanket for a battery bank for northern ontario?
On May 1st I sent a message to you regarding my situation and asking you for advise. As of this date I still have not received a reply. Could you maybe review the question and reply. Thank you.
Allen, Try searcing for a battery sizing calculator online or through a battery manufacturer to find more accurate information for your situation.
Great info. I have an off grid system on a trailer that is only used during summer from May 1 to Nov 1. Currently I bring all 6 six volt batteries home. I plan to buy a metal box, insulate it and hook a solar panel to it to maintain my batteries. I will leave the system up there on the trailer. What power of panel would you suggest to maintain those batteries and I do believe a solar controller is a must. I unhook and shut off all the power to those batteries when I shut down so when I get the new panel and solar charger that would be all that would be attached to the system. The battery bank is hooked in series and parallel
Hi Kathie,
I have an off grid seasonal cabin with solar (4K inverter for the cabin) in Montana (close to Craig, MT). I just want to protect the batts over the winter. They’re not used in the winter. But, low temp in the winter is an issue…They’re currently in a shack that is half insulated. There are 4 235 watt panels online faced to the south. I can’t help the power coming in from the panels during the winter. What do you suggest to keep the batts in good shape over the winter?
Thanks so much for your time and help
Kevin
@Kevin What kind of batteries do you have? Maintenance free batteries (AGM, gel, lithium ion) will be fine as long as they are hooked up to your solar panels and maintain a full charge. If your charge controller has a temperature sensor port it would be a good idea to invents in a temp sensor.
What if you cannot ensure the panels are not covered in snow and not working.
Can the flooded batteries be charged to 100 percent and disconscted from the inverter and the charge controllerand left for four months. The temp at the off grid cabin can go down to -35 c