Charger with 90% efficiency for nickel-cadmium batteries. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells
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The built-in battery charger must have particularly low power loss, as it is necessary to minimize heat generation in compact portable systems. The figure shows a diagram of a device that can charge a rechargeable battery of four nickel-cadmium cells and operate in a fast charge current of 1,3 A or a trickle charge current of 100 mA, while having a conversion efficiency of up to 90%.
(click to enlarge)
The LTC1148 is a stabilized buck switching voltage converter. It also controls the charge mode, monitoring the level of the output current using an external shunt on the resistor R3. The current in the fast charge mode is determined by the resistance of the resistor R3 (according to the formula for fast charge shown in the figure) and in this case is chosen to be 1,3 A.
The feedback divider across resistors R4 and R5 determines the output voltage level of 8,1 V at idle when the battery is disconnected. On a battery of four nickel-cadmium cells, the voltage can be from 3,6 to 6 V, depending on the state of charge.
When the battery is connected, the output voltage of the device will drop below 8,1 V, and the LTC1148 chip will start operating in a mode with output current limiting at 1,3A. A stable current of this magnitude will be supplied until the external end-of-charge controller issues a command to switch to the compensation charge mode or the charged battery is simply removed. Transistor Q3 turns on the compensation charge mode, and the current is determined by the resistance of resistor R1. Diode D2 prevents the battery from being discharged through the feedback divider circuit when the LTC1148 is turned off.
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