Battery manufacturers recommend to slow charge a new NiCd battery for 24 hours before use. This initial trickle charge helps to redistribute the electrolyte to remedy dry spots on the separator that may appear when the electrolyte gravitates to the bottom of the cell during long storage. A slow charge also helps to bring all the individual cells within a battery pack up to an equal charge level because each cell may have self-discharged to different capacity levels during storage.

Some battery manufacturers do not fully form their NiCd batteries before shipment. These batteries will reach their full potential only after the customer has primed them through several charge/discharge cycles, either with a battery analyzer or through normal use. In many cases, 50 to 100 discharge/charge cycles are needed to fully form the battery. Some Sanyo and Panasonic NiCd cells are known to perform to full specification after as few as 5 to 7 discharge/charge cycles (priming). Early readings may be inconsistent but the capacity levels become very steady once fully primed.

Although most NiCd batteries are suited for fast charging in an hour or so, fast charge should only be applied between 5C and 45C (41F and 113F). When charging a NiCd below 5C (41F), the efficiency of oxygen recombination is greatly reduced and pressure build up occurs . Sometimes, hydrogen can be generated as well.

To compensate for the slower metabolism at cold temperatures, a low charge of 0.1C must be applied, especially at the beginning and end of the charge cycle. Special methods are available for charging at cold temperatures. The NiCd is the only commercial battery that can accept charge at extremely low temperatures.

The charge acceptance of a NiCd at higher temperatures is drastically reduced. A battery that provides a capacity of 100% if charged at a moderate room temperature can only accept 70% if charged at 45 C (113 F) and 45% if charged at 60 C (140 F) (see below). This is demonstrated by the typically poor summer performance of vehicular mounted chargers. Special methods are available for charging at high temperatures.

Most rechargeable cells are equipped with a safety vent to release excess pressure if incorrectly charged. The safety vent on a NiCd cell opens at 150 to 200 psi (the pressure of a car tire is 30 psi). With a resealable vent, no damage occurs on venting but some electrolyte is lost and the seal may leak afterwards. When this happens, a white powder appears over time at the vent opening.

STATE OF CHARGE ( % of nominal 25 degrees C capacity)

Charge acceptance of a NiCd as a function of temperature

Commercial fast-chargers are often not designed in the best interests of the battery. This is especially true of NiCd chargers that measure the batterys charge state solely through temperature sensing. Temperature sensing is inaccurate because of the wide tolerances of the sensing thermistor or thermoswitch and its positioning with respect to the cells. Ambient temperatures and exposure to the sun while charging also play a role. To assure full charge under all conditions, charger manufacturers tend to "cook" the batteries. Any prolonged temperature above 45C (113F) is harmful to the battery and must be avoided. Chargers that depend primarily on an absolute temperature such as 50 C (122 F) to terminate charge significantly shorten battery life.

More advanced NiCd chargers observe a change in cell temperature rather than an absolute temperature to terminate full charge. This type of charger is kinder to the batteries but the cells still need to provide a sufficient temperature swing to trigger detection.

Harmful overcharge may occur if a fully charged battery is repeatedly inserted for topping charge. Vehicular or base station chargers that require the removal of the portable radio with each use are especially hard on the batteries because each reconnection initiates a fast-charge cycle.

With a microcontroller, more precise full charge detection can be achieved by monitoring the battery voltage, current or other variables and terminating the charge when a certain signature occurs. One such a signature is the drop in voltage known as Negative Delta V (NDV).

The NDV is the recommended full-charge detection method for open-lead NiCd chargers and analyzers that service batteries which do not have (or require) a thermistor. In order to obtain a sufficient voltage drop for a reliable measurement, the charge rate must be 0.5C and higher. In addition, all cells in a pack must be well-matched. Failing to achieve a sufficient negative slope would allow the fast-charge to continue, causing excessive heat due to overcharge. Chargers using the NDV must include other charge-termination methods to provide safe charging under all conditions.

The charge efficiency factor of a standard NiCd is better on fast charge than slow charge. At a 1C charge rate, the typical charge efficiency is 1.1 or 91%. On an overnight slow charge (0.1C), the efficiency drops to 1.4 or 71%. This means that only the "1.0" part of the charge energy is absorbed by the battery and the other 0.1 or 0.4 is lost, mainly through dissipation into heat.

At a rate of 1C, the charge time of a NiCd capable of holding 100% charge is slightly longer than 60 minutes (66 minutes at a charge efficiency of 1.1). The charge time on a battery that is partially discharged or cannot hold full capacity due to memory or other degradation is shorter accordingly. At a 0.1C charge rate, the charge time of an empty NiCd is about 14 hours, which relates to the charge efficiency of 1.4.

During the first 70% of the charge cycle, a NiCd battery absorbs almost all of the energy and the battery remains cool. In fact, currents of several times the C-rating can be applied to a NiCd battery designed for fast-charging. Ultra-fast chargers make use of this unique phenomenon and charge a battery to the 70% charge level within minutes. If sufficient time is available, the charge continues until the battery is fully charged, but at a much reduced current.

After the 70% charge threshold is reached, less and less energy is absorbed. The cells start to generate excess gases, the pressure rises and the temperature increases. The charge acceptance drops further and once full charge is reached, the battery goes into overcharge.

Ultra-high capacity NiCd batteries tend to heat up more than the standard NiCd if charged at 1C. This phenomenon is partly due to the increased internal impedance of the battery. Optimum charge performance can be achieved by applying higher currents at the initial charge stage, then tapering it to a lower rate as the charge acceptance decreases. This method avoids excess temperature rise and yet assures fully charged batteries.

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