Nickel chemistry rechargeable batteries exhibit two distinct phases during the primary charge acceptance period. Initially, the battery chemical reaction is endothermic, and the majority of the electrical energy going to the battery is converted into chemically stored electrical energy. Following this period the chemical reaction becomes exothermic, at this time a rapid rate of change in the battery cell temperature becomes apparent. The primary objective of the charger then becomes to differentiate between the slow increase in temperature which occurs in the endothermic period due to various resistances which are normally present in the battery, and the greater rate of change which occurs during the exothermic period.

Typical Charging Profile

Additionally, slow variations in ambient temperature must not trip the dT/dt circuit. Jerome Industries typically designs the dT/dt sensor to have a 0.9°C / minute nominal sensitivity, with a 1.0°C / minute maximum. The dT/dt sensitivity can be increased somewhat for special situations, however the operational temperature range will consequently be narrowed. When using the delta T technique the following precautions should be addressed: 1) Do not allow discharge to below 1V / cell for any cell in the battery pack. In order to accomplish this your system will probably require a discharge cutoff circuit. 2) Make sure the cells are balanced in AHr capacity. Imbalanced Ahr values will cause voltage imbalance at the end of discharge state, consequently causing cell voltages less than 1V. Additionally, syncronization problems can develop for batteries with many cells, at the delta T point. Bear in mind that two additional charge termination methods are available to prevent continuous overcharging of the battery. The timeout circuit is standard on all chargers, and will cause the charger to switchback to the trickle charge state after a fixed amount of time. A high temperature cutoff circuit, is standard on all dTdt chargers. This circuit will cause the charger to switchback to the trickle charge state if the battery pack temperature reaches 50 degrees centigrade.