Frequently Asked Questions About Nicad Batteries QUESTIONS 1 Can the charger detect a level of discharge ? Can the charger system discharge a partially charged battery to 1V/cell potential before starting the charge cycle? 2 Is a proper current level used in charging the battery? 3 Is a trickle current applied to the battery after the charge cycle is complete? 4 Is the technique to determine when a full charge has been obtained effective and safe? 5 Is positive contact maintained between battery and charger terminals? **Discharge cycles** Discharging a battery before applying a charging current cures the battery of at least three potential problems, including long-term storage, long-term over-charging and memory effects. A battery subjected to shallow discharges followed by full charges is likely to exhibit a phenomenon known as memory effect. Memory is not a loss of capacity, it is a voltage depression. Each cell in the battery rapidly loses voltage during subsequent use, suffering a reduction from 1.2V/cell to about 1V/cell. The reduced voltage mimics the appearance of a loss of capacity. The memory effort is avoided when the battery is fully discharged before each charge cycle. Long-term over-charging causes the loss of 30-40% of the battery's rated capacity the first time it is used after being subjected to the over-charging. One discharge-charge cycle can restore the abused battery to 75-90% of its rated capacity for its next use. Long-term storage causes the battery electrolyte to become unevenly distributed within the cells, reducing capacity. During long-term storage a passivation layer may form on the battery's anode. The layer acts as an insulator, diminishing the capacity as much as 55%. After repeated discharge-charge cycles, the long-term over-charging and storage effects are overcome and the battery will deliver its rated capacity. A battery is considered fully discharged if the terminal voltage is 1V/cell or less when the battery is under a 1C load. A 1C load draws a constant current while discharging a battery and reduces a fully charged battery to a 1V/cell level in one hour. The load normally is specified by the battery manufacturer. The charger should not discharge the battery below 1V/cell. If it does, weak cells in the stack may undergo a polarity reversal that would shorten the battery's useful life. **Charging currents** A charger should deliver a constant current at a level that charges the battery fastest. For rapid-charge batteries, a 1C charging rate is preferred. On standard batteries, a C/4 charge rate should be used, provided the charger can monitor the state of the charge and reduce the current to a trickle or remove current from the battery completely when a full charge is attained. **Trickle charges** A trickle current prevents the battery from discharging itself it is left in the charger for an extended period. A battery charged with the maximum safe current accepts the charge more efficiently. The battery will deliver as much as 10% greater capacity for a given amount of charge - comparing a 1C charging rate to a 0.02C rate, for example. A charger with 1C discharge and 1C and C/4 rates theoretically can discharge and charge a rapid-charge battery fully in less than 2.5 hours. A standard-charge battery would need less than seven hours. **Determining full charge** Three methods are common in determining when a battery has reached a state of full charge. One involves using the battery specifications and applying a controlled charge; the other two involve measurements. The controlled-charge method applies a constant current to a battery that has been discharged to 1V/cell. The current is applied for a specified period that satisfies the battery's specification. Measurement methods monitor the battery voltage during charging to detect a negative slope. As a battery reaches full charge, its terminal voltage first slightly increases, then decreases. The transition from the higher to lower voltage is known as the negative slope and indicates a full charge. Both methods are acceptable. But when the negative slope method is used, a discharge cycle should still be included prior to charging. Another method measures battery temperature. A thermal detection device placed against the cells reveals an increase in temperature. A nicad battery's temperature rises only after it reaches full charge. Therefore, to some extent, the battery is overcharged with each charge cycle. The thermal method may be acceptable if it is engineered properly, but it is not the preferred technique. **Positive contacts** The charger terminals must make positive contact with the battery terminals to ensure a low resistance connection for an efficient discharge-charge cycle. Positive contact also is a safety consideration with chargers that use thermal sensing of the full charge. If proper contact is not made, the battery could overcharge. A rapid-charge battery subjected to an overcharge could reach temperatures high enough to melt the battery case.