05. Discharge Methods 5

GUIDE: Batteries in a portable world. 5. Discharge Methods 5

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5.4 Discharging at High and Low Temperature

Batteries function best at room temperature. Operating batteries at an elevated temperature dramatically shortens their life. Although a lead acid battery may deliver the highest capacity at temperatures above 30°C (86°F), prolonged use under such conditions decreases the life of the battery.

Similarly, a Li-ion performs better at high temperatures. Elevated temperatures temporarily counteracts the battery’s internal resistance, which is a result of aging. The energy gain is short-lived because elevated temperature promotes aging by further increasing the internal resistance.

There is one exception to running a battery at high temperature — it is the lithium polymer with dry solid polymer electrolyte, the true ‘plastic battery’. While the commercial Li-ion polymer uses some moist electrolyte to enhance conductivity, the dry solid polymer version depends on heat to enable ion flow. This requires that the battery core be kept at an operation temperature of 60°C to 100°C.

The dry solid polymer battery has found a niche market as backup power in warm climates. The battery is kept at the operating temperature with built-in heating elements. During normal operation, the core is kept warm with power derived from the utility grid. Only on a power outage would the battery need to provide power to maintain its own heat. To minimize heat loss, the battery is insulated.

The Li-ion polymer as standby battery is said to outperform VRLA batteries in terms of size and longevity, especially in shelters in which the temperature cannot be controlled. The high price of the Li-ion polymer battery remains an obstacle.

The NiMH chemistry degrades rapidly if cycled at higher ambient temperatures. Optimum battery life and cycle count are achieved at 20°C (68°F). Repeated charging and discharging at higher temperatures will cause irreversible capacity loss. For example, if operated at 30°C (86°F), the cycle life is reduced by 20 percent. At 40°C (104°F), the loss jumps to a whopping 40 percent. If charged and discharged at 45°C (113°F), the cycle life is only half of what can be expected if used at moderate room temperature. The NiCd is also affected by high temperature operation, but to a lesser degree.

At low temperatures, the performance of all battery chemistries drops drastically. While -20°C (-4°F) is threshold at which the NiMH, SLA and Li-ion battery stop functioning, the NiCd can go down to -40°C (-40°F). At that frigid temperature, the NiCd is limited to a discharge rate of 0.2C (5 hour rate). There are new types of Li-ion batteries that are said to operate down to -40°C.

It is important to remember that although a battery may be capable of operating at cold temperatures, this does not automatically mean it can also be charged under those conditions. The charge acceptance for most batteries at very low temperatures is extremely confined. Most batteries need to be brought up to temperatures above the freezing point for charging. The NiCd can be recharged at below freezing provided the charge rate is reduced to 0.1C.

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