Automatic Nicad charger

by Eric Gaze, G8NKA

One of my favourite bits of kit is a handheld transceiver . If like me , you were startled at the cost of manufacturers battery packs, you probably opted for an empty case and at the next rally bought yourself enough batteries to fill it. The most common packs hold six 1.2v batteries giving a teminal voltage of 7.2v at capacities ranging from 500-900mA/hr. The commonest is 700mA/hrs and all reference to batteries in this article refer to this variety.

The calculations are simple and easily altered to to whatever type you have. To keep the batteries in top condition it is necessary to discharge them fully before recharging. This charge/discharge facility along with the built in one or 12 hour timer is the essence of this project.

Most nicads have their capacity written on them in the form current (mA) / time (hours). In a perfect world, if we take a fully charged battery and place a variable load across it,then adjust the load so a 800mA current  flows, the battery would sit there supplying its 800mA at 7.2v for one hour. the voltage and current would fall to zero leaving the battery fully discharged.

If we then remove the load and connect the battery to a constant current source of 800mA for one hour the battery would be full. In fact its a bit more complicated, Batteries exhibit a characteristic called "memory" if for example our standard battery is repeatedly discharged  to say , half its capacity, the battery remembers and over time the effective capacity falls to 400mA/hrs.
Most handhelds do not discharge the battery fully or they are recharged are before are completely discharged. The practical outcome is a gradual reduction in the working life of the battery

To overcome this problem the battery should be discharged to about 0.9V per cell before recharging starts. After several cycles this reforms the battery pack to its original capacity. It is also standard practice to over charge the battery by about 10%, ie 770mA for one hour , this is ok but it is better to take a bit longer and do the job properly.
Charging the battery at 10% of its amp/hr rate plus 10% (77mA) for 12 hours is better as the charge is low enough not to generate internal heat. Also it can be kept topped up by trickle charging, ie reducing the current to approximatley 3% of its amp/hr rating(21mA) This can be maintained for about a week or so.

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So now we have our charger requirements as follows.

1. Discharge the batteries

2. Charge the batteries for one hour at their mA/hr rating or 12 hours at 10% of their mA/hr rating plus 10%

3. Trickle charge the batteries at 3% of the mA/hr rating.

How it works

We know what is required So how is it done? refering to fig1 and ignoring the ic and surrounding components, Look at the 2 transistors and presume they are both off.
Insert the battery pack, nothing happens! connect the base of TR1 to a positive supply and it will turn on.

Think of it simply as aswitch. When TR1 is switched on it puts R17 ( 8R2 ) across the batteries. Assuming the batteries are fully charged and have a terminal voltage of 8.2V ( to keep the figures simple) there will be a discharge current of 1 amp, (I = V/R) Discharging the battery in about 1 hour, in practice it will take about 50 minutes for a full battery and about 10 -15 minutes when my handheld blanks out.

Transformer wiring diagram

Click diagram for full  circuit  view

We then remove the positive voltage from the base of TR1 , turning it of. Applying a negative voltage to to the base of TR2 (as it is a pnp device it switches on) the Darlington transistor and associated components forma simple constant current source, the current being limited by R16, Its value was  chosen to give a maximum current of about 1300mA (this is about the limit of the transformer  and should be adequate for any increase in battery capacity).
That is really the heart of the charger. All we need to do is build some circuitry that will monitor the voltage of the discharging battery, switch it to charge, adjust the charge current to suit a range of batteries, and build a timer for one or twelve hours, then arrange for the batteries to be trickle charged until we need them. Panic not! the ic does all of these jobs.

The ic has 3 voltage comparitors which, with suitable voltage divider resistors, monitor the state of the battery. Pin 5 of the ic would normally go to a positive temperature coefficient resistor in close proximity to the battery, so if it became hot the ic would detect this and switch of the charge current. This was dispensed with as i did not want to keep removing the batteries from the case and did not intend using the brutal half hour charge range. A simple potential divider R2 /R3  takes care of this

Pin 6 via divider R13/R10 monitors the falling battery voltage on discharge. When the battery voltage flls to about 5.4V (0.9V x 6 cells) pin 10goes low, turning of TR1 . Pin12 also goes low , turning on TR2  and the charging starts.

under construction  finished soon

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