Charging at High and Low Temperatures

Rechargeable batteries operate in a wide temperature range but this does not give license to charge them at extreme temperatures. Extreme cold and high heat reduce charge acceptance, and the battery must be brought into moderate temperature conditions before charging.

Older battery technologies, such as lead acid and NiCd, have higher charging tolerances than newer systems and can be charged below freezing at a reduced 0.1C rate. This is not possible with most NiMH and lithium-ion systems. Table 1 summarizes the permissible charge and discharge temperatures of common lead acid, NiCd, NiMH and Li‑ion. We exclude specialty batteries designed to charge outside these parameters.
 

Table 1: Permissible temperature limits for various batteries. Batteries can be discharged over a large temperature range but charge temperature is limited. For best results, charge between 10°C and 30°C (50°F and 86°F). Lower the charge current when cold.

Low-temperature Charge

Fast charging of most batteries is limited to a temperature of 5 to 45°C (41 to 113°F); for best results consider narrowing the temperature bandwidth to between 10°C and 30°C (50°F and 86°F). Nickel-based batteries are most forgiving in accepting charge at low temperatures, however, when charging below 5°C (41°F), the ability to recombine oxygen and hydrogen diminishes. If NiCd and NiMH are charged too rapidly, pressure builds up in the cell that will lead to venting. Not only do escaping gases deplete the electrolyte, the hydrogen released is highly flammable. The charge current of all nickel-based batteries should be reduced to 0.1C below freezing.

Nickel-based chargers with NDV full-charge detection offer some protection when fast-charging at low temperatures. The resulting poor charge acceptance mimics a fully charged battery. This is in part due to the pressure buildup caused by gas recombination problems. Pressure rise and a voltage drop at full charge appear to be synonymous.

To enable fast-charging at all temperatures, some industrial batteries include a thermal blanket that heats the battery to an acceptable temperature; other chargers adjust the charge rate to prevailing temperatures. Consumer chargers do not have these provisions and users should make all attempts to only charge batteries at room temperatures.

Lead acid is reasonably forgiving when it comes to temperature extremes, as we know from the starter batteries in our cars. Part of this tolerance is their sluggish behavior. The recommended charge rate at low temperature is 0.3C, which is almost the same as under normal conditions. At a comfortable temperature of 20°C (68°F), gassing starts at 2.415V/cell, and by lowering the temperature to –20°C (0°F), the gassing voltage rises to 2.97V/cell.

Do not freeze a lead acid battery. This would causes permanent damage. Always keep the batteries fully charged. In the discharged state the electrolyte becomes more water-like and freezes earlier than a fully charged battery. According to BCI, a specific gravity of 1.15 has a freezing temperature of –15°C (5°F). This compares to 1.265 of a fully charged starter battery. 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 a replacement is necessary.

Li‑ion batteries offer reasonably good charging performance at cooler temperatures and allow fast-charging in a temperature bandwidth of 5 to 45°C (41 to 113°F). Below 5°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. With all batteries, cold temperature raises the internal resistance.

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 can occur on the anode during a subfreezing charge. The plating is permanent and cannot be removed with cycling. Batteries with lithium plating are known to be more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers, such as those made by Cadex, prevent charging Li-ion below freezing.

Manufactures continue to seek ways to charge Li-ion below freezing and low-rate charging is indeed possible with most lithium-ion cells; however, it is outside the specified (and tested) limits of most manufacturers’ products. Low-temperature charging would need to be addressed on a case-by-case basis and would be manufacturer and application dependent. According to information received from university research centers, the allowable charge rate at –30°C (–22°F) is 0.02C. At this low current, a 1,000mAh Li-ion could only charge at 20mA, and this would take more than 50 hours to reach full charge.

Some Li-ion cells developed for power tool and EV applications can be charged at temperatures down to –10°C (14°F) at a reduced rate. To charge at a higher rate, Li-ion systems for automotive propulsion systems require a heating blanket. Some hybrid cars circulate warm cabin air through the batteries to raise the battery temperature, while high-performance electric cars heat and cool the battery with a liquid agent.

High-temperature Charge

Heat is the worst enemy of most batteries, including lead acid. Adding temperature compensation on a lead acid charger to adjust for temperature variations prolongs battery life by up to 15 percent. The recommended compensation is 3mV per cell per degree Celsius applied on a negative coefficient, meaning that the voltage threshold drops as the temperature increases. For example, if the continued float voltage were set to 2.30V/cell at 25°C (77°F), the recommended setting would be 2.27V/cell at 35°C (95°F) and 2.33V/cell at 15°C (59°F). This represents a 30mV correction per cell per 10°C (18°F). Table 2 indicates the optimal peak voltage at various temperatures when charging lead acid batteries. The table also includes the recommended float voltage while in standby mode.
 

Table 2: Recommended voltage limits when recharging and maintaining stationary lead acid batteries on float charge. Voltage compensation prolongs battery life when operating at temperature extremes.

Charging nickel-based batteries at high temperatures lowers oxygen generation, which reduces charge acceptance. Heat fools the charger into thinking that the battery is fully charged when it’s not.

NiCd has the largest pool of published information on this subject, and Figure 3 demonstrates a strong decrease in charge efficiency above 30°C (86°F). At 45°C (113°F), the battery can only accept 70 percent of its full capacity; at 60°C (140°F) the charge acceptance is reduced to 45 percent. NDV for a full-charge detection becomes unreliable at higher temperature and temperature sensing is essential for backup. Newer type NiMH batteries perform better at elevated temperatures than NiCd.

Figure 3: NiCd charge acceptance as a function of temperature. High temperature reduces charge acceptance. At 55°C, commercial NiMH has a charge efficiency of 35–40%; newer industrial NiMH attains 75–80%.

Courtesy of Cadex

Lithium-ion performs well at elevated temperatures; however, prolonged exposure to heat reduces longevity. The charge efficiency is 97 to 99 percent, regardless of temperature. In fact, high temperature increases charge effectiveness slightly by improving the internal resistance.

While other chemistries can tolerate stepping outside set boundaries once in a while, there are limitations with Li-ion. Safety concerns dictate that Li-ion remains within specified limits because of possible thermal runaway if stressed. A fully charged Li-ion is more sensitive to a thermal runaway than an empty one; the thermal runaway temperature moves lower with higher charge. In spite of this, specialty Li-ion batteries serve in applications that go to momentary high temperatures, and surgical tools that undergo steam sterilization at 137°C (280°F) are such an example. Other uses that reach similar temperatures are batteries in drilling bits for mining.
 

Comments

On January 12, 2011 at 11:02pm

Girish.K wrote:

How to calculate the connected load of battery chargers
360V-150A-80% efficency

On August 24, 2011 at 10:33am

Janusz Konopka wrote:

Battery means several cells connected in series.  I the course of work (after several charging and discharging cycles) one may note important differences between particular cells.  Despite of   monitoring particular cells during discharge it chappens that battery become useless because e.g. one cell is much weaker. I founfd it in the case of my electric bike. After, say, some 30 cycles battery become usless. When I performed separate charging of weaker cells the baterry recovered and could be used again.
Plese give me the approximate internal resistance of Li-Ion cell (dV/dI).  This parameter will may be decisive in controlling quality of each cell.

On October 31, 2011 at 7:48am

Max Dirnberger wrote:

Dear Janusz,

This will be because your charger is not doing charge balancing. When Li-Ion batteries are in series (most battery packs) this step is often not implemented but must be to avoid the problem you mention

On December 24, 2011 at 9:03pm

Omair wrote:

So basically, if I don’t requite the batteries to fast charge, and since I plug my phone to the charger before bed, I actually prefer it to charge slowly in order to reduce stress, wouldnt cooling the phone to about 15°every night
help elongate my battery life? (Accodibg to your other post, by uptill 15%).
The battery in consideration is a 3.7v 1230mAh Li-ion battery.

On March 13, 2012 at 7:23pm

Happy wrote:

So i have already read all the lesson above. my question is which one is the best metode, charging at the lowest or highest temperature for range 10 - 30C that you mentioned?

thanks for the answer

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