NBS designs and manufactures a full line of custom NiMH / NiCd battery chargers for a variety of commercial, industrial, and military applications.
-DV – Negative Delta Voltage
-DV Refers to the termination or the switching moment of a charge cycle. The -DV is the point at which the battery voltage under charge reaches a level where the voltage dips in a negative slope, hence Negative Delta Voltage. The -DV method is a result of a rapid constant current applied to a battery. A rapid charge current would be a current which is equivalent or exceeds a 0.5C rate. A -DV point would not be apparent if the charge current is less then a rapid rate. In this case, a -DV would not be a recommended choice. A -DV charge method is the recommended choice for Nickel Cadmium battery packs. It can also be used with Nickel Metal Hydride batteries. However, when used with Nickel Metal Hydride batteries, the -DV value is almost ½ that of Nickel Cadmium batteries and is usually as low as 8mV/cell.
Trickle charging is a method of charge where the charge current is very low. Usually this value is 0.1C of the rated current capacity of the battery. Trickle charging is an acceptable charging method for Nickel Cadmium batteries. In most cases, when applying a trickle charge method as the method of charge, a termination method is not necessary. Most Nickel Cadmium cells are capable withstanding a 0.1C charge rate for extended periods of time. Trickle charging is also a secondary method of charging for both a -DV and DT/dt methods.
∆T/dt – Delta Temperature per Time
=∆T/dt is the recommended method of charge and termination for NiMH batteries. The concept is based upon monitoring the change in temperature of the cells within the pack over a pre-defined time period, e.g., a change of 1° per minute. Similar to the -DV method, this method applies when a Rapid Charge current is being used. Secondly, a DT/dt method does NOT provide a 100% fully charged battery. A DT/dt point usually occurs at a point where the battery has acquired 90% of its capacity.
The use of the DT/dt method requires the monitoring of cell surface temperature. This is accomplished by way of a NTC thermistor. A NTC (Negative Temperature Coefficient) thermistor provides an ohmic value which decreases as the temperature increases.
DT/dt can also be used for NiCd batteries.
TCO – Temperature Cut Off
TCO is another temperature based method which requires the use of a thermistor and a rapid charge rate. The concept relies on the surface temperature of the cells and a specific temperature value at which a charge current is terminated, or switched from a rapid to a trickle charge rate.
TCO is not the most recommended method of charge for NiMH batteries, but it can be utilized.