A constant current discharge can be arranged electronically, as can the voltage monitoring which stops the clock when the end voltage is reached.Such circuits can be completed with an 8-pin dual op-amp type LM358NS and a few other components.
The measurement of capacity is often based on a 10-hour discharge rate.To increase the discharge current further will usually result in less recovered capacity.
Time indication could employ a digital display and a small processor, such as a PIC, but there is a fair amount of time needed for the software development and the construction.I side stepped that by using a small AA powered clock. The battery was removed and it was powered from the voltage monitor so as to stop when the end voltage was reached.
Such an analogue display is less than ideal because of the limitations of the 12 hour display.That is unlikely to be a problem when testing AAA cells, using a discharge current of 100mA, which were the main interest here.
The discharge current could be increased to avoid exceeding 12 hours with larger cells, but also the circuit could include a small xtal counter to light an LED after 12 hours so as to extend the unambiguous time period to 24 hours. Something for later, perhaps.
The Ni-MH AAA cell has seen a big increase in its capacity over recent years and it is now in much use in small equipments like phones and radio earphones.That was what inspired this design.
The two main elements of the present design are firstly the cell voltage monitor that provides the clock supply and secondly the constant current discharge circuit.
A basic cell voltage monitor is shown in figure 1. It operates as a comparator.The end-voltage indication level is set by VR1 and is supplied to pin 2 .The actual cell voltage Vb is applied to pin 3.While pin 3 voltage exceeds pin 2 voltage the output of the LM358 at pin 1 is a high of typically 5.4 volts.That voltage is used both to light a green LED and to provide about 1.5 volts for the clock.
The other half of the LM358 is used to control the FET current discharge, figure 2. A small resistor, R5, in the FET source produces a voltage proportional to current flow.That voltage is compared with the voltage set by VR2 and the output of the comparator drives the FET gate to make those two voltages equal.
The complete circuit is shown in Fig. 3 and uses only about 14 mA from a nominal 12 volt supply.An adequately stabilized working voltage of 6.8V for the circuit is provided by a simple 6V8 Zener diode.Text on the diagram indicates settings.
The battery voltage monitor is substantially as previously described, except for some positive feedback that uses R4 and R1 to add 50mV to the battery voltage via feedback from the high output so as to ensure a clean snap-off action when the battery voltage falls to 0.9V. If you use the voltage at pin 2 to set up a correct working, that voltage is set 50mV higher than the 0.9V battery voltage.
With the battery voltage above the minimum, IC1 output at pin 1 is typically +5.4V and is used to turn on the green LED D2. It also supplies the 1.5 volt, or so, clock supply that is derived across the 3 forward biased diodes D4, D5 and D6.The 220uF, across those diodes, provides sufficient impulse power for the clock pulses that occur each second and enables the clock supply to need only 1mA current feed to the diodes via R12.
A red LED is used to indicate the circuit is powered
Battery discharge
The battery is discharged by a V10LNF FET, Q1.In its source is the 3.3 ohms resistor R5 and with a 100mA flowing it has 330mV across it.That voltage is fed to pin 6 via R7 but, due to the current via R14 flowing through R7, another 50mV is added to the 330mV, for reasons to be explained later.
The FET current flow is set by VR2 which varies the voltage on pin 5 and so determines the FET gate bias.If the setting is made while monitoring the voltage at pin 5, that should be set to 50mV more than the wanted voltage across R5. But you can also just set for a 100mA discharge by the voltage across R5 being 330mV whilst discharging a cell.
There are two options for connecting the upper end of R9.Link 1 connects it to the 6V8 zener supply. Link 2 connects it to the output voltage provided by pin 1 of the LM358.
The first option sets the discharge current to continue indefinitely after the clock stops when the cell voltage falls to 0.9V -- or whatever other end point you care to set.
The second option causes the discharge to depend on whether the cell voltage is above 0.9V when pin1 output voltage will be high (5.4 volts).It was the reason that 50mV was added to pin 6 so that IC2 would turn the FET off properly.
However, in practice, when the discharge current is switched off the cell voltage recovers somewhat and the voltage rises up by more than 50mV such that the battery has a 100mA discharge again for a short period.That process repeats at a frequency that has been found to vary from about once every 3 seconds to as quick as 10 times per second, depending on how a particular battery recovers.Even if you increase the 50mV to 100mV it still happens, but at a lower frequency.
Mostly the clock either does not run during that oscillation or, if it does, the time increases by only a few minutes.The mode, as yet, is somewhat experimental.
I used it when I wanted to obtain a battery shortly after it had reached its 0.9V level, to see what a cheap, red/green, battery tester would indicate.
FET suitability
In order that the FET will maintain the 100mA discharge down to a cell voltage of 0.9V, the discharge loop resistance, which includes R5, must be sufficiently low and R5 was restricted to 3.3 ohms.More resistance is in the FET.According to the data sheet the FET has a typical resistance of 4 ohms with a maximum of 7 ohms.Thus a typical loop resistance will be 7.3 ohms total needing a cell voltage of not less than 0.73V.Which is OK for our 0.9V minimum.
However, a 7 ohms FET will require 1.03 volts and is not OK.
There is a choice here in what to do. Either use two FETs in parallel as one, which will always be OK for 100mA,or riskhaving to change the FET. If you want to be economical, make a circuit to take two, use one to start with and test it.
FET Test
The FET test is fairly simple to do.First set up the pre-sets to discharge a fully charged cell (1.2 volts or so) at 100mA and then remove the cell and apply just above the minimum 0.9 volts in its place and measure the FET gate voltage to see that it is still being controlled.One of the link options switches the FET off at the 0.9V minimum.
A test like that on the prototype, using a single FET, showed a gate voltage of 3.4V with a fully charge cell which rose to 3.8V with 0.9V cell voltage.The 3.8V is healthily less than the control voltage maximum of 5.4V and so no problem is expected.
In general you should perhaps change the FET if the gate voltage needed is more than about 4.5V.
For the design to be able to discharge at 200 mA it needs more FETs in parallel and also the 3.3 ohm R5 should be reduced to half that (1.65 ohms).The pre-set settings will be much the same as now.You could have two FETs each using 3.3 ohms, and switch the second gate on for the higher current.But be careful not to damage the gate which is static sensitive.
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Why anLM35
The LM358 has inputs that go down to zero volts and so it is able to monitor the 330 mV across the 3.3 ohm.Its output voltage ranges between 0.7V and 5.4V when powered with the 6V8 supply voltage.
The output will actually go down to 0V but it is not practical to use that because even 1mA was found to cause an 0.7V minimum.
Test Results
So far 12 Ni-MH cells have been tested.A 500mAH cell taken from each of two 4 year old house phones lasted just 10 minutes.Several 2 year old900mAH cells that had seen small use showed only 600mAH.More recent versions of those cells by the same manufacturer, with the same type number, but having a different colour sleeve, and now claiming 850mAH, returned figures of 800mAH.Had I discharged them at 85mA quite possibly they would have produced the claimed 850 figure.
Ken Holford (G1ACH)
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