Power Supplies and Battery Chargers by E.D.S. | NiMH

E.D.S. offers a fully controlled two stage temperature charger designed to properly return capacity to a nickel metal hydride battery without fear of overcharge. This product is offered to be an affordable option for controlled NiMH battery charging. The battery end-of-charge voltage is a fixed voltage based upon the applications' intital charge rate. The charger is designed around a 45 degree C pack temperature (1) which acts upon this voltage when the battery heats to bring the charging current down into a C/20 trickle mode. Each charger is equipped with an LED, which indicates charge status.

This product is used in conjunction with a thermistor placed in the battery pack. The thermistor will pick up heat in the battery pack and trigger the charger to switch from the initial rate to the low trickle rate, thus preventing excessive heating and potential cell damage. The addition of the thermistor to the battery pack requires the use of a three-terminal connector between the battery and charger. A battery is required by E.D.S. in order to test and sample this unit.

This charger has two distinct states throughout the charge of the battery.

State1
When first attached to the battery, the charger charges at the designated high rate. The red LED is fully illuminated while in this stage.

State 2
When the battery reaches full capacity as determined by the thermistor sensing temperature in the battery pack, the charger switches into a c/20 maintenance trickle. The LED will not be illuminated.

The charge data below illustrates the typical operation of the two stage temperature charger

Charge Voltage vs Time	Charge Current vs Time
Cell Tempertature vs Time	Discharge Caoacity

click on charts to enlarge

Resources
• Approved Thermistor List
• Suggested Thermistor Assembly Technique
• Why Use a Controlled Charger?
• Patented Design Press Release

(1) E.D.S. chargers are intended for office environments which generally have 25-30 degree C ambient operating temperature. If charging in a hotter ambient, the charge may terminate before the battery reaches capacity. The rationale here is that heat is heat and the battery may be affected in a detrimental way regardless of whether the heat is coming from charging or ambient air. We feel it is better to not fully charge the battery than to subject the pack to temperatures that will shorten it's life.