Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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4.6 Charging the Lithium Polymer Battery

The charge process of a Li-Polymer is similar to that of the Li-ion. Li-Polymer uses dry electrolyte and takes 3 to 5 hours to charge. Li-ion polymer with gelled electrolyte, on the other hand, is almost identical to that of Li-ion. In fact, the same charge algorithm can be applied. With most chargers, the user does not need to know whether the battery being charged is Li-ion or Li-ion polymer.

Almost all commercial batteries sold under the so-called ‘Polymer’ category are a variety of the Li-ion polymer using some sort of gelled electrolyte. A low-cost dry polymer battery operating at ambient temperatures is still some years away.

4.7 Charging at High and Low Temperatures

Rechargeable batteries can be used under a reasonably wide temperature range. This, however, does not automatically mean that the batteries can also be charged at these temperature conditions. While the use of batteries under hot or cold conditions cannot always be avoided, recharging time is controlled by the user. Efforts should be made to charge the batteries only at room temperatures.

In general, older battery technologies such as the NiCd are more tolerant to charging at low and high temperatures than the more advanced systems. Figure 4-6 indicates the permissible slow and fast charge temperatures of the NiCd, NiMH, SLA and Li-ion.

	Slow Charge (0.1)	Fast Charge (0.5-1C)
Nickel Cadmium	0°C to 45°C (32°F to 113°F)	5°C to 45°C (41°F to 113°F)
Nickel-Metal Hydride	0°C to 45°C (32°F to 113°F)	10C° to 45°C (50°F to 113°F)
Lead Acid	0°C to 45°C (32°F to 113°F)	5C° to 45°C (41°F to 113°F)
Lithium Ion	0°C to 45°C (32°F to 113°F)	5C° to 45°C (41°F to 113°F)

Figure 4-6: Permissible temperature limits for various batteries.
Older battery technologies are more tolerant to charging at extreme temperatures than newer, more advanced systems.

NiCd batteries can be fast-charged in an hour or so, however, such a fast charge can only be applied within temperatures of 5°C and 45°C (41°F and 113°F). More moderate temperatures of 10°C to 30°C (50°F to 86°F) produce better results. When charging a NiCd below 5°C (41°F), the ability to recombine oxygen and hydrogen is greatly reduced and pressure build up occurs as a result. In some cases, the cells vent, releasing oxygen and hydrogen. Not only do the escaping gases deplete the electrolyte, hydrogen is highly flammable!

Chargers featuring NDV to terminate full-charge provide some level of protection when fast-charging at low temperatures. Because of the battery’s poor charge acceptance at low temperatures, the charge energy is turned into oxygen and to a lesser amount hydrogen. This reaction causes cell voltage drop, terminating the charge through NDV detection. When this occurs, the battery may not be fully charged, but venting is avoided or minimized.

To compensate for the slower reaction at temperatures below 5°C, a low charge rate of 0.1C must be applied. Special charge methods are available for charging at cold temperatures. Industrial batteries that need to be fast-charged at low temperatures include a thermal blanket that heats the battery to an acceptable temperature. Among commercial batteries, the NiCd is the only battery that can accept charge at extremely low temperatures.

Charging at high temperatures reduces the oxygen generation. This reduces the NDV effect and accurate full-charge detection using this method becomes difficult. To avoid overcharge, charge termination by temperature measurement becomes more practical.

The charge acceptance of a NiCd at higher temperatures is drastically reduced. A battery that provides a capacity of 100 percent if charged at moderate room temperature can only accept 70 percent if charged at 45°C (113°F), and 45 percent if charged at 60°C (140°F) (see Figure 4-7). Similar conditions apply to the NiMH battery. This demonstrates the typically poor summer performance of vehicular mounted chargers using nickel-based batteries.

Another reason for poor battery performance, especially if charged at high ambient temperatures, is premature charge cutoff. This is common with chargers that use absolute temperature to terminate the fast charge. These chargers read the SoC on battery temperature alone and are fooled when the room temperature is high. The battery may not be fully charged, but a timely charge cut-off protects the battery from damage due to excess heat.

The NiMH is less forgiving than the NiCd if charged under high and low temperatures. The NiMH cannot be fast charged below 10°C (45°F), neither can it be slow charged below 0°C (32°F). Some industrial chargers adjust the charge rate to prevailing temperatures. Price sensitivity on consumer chargers does not permit elaborate temperature control features.

Figure 4-7:    Effects of temperature on NiCd charge acceptance.
Charge acceptance is much reduced at higher temperatures. NiMH cells follow a similar pattern.

The lead acid battery is reasonably forgiving when it comes to temperature extremes, as in the case of car batteries. Part of this tolerance is credited to the sluggishness of the lead acid battery. A full charge under ten hours is difficult, if not impossible. The recommended charge rate at low temperature is 0.3C.

Figure 4-8 indicates the optimal peak voltage at various temperatures when recharging and float charging an SLA battery. Implementing temperature compensation on the charger to adjust to temperature extremes prolongs the battery life by up to 15 percent. This is especially true when operating at higher temperatures.

An SLA battery should never be allowed to freeze. If this were to occur, the battery would be permanently damaged and would only provide a few cycles when it returned to normal temperature.

	0°C (32°F)	25°C (77°F)	40°C (104°F)
Voltage limit on recharge	2.55V/cell	2.45V/cell	2.35V/cell
Continuous float voltage	2.35V/cell or lower	2.30V/cell or lower	2.25V/cell or lower

Figure 4-8: Recommended voltage limits on recharge and float charge of SLAs.
These voltage limits should be applied when operating at temperature extremes.

To improve charge acceptance of SLA batteries in colder temperatures, and avoid thermal runaway in warmer temperatures, the voltage limit of a charger should be compensated by approximately 3mV per cell per degree Celsius. The voltage adjustment has a negative coefficient, meaning that the voltage threshold drops as the temperature increases. For example, if the voltage limit is set to 2.40V/cell at 20°C, the setting should be lowered to 2.37V/cell at 30°C and raised to 2.43V/cell at 10°C. This represents a 30mV correction per cell per 10 degrees Celsius.

The Li-ion batteries offer good cold and hot temperature charging performance. Some cells allow charging at 1C from 0°C to 45°C (32°F to 113°F). Most Li-ion cells prefer a lower charge current when the temperature gets down to 5°C (41°F) or colder. Charging below freezing must be avoided because plating of lithium metal could occur.

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