Switchless NiCd-NiMH Battery Charger

Posted on Feb 4, 2014

This circuit may be used to replace the single current limiting resistor often found in dirt cheap battery chargers. The alternative shown here will eventually pay off because you no longer have to throw away your NiCds after three months or so of maltreatment in the original charger. The circuit diagram shows an LM317 in constant-current configur

Switchless NiCd-NiMH Battery Charger
Click here to download the full size of the above Circuit.

ation but without the usual fixed or variable resistor at the ADJ pin to determine the amount of output current. Also, there is no switch with an array of different resistors to select the charge currents for three cell or battery types we wish to charge: AAA, AA and PP3 (6F22). When, for example, an empty AAA cell is connected, the voltage developed across R1 causes T1 to be biased via voltage dropper D1. This results in about 50 µA flowing from the LM317`s ADJ pin into the cell, activating the circuit into constant-current mode. D4 is included to prevent the battery being discharged when the charger is switched off or without a supply voltage. The charging current I is determined by R1/R3/R3 as in R(n) = (1. 25 + Vsat) / I where Vsat is 0. 1 V. The current should be one tenth of the nominal battery capacity ” for example, 170 mA for a 1700-mAh NiCd AA cell. It should be noted that PP3` rechargeable batteries usually contain seven NiCd cells so their nominal voltage is 8. 4 V and not 9V as is often thought. If relatively high currents are needed, the power dissipation in R1/R2/R3 becomes an issue. As a rule of thumb, the input voltage required by the charger should be greater than three times the cell or battery (pack) voltage. This is necessary to cover the LM317`s dropout voltage and the voltage across R(n). Two final notes: the LM317 should be fitted with a small heat sink. With electrical safety in mind the use of a...

Leave Comment

characters left:

New Circuits



Popular Circuits

Pulsed Charger for Lead Acid batteries (NE555)
Digital Car Voltage Meter
Prototype: Openbench Logic Sniffer logic analyzer
USB08 HC908JB8 USB Reference Design
mini drill speed regulator
light activated police siren using 555 timer
Digital Theremin
RXTX V6.3 TX Mixer
Circuit FM transmitter for MP3 and MP4 Player 2n2218 and 741 opamp
JEC-2 delay circuit diagram consisting of b