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You need either a specialized BMS (Battery Management System) that can block charging at low temperatures or highly specialized cells that are very expensive.
Charging Li-ION batteries below ~5°C is dangerous, and it can be done only with a system designed for it. You really want an expert to design and build that system for you. You should consult companies that do that professionally, with adequate safeguards in place to avoid fires and property damage.
It all depends on how the batteries are connected to the solar systems. Most probably you would want to have a professionally designed storage system, that is designed with your environmental situation in mind (temperature excursions, humidity, salt in sea breeze, etc.).
In general any kind of battery can be discharged over a given temperature range that is wider than the temperature range suitable for charging it. Lithium cells are no exception.
Discharging Li-ION the cells well below 0°C is possible (albeit you lose some capacity) without damaging the cell. However 0°C is the limit for charging them safely, at least common cells not designed for sub-zero charging.
Here is an excerpt from Battery University(emphasis mine):
Batteries operate over a wide temperature range, but this does not give permission to also charge them at these conditions. The charging process is more delicate than discharging and special care must be taken. Extreme cold and high heat reduce charge acceptance and the battery should be brought to a moderate temperature before charging. [...]
Low-temperature Charge
[...] Lithium Ion: Li-ion can be fast charged from 5°C to 45°C (41 to 113°F). Below 5°C, the charge current should be reduced, and no charging is permitted at freezing temperatures because of the reduced diffusion rates on the anode. During charge, the internal cell resistance causes a slight temperature rise that compensates for some of the cold. The internal resistance of all batteries rises when cold, prolonging charge times noticeably. This also affects discharge performance noticeably with Li-ion.
Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the pack appears to be charging normally, plating of metallic lithium occurs on the anode during a sub-freezing charge that leads to a permanent degradation in performance and safety. Batteries with lithium plating are more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers (Cadex) prevent charging Li-ion below freezing.
Advancements are being made to charge Li-ion below freezing temperatures. Charging is indeed possible with most lithium-ion cells but only at very low currents. According to research papers, the allowable charge rate at &#;30°C (&#;22°F) is 0.02C. At this low current, the charge time would stretch to over 50 hours, a time that is deemed impractical. There are, however, specialty Li-ions that can charge down to &#;10°C (14°F) at a reduced rate.
Some Li-ion manufacturers offer custom-made cells for cold-charging. Specialty chargers will also be needed that decrease the C-rate according to temperature and charge the battery to a lower voltage peak; 4.00V/cell rather than the customary 4.20V/cell for example. Such limitations decrease the energy a Li-ion battery can hold to roughly 80% instead of the customary 100%. Charge times will also be prolonged and can last 12 hours and longer when cold.
Li-ion batteries charging below 0°C (32°F) must undergo regulatory issue to certify that no lithium plating will occur. In addition, a specially designed charger will keep the allotted current and voltage within a safe limit throughout the temperature bandwidth. Certification of such batteries and chargers are very costly that will reflect in the price. Similar regulatory requirements also apply to intrinsically safe batteries (See BU-304: Why Are Protection Circuits Needed?)
There are cell and charger manufacturers claiming to charge Li-ion at low temperatures; however, most companies do not want to take the risk of potential failure and assume liability. Yes, Li-ion will charge at low temperature but research labs dissecting these batteries see concerning results.
For more information, please visit Wide temperature range low voltage lithium battery services.
BTW, you say that (emphasis mine):
They are in a basement that rarely falls below 0°C (32°F).
The problem is that it takes just one single time the temperature falling below ~5°C and if your system tries to charge the batteries they can be damaged or they can even vent with fire, unless the charger system is designed to handle the event (either refusing to charge or charging with drastically reduced current).
https://dcbattery.tech/charging_your_lithium_battery_below_zero
https://www.aceongroup.com/news/why-does-low-temperature-affect-li-ion-batteries
https://www6.slac.stanford.edu/news/-08-26-how-extreme-cold-can-crack-lithium-ion-battery-materials-degrading-performance
https://www.continentalbattery.com/blog/how-does-cold-weather-affect-lithium-ion-batteries
Lithium Battery Operating Temperature Range
The operating temperature range of lithium batteries is generally known as the absolute temperature range, the common absolute temperature range is -20 ° C ~60 ° C, of which the lowest temperature of -20 ° C is the lowest usable temperature of the battery, the highest temperature of 60 ° C is the highest temperature that the battery can withstand, beyond this range will damage the battery.
The most ideal operating temperature of lithium batteries is 0 ° C ~ 45 ° C, and the battery discharge in this range can achieve excellent results. High temperature has a negative effect on the battery, higher than 60&#; will accelerate the aging of the battery, the principle is in the traction tank storage process history of chlorine collection.(SEI) history! And the organic material based on it will hydrolysate: produce a large amount of hydrogen: damage the performance of the battery.
Similarly, low temperature will also harm the reliability of the battery, less than -10 ° C will lead to battery performance damage, resulting in poor charge and discharge performance, capacity loss will become larger and larger, but also damage the structure of the battery. In addition, the low temperature environment will also make the electrolyte solidify, affecting the charging of air power.
Therefore, when operating lithium batteries, it is strictly necessary to follow the operating temperature between -20 ° C and 60 ° C, and the optimal operating temperature of the battery is between 0 ° C and 45 ° C. In order to avoid the negative impact of high and low temperatures on battery performance, it is also necessary to pay attention not to apply external pressure and vibration to the battery, otherwise it will affect the service life of the battery.
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