Note: Most of the information in this article comes from the Solar Living Sourcebook.
Sunlight doesn't just power your solar panels - it heats your batteries. With fewer hours of sunlight per day, winter is the time of year when it's most important to make sure the energy storage component of an off-grid solar system or grid-tie solar system with battery backup is working efficiently. Unfortunately for those of us in the northern U.S. and Canada, winter can bring extreme cold, creating the worst environmental conditions for those deep-cycle batteries.
But just because deep cycle solar batteries have a harder time in cold temperatures doesn't mean you need to suffer through the winter with them. Learning a few simple tricks to keep solar batteries warm in winter will substantially improve their performance during the part of the year in which you rely on them most.
Note: Lithium-ion and sealed lead acid batteries (AGM and gel cell) fare much better in cold weather than traditional flooded lead acid batteries do, and are now so safe that they can - and should - be installed indoors. If you've got these batteries and they're installed indoors, you should have nothing to worry about when it comes to cold weather!
Why Are Cold Temperatures Bad for Solar Batteries?
First let's clarify that cold temperatures are actually good for batteries when it comes to holding a charge; a battery placed in a refrigerator will self-discharge more slowly than the same battery in a warmer setting (and in fact, some people keep their household AA and 9V batteries in the fridge). Where cold becomes a detriment to batteries is in situations where charging and discharging is frequent and the cycles are deep - like in a renewable energy system. This is because cold slows down the chemical reactions that take place inside a battery as it supplies electrical current. Colder temperatures reduce current, which in turn lowers energy storage capacity.
The cold weather problem is compounded with deep cycle batteries used for solar storage because cold weather usually coincides with storms or the shorter days of winter. In both cases, PV production is lower due to the reduced number of sun hours, and the loads in the house are often higher due to active heating.
How Cold is "Cold" When it Comes to Solar Batteries?
Most batteries are rated at 77°F (25°C), meaning their technical specs are based on how the battery's cells perform at 77 degrees. As a rule of thumb, batteries lose about 10% of their rated capacity for every 15-20 degrees below 80°F as measured in the cells.
How Can You Keep Your Solar Batteries Warm?
The best way to keep deep cycle solar batteries warm is simply to give them shelter and insulation. Let's look at how to best do this, depending on what kind of deep cycle batteries you have.
How to Keep Lithium-Ion Solar Batteries Warm in Winter
Lithium-ion solar batteries can be safely installed inside your house, meaning the "shelter" and "insulation" boxes are already checked and no additional action needs to be taken. However, if they are installed someplace where there's a risk of freezing, special care must be taken because - while they can safely discharge in temperatures as low 0°F (-18°C) - lithium-ion batteries should never be charged in sub-freezing temperatures (below 32°F or 0°C.
Nearly all lithium-ion solar batteries on the market today include at least a basic built-in battery management system (BMS) that can activate an external heating source when ambient temperatures approach freezing, ensuring the battery's environment stays warm enough for safe charging. A few more advanced lithium batteries even have internal heaters that are managed by their BMS.
How to Keep Flooded Lead Acid Solar Batteries Warm in Winter
Even though flooded - or "wet cell" - lead acid batteries can charge at lower temperatures than lithium-ion batteries (as low as -4°F or -20°C), they typically require the most attention in winter because they cannot be installed inside your house where temperatures are nicely regulated. (When in use these batteries release hydrogen gas - which is explosive at even a modest 4% concentration.)
Instead, flooded lead acid batteries should be kept inside a battery box or enclosure with 2" holes near the top and bottom for ventilation. If the enclosure is inside an insulated garage or shed, they most likely won't need any extra help staying warm. But if the enclosure itself is not in an insulated area, it should be "reinforced" with insulation and thermal mass to stabilize temperature swings. When adding insulation, make sure to leave an inch or two between the batteries and the insulation (as well as between the batteries themselves) to allow air to circulate and keep temperatures as similar as possible across all the batteries. Because the batteries generate a bit of heat when they are charging, the insulation will keep that heat inside the enclosure, keeping the batteries warmer. Be sure to remove the insulation when the weather warms up. You don’t want the batteries to overheat in the summer.
How to Keep AGM/Sealed Lead Acid Solar Batteries Warm in Winter
Like lithium-ion batteries, sealed lead acid batteries (AGM and gel cell) are safe enough to be installed indoors, giving you a huge leg up on temperature regulation. Also working in your favor is the fact that sealed battery cells freeze at lower temperatures than flooded/wet cells do. It's best to check the manufacturer's specs for your specific batteries regarding minimum charging and discharging temperatures. If your batteries are going to be exposed to temperatures lower than the minimums listed by the manufacturers, the same techniques suggested for lithium-ion and flooded lead acid batteries can be used to keep your sealed-cell batteries warm enough for safe use (insulated enclosure and a BMS/heater).
Tools and Tech to Help Keep Solar Batteries Healthy in Winter
MPPT Charge Control Technology
A charge controller with maximum power point tracking (MPPT) technology is probably already part of your battery-based solar system, but those short winter days are when you'll be happiest to have it. MPPT solar charge controllers squeeze 15-30% more power out of a PV array on average than non-MPPT controllers by converting excess PV voltage into amperage. MPPT controllers perform best with cold PV modules and hungry batteries (sounds like winter conditions!), and can adjust the battery voltage based on battery temperature to help the cells reach 100% state of charge.
Battery Temperature Sensor
A battery temperature sensor can provide an early warning if battery temperatures drop below recommended levels. Some charge controllers will stop charging at certain low temperatures.
Battery Management Systems (BMS)
A battery management system includes functions beyond just temperature sensing, typically including features like voltage and depth of discharge at both the cell and battery bank level. This level of insight can help you identify problem areas in your battery bank, some of which may be cold-related.
A BMS is an absolute necessity for lithium-ion solar batteries (to prevent overcharging and thermal runaway), but most Li-ion batteries on the market today already have a BMS built in.
These deep cycle battery maintenance tools simply connect to the battery terminals and then work automatically to remove lead sulphate buildup on the plates. Lead sulphate will otherwise build up on the plates over time, reducing a lead acid battery's capacity and ultimately shortening its life. When you're already contending with cold weather reducing battery capacity during the winter, you don't need sulphate buildup compounding the problem.
Keep the Water Topped Off
It is important year-round to not let your flooded lead acid batteries go dry. In winter it's especially critical to make sure there is plenty of water in them. A battery with low water levels is more likely to freeze.
Likewise, keep them charged up. The acid in the water prevents freezing, so a battery with a low charge is more likely to freeze than a charged one.
Adding More Battery Storage
Sometimes all the tricks in the book just aren't enough to keep an older lead acid battery bank powerful enough to get you through the winter comfortably. Over time, batteries age and their capacity falls, and we add more electric loads to our homes. Sometimes the best way to combat the problem of cold weather limiting your batteries' performance is to simply add capacity with a couple of new deep cycle lead acid batteries.
Winter or Not, Don't Forget to Perform Regular Maintenance on Your Lead Acid Deep Cycle Batteries
For lithium-ion and sealed cell lead acid batteries (AGM and gel cell), a simple annual maintenance routine of cleaning the tops of the batteries with baking soda and water (then rinsing with water), and then cleaning/tightening the terminals is sufficient.
For wet cell (flooded) lead acid batteries, read Real Goods Solar Tech Noah Mervine’s battery maintenance checklist.