NOTE: For NiMH batteries, please read our Charging Nickel-metal-hydride article.
Battery manufacturers recommend that new batteries be slow-charged for 16 to 24 hours before use. A slow charge brings all cells in a battery pack to an equal charge level. This is important because each cell within the nickel-cadmium battery may have self-discharged at its own rate. Furthermore, during long storage the electrolyte tends to gravitate to the bottom of the cell and the initial trickle charge helps redistribute the electrolyte to eliminate dry spots on the separator.
Battery manufacturers do not fully format the batteries before shipment. The cells reach optimal performance after priming that involves several charge/discharge cycles. This is part of normal use and can also be done with a battery analyzer. Early readings are often inconsistent and a battery may require 50–100 charge/discharge cycles to reach the best formation. Quality cells are known to perform to full specifications after only 5–7 cycles. Peak capacity occurs between 100–300 cycles, after which the performance starts to drop gradually.
Most rechargeable cells include a safety vent that releases excess pressure if incorrectly charged. The vent on a NiCd cell opens at 1,000–1,400kPa (150–200psi). Pressure release through a re-sealable vent causes no damage; however, with each venting, some electrolyte escapes and the seal may begin leaking. The formation of a white powder at the vent opening makes this visible, and multiple venting will eventually result in a dry-out condition. A battery should never be stressed to the point of venting.
Full-charge detection of sealed nickel-based batteries is more complex than that of lead acid and lithium-ion. Low-cost chargers often use temperature sensing to end the fast-charge, but this can be inaccurate. The core of a cell is several degrees warmer than the skin where the temperature is measured, and the delay that occurs causes over-charge. Charger manufacturers use 50°C (122°F) as temperature cut-off. Although any prolonged temperature above 45°C (113°F) is harmful to the battery, a brief overshoot is acceptable as long as the battery temperature will drop quickly when the “ready” light appears.
With microprocessors, advanced chargers no longer rely on a fixed temperature threshold, but sense the rate of temperature increase over time, also known as delta Temperature over delta time, or dT/dt. Rather than waiting for an absolute temperature to occur, this method uses the rapid temperature increase towards the end of charge to trigger the “ready” light. The delta Temperature method keeps the battery cooler than a fixed temperature cut-off, but the cells need to charge reasonably fast to trigger the temperature rise. Charge termination occurs when the temperature rises 1°C (1.8°F) per minute. If the battery cannot achieve the pace of temperature rise, an absolute temperature cut-off set to 60°C (140°F) terminates the charge.
Chargers relying on temperature inflict harmful overcharges when a fully charged battery is removed and reinserted. This is the case with chargers in vehicles and desktop stations where a two-way radio is being removed with each use. Every reconnection initiates a fast-charge cycle that raises the battery temperature to the triggering point again. Li‑ion systems have an advantage in that state-of-charge is being detected by voltage. Reinserting a fully charged Li-ion battery pushes the voltage to the full-charge threshold, and the charger turns off shortly without needing to create a temperature signature.
Advanced chargers terminate charge when a defined voltage signature occurs. This provides more precise full-charge detection of nickel-based batteries than temperature-based methods. Monitoring time and voltage, a microcontroller in the charger looks for a voltage drop that occurs when the battery has reached full charge. This method is called negative delta V (NDV).
NDV is the recommended full-charge detection for “open-lead” nickel-based chargers. “Open-lead” refers to batteries that have no thermistor. NDV offers a quick response time and works well with a partially or fully charged battery. When inserting a fully charged battery, the terminal voltage rises quickly, and then drops sharply to trigger the ready state. The charge in this case lasts only a few minutes and the cells remain cool. NiCd chargers based on the NDV full-charge detection typically respond to a voltage drop of 10mV per cell.
To obtain voltage drop of 10mV per cell, the charge rate must be 0.5C and higher. Slower charging produces a less defined voltage drop and this becomes difficult to measure, especially if the cells are mismatched. In this case, each cell in a mismatched pack reaches the full charge at a different time and the voltage curve flattens out.
Failing to achieve a sufficient negative slope would allow the fast charge to continue. To prevent this, most chargers combine NDV with a voltage plateau detector that terminates the charge when the voltage remains in a steady state for a given time. For additional safety, most advanced chargers also include delta temperature, absolute temperature and a time-out timer.
NDV works best with fast charging. A fast charge also improves charge efficiency. At a 1C charge rate, the charge efficiency of a standard NiCd is 91 percent, and the charge time is about an hour (66 minutes at an assumed charge efficiency of 91 percent). A battery that is partially charged or has reduced capacity due to age will have a shorter charge time because there is less to fill. In comparison, the efficiency on a slow charger drops to 71 percent. At a charge rate of 0.1C, the charge time is about 14 hours.
During the first 70 percent of charge, the efficiency of a NiCd is close to 100 percent; the battery absorbs almost all energy and the pack remains cool. NiCd batteries designed for fast charging can be charged with currents that are several times the C-rating without much heat buildup. Ultra-fast chargers use this quality and charge to 70 percent in minutes. Read more about Ultra-fast Chargers. The full charge must be done with a reduced current.
Figure 1 illustrates the relationship of cell voltage, pressure and temperature of a charging NiCd. We observe an almost perfect charge behavior up to about 70 percent, after which the battery loses the ability to accept charge. The cells begin to generate gases, the pressure rises and the temperature increases rapidly. One can appreciate the importance of accurate full-charge detection to terminate the fast charge before damaging overcharge occurs. In an attempt to gain a few extra capacity points, however, some chargers allow a limited amount of overcharge.
Figure 1: Charge characteristics of a NiCd cell
NiMH batteries exhibits similar characteristics to NiCd.
Courtesy of Cadex
Ultra-high-capacity NiCd batteries tend to heat up more than standard NiCds when charging at 1C and higher, and this is partly due to the higher internal resistance. Applying a high current at the initial charge and then tapering to a lower rate as the charge acceptance decreases achieves good results with all nickel-based batteries. This moderates excess temperature rise while assuring fully charged batteries.
Interspersing discharge pulses between charge pulses is known to improve charge acceptance of nickel-based batteries. Commonly referred to as a “burp” or “reverseload” charge, this method assists in the recombination of gases generated during charge. The result is a cooler and more effective charge than with conventional DC chargers. There is also the believed benefit of reduced “memory” effect, as the battery is being exercised while charging with pulses. Read about Memory: Myth or Fact? While pulse charging may be valuable for NiCd and NiMH batteries, this type of charge does not apply to lead- and lithium-based systems. These batteries work best with a pure DC charge voltage.
After full charge, the NiCd battery receives a trickle charge of between 0.05C and 0.1C to compensate for the self-discharge. To reduce possible overcharge, charger designers aim for the lowest possible trickle charge current. Even though the trickle charge is carefully measured, it is best not to leave nickel-based batteries in a charger for more than a few days. Remove them and recharge before use.
The flooded NiCd is charged with a constant voltage to about 1.55V/cell. The current is then reduced to 0.1C-rate and the charge continues until 1.55V/cell is reached again. At this point, a trickle charge is applied and the voltage is allowed to float freely. Higher charge voltages are possible but this generates excess gas and causes rapid water depletion.