-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.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.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 ventingarge Detection by Temperaturecharge 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.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 Temperaturemethod 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 can not achieve the pace of temperature rise, an absolute temperature cut-off set to 60 ° C (140 ° F) terminates the charge.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 signaturearge Detection by Voltage Signaturechargers 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) .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.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.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.works best with fast charging. A fast charge also i...
