Low-temperature Batteries Charge/Discharge
Fast charging of most batteries is limited from +5o C to +45o C; for best results consider narrowing the temperature bandwidth to between +10o C and +30o C as the ability to recombine oxygen and hydrogen diminishes for nickel-based batteries for example. If batteries are charged too quickly, pressure builds up in the cell that can lead to venting. It is very important to reduce the charge current of all nickel-based batteries to 0.1C when charging is below 0o C. For a quick explanation of the “C”: If the battery has the capacity of 1000mAh. “C” will be defined as having a discharge of 1000mA for 1 hour.
Nickel-based chargers with Negative Delta V (NDV) full-charge detection offer some protection when fast charging at low temperatures; the poor charge acceptance happens when cold temperatures mimic a fully charged battery. This is in part caused by a high pressure buildup due to the reduced ability to recombine gases at low temperature. Pressure rise and a voltage drop at full charge appear synonymous.
To enable fast charging at all temperatures, some industrial batteries add a thermal blanket that heats the battery to an acceptable temperature; smart chargers adjust the charge rate automatically to prevailing temperatures. Consumer chargers do not have these abilities and the end user is advised to only charge at room temperature.
Lead acid batteries are reasonably forgiving when it comes to temperature extremes, as the starter batteries in our cars reveal. The recommended charge rate at low temperature is 0.3C, which is almost identical to normal conditions. At a comfortable temperature of +20o C, gassing starts at charge a voltage of 2.41V/cell. When going to –20o C, the gassing threshold rises to 2.97V/cell.
Freezing a lead acid battery leads to permanent damage. Always keep the batteries fully charged because in the discharged state the electrolyte becomes more water-like and freezes earlier than when fully charged. Flooded lead acid batteries tend to crack the case and cause leakage if frozen; sealed lead acid packs lose potency and only deliver a few cycles before they fade and need replacement.
Li-ion batteries offer reasonably good charging performance at cooler temperatures and allow fast-charging in a temperature bandwidth of +5 to +45o C. Below +5o C, the charge current should be reduced, and no charging is permitted at freezing temperatures. 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.
Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0o C. Although the pack appears to be charging normally, plating of metallic lithium can occur on the anode during a sub-freezing charge. This is permanent and cannot be removed with cycling. Batteries with lithium plating are more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers 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 –30o C is 0.02C. For example: 2500mAh battery has to be charged by 50mA at –30o C. At this low current, the charge time would stretch to over 50 hours, a time that is seemed impractical but maybe is practical at the northern oil/gas fields for keeping Li-ion battery packs ready and fully charged to power the systems that control the pressure and other parameters inside the pipes. Usually these battery packs are used as back-up power sources.
The performance of all batteries drops drastically at low temperatures; however, the elevated internal DC resistance will cause some warming effect because of efficiency loss during use. At –20o C most batteries stop functioning. Although Ni-Cd can go down to –40o C, the permissible discharge is only 0.2C. Li-ion can operate to a temperature of –40o C but only at a reduced discharge rate; charging at this temperature is out of the question. With lead acid there is the danger of the electrolyte freezing, which can crack the enclosure. Lead acid freezes quicker with a low charge when the specific gravity is more like water when fully charged.
Matched cells with identical capacities play an important role when discharging at low temperature and under heavy load. Since the cells in a battery pack can never be perfectly matched, a negative voltage potential can occur across a weaker cell in a multi-cell pack if the discharge is allowed to continue beyond a safe low battery cut-off point. Known as cell reversal, the weak cell gets stressed to the point of developing a permanent electrical short. The larger the cell-count, the greater is the likelihood of cell reversal under load. Over-discharge at a low temperature and heavy load is a large contributor of battery powered device/system failure.
The capacity may also be reduced if it is cycled at temperatures significantly below −10o C, which coincides with significant lithium plating on charging. At these low temperatures, the extractable energy is also significantly reduced.
What are your experiences with batteries at low temperatures?