Discharger for NICD and NIMH cells by Rodney King

A Design by Rodney King

After a while one accumulates increasing numbers of Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) rechargeable  cells. Over time they lose capacity and, particularly in the case of  NiCd, develop unacceptably high self-discharge rates. Unfortunately cells do not deteriorate at equal rates when used in series connected applications as the weakest cell always suffers first.

The author investigated value-for-money commercially produced dischargers but was very disappointed with the facilities offered. It was then decided to design something new. All components were basic with discrete transistors being used in preference to more complex and expensive ICs. Whilst a transistor’s base-emitter voltage used to define the switch-off threshold is slightly temperature dependant , the end-of-discharge voltage for these cells is usually defined as anywhere between 1 volt and 1.1 volts. Furthermore the voltage is always decreasing fairly rapidly at this point.

rodney-king-discharger

Transistors Tr1 and Tr2 are connected in a positive feedback loop with two stable states – SET and RESET. The SET state with Tr1 ON and Tr2 OFF is initiated by pressing the ‘start’ button SW1. This switches off Tr2 and energises the relay which in turn connects the ‘cell under test’ to a torch bulb load B1 and preset potentiometer R1. With a charged cell the output from R1 turns on Tr1 allowing the button to be released.

R1 must be adjusted so that Tr1 starts turning off immediately the cell’s voltage drops below a 1.05V threshold. Positive feedback via Tr2 expedites the switching process causing Tr1 to snap off, the relay then de-energises and the cell  becomes completely disconnected preventing any further discharge. With Tr1 OFF and Tr2 ON the circuit remains in its RESET or standby state.

The quartz clock ticks when the relay coil is powered allowing the discharge period to be accurately timed.

The constructor may wish to use a different relay and perhaps raise the supply voltage. In this case the value of R4 may have to be raised to ensure that the relay releases reliably whenever Tr1 turns off.

Each cell should be discharged immediately following a full charging to determine its Ah capacity (bulb current in amps X discharge time in hours). After a further full recharging, each cell should be left for a couple of weeks before re-testing. Any significant reduction in recovered charge will indicate self-discharge.

Rodney King – Vital Spark April 2008.

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Popular pages

Get your amateur radio licence - Find out more about amateur radio licence training.
Month on the Air - G3MGQ's popular monthly DX contest/expedition list.
Wilf Gaye Memorial Cup - The clubs annual operating event in the memory of Wilf Gaye M0GYE.
St. Richard's College Buildathon/STEM/ARISS - HERC attends St. Richard's Catholic College for their various events surrounding the Tim Peake ARISS contact.
G3BDQ - John Hey's Rare QSL Cards.
Sussex Electronics Radio Fair - SERF Sussex Electronics Radio Fair 2016.
Vital Spark - A selection of articles re-published from the Vital Spark.
RSGB News - Find out how to get RSGB news on your mobile or PC.
Experimenters Corner - A selection of Proteus projects by Bob Gornal (G7DME)
BBADL - Bath Based Distance Learning Course.
Conquest Hospital Radio - Presented by HERC member Antony (G4CUS).
Radio Rallies 2016 - An up to date list of radio rallies scheduled for 2016.
Club QSL Cards - A selection of QSL cards the club has received over the years.
Other Newsletters - Excellent newsletters and magazines from other clubs.
TX Factor episodes - Take a look at the TX Factors YouTube videos.
John Taplin - A bio of the late John Taplin.

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