Discharging  01




Q: What are considerations for discharging at high and low temperature

(May 2003) 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 Lithiumion performs better at high temperatures. Elevated temperatures temporarily counteract the battery's internal resistance, which may have advanced as 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 Lithiumion polymer uses some moist electrolyte to enhance conductivity, the dry solid polymer version depends on heat to enable sufficient ion flow. This requires that the battery core be kept at an operation temperature of 60°C to 100°C (140°F to 212°F).

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 that is fed by the utility grid during normal operation. On a power outage, the battery would need to provide its own power to maintain the temperature. Although said to be long lasting, price is an obstacle.

Nickel-metal-hydride degrades rapidly if cycled at higher ambient temperatures. For example, if operated at 30°C (86°F), the cycle life is reduced by 20%. At 40°C (104°F), the loss jumps to a whopping 40%. 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 Nickel-cadmium 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 Nickel-metal-hydride, Sealed Lead-acid and Lithiumion battery cease to function, the Nickel-cadmium 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 Liion 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 allow charging 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. Nickel-cadmium can be recharged at below freezing provided the charge rate is reduced to 0.1C.

Pulse discharge

Battery chemistries react differently to specific loading requirements. Discharge loads range from a low and steady current used in a flashlight, to sharp current pulses for digital communications equipment, to intermittent high current bursts in a power tool and to a prolonged high current load for an electric vehicle traveling at highway speed. Because batteries are chemical devices that must convert higher-level active materials into an alternate state during discharge, the speed of such transaction determines the load characteristics of a battery. Also referred to as concentration polarization, the nickel and lithium-based batteries are superior to lead-based batteries in reaction speed.

The Lead-acid battery performs best at a slow 20-hour discharge. A pulse discharge also works well because the rest periods between the pulses help to disperse the depleted acid concentrations back into the electrode plate. A discharge at 1C of the rated capacity yields the poorest efficiency. The lower level of conversion, or increased polarization, manifests itself in a momentary higher internal resistance due to the depletion of active material in the reaction.

Different discharge methods, notably pulse discharging, affect the longevity of some battery chemistries. While Nickel-cadmium and Lithiumion are robust and show minimal deterioration when pulse discharged, the Nickel-metal-hydride exhibits a reduced cycle life when powering a digital load.
In a recent study, the longevity of Nickel-meal-hydride was observed by discharging with analog and digital loads to 1.04V/cell. The analog discharge current was 500mA; the digital mode simulated the load requirements of the Global System for Mobile Communications (GSM) protocol and applied 1.65-Ampere peak current for 12 ms every 100 ms and a standby current of 270mA. (Note that the GSM pulse for voice is about 550 ms every 4.5 ms.)

With the analog discharge, the Nickel-metal-hydride provided an above average service life. At 700 cycles, the battery still provided 80% capacity. By contrast, the cells faded more rapidly with a digital discharge. The 80% capacity threshold was reached after only 300 cycles. This phenomenon indicates that the kinetic characteristics for the Nickel-metal-hydride deteriorate more rapidly with a digital rather than an analog load. Lithium and Lead-acid systems are less sensitive to pulsed discharge than Nickel-metal-hydride.


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