The simple automatic charger we are talking about is assembled from readily available parts, the total cost of which does not exceed the price of a single AA nickel-metal hydride rechargeable cell, which the device is designed to charge. You can assemble it on a printed circuit board, the dimensions of which are not much larger than the dimensions of the rechargeable cell itself.
In their technical documentation, manufacturers specify the parameters of the optimal charge mode, and also provide charging and discharging characteristics of the rechargeable cell. Their comparative analysis, including the experimental one, made it possible to note two features that later became the starting point for the development of this charger, namely:
During fast charging, it is necessary to monitor the voltage on the rechargeable cell so as not to miss the moment when this voltage stops growing and begins to decline along with the rechargeable cell temperature rise – this is the moment when the charge ends with increased current. To ensure such a charge mode, either chips specially designed for this are used, or the use of a microcontroller becomes necessary – ugly and expensive.
We also note that even rechargeable cells of the same type from the same manufacturer with the same release date differ in their parameters to one degree or another from each other. This cannot be ignored, since, for example, different internal resistance does not allow either discharging or then charging the rechargeable cells in the battery to the same extent.
Based on the above arguments, an “individual” charger was developed based on a cheap LM2903P dual comparator chip. After connecting the rechargeable cell and applying the supply voltage, the device first completely discharges the cell, and then charges it with the rated charge current. Shortly before the end of the charge, the device automatically switches to the mode of booster charging the cell with a small current. In this mode, even a fully charged cell can stay long enough without any damage, and therefore any shutdown timer is redundant. All three stages of operation of the charger indicates by LEDs.
The schematic diagram of the charger is shown in Fig.1. Two voltage comparators provide automatic operation of the charger. Comparator D2:2 controls the discharge of the cell, and comparator D2:1 controls its charge. We note right away that the operation of this charger assumes that the rechargeable cell is connected to it first, and only then the supply voltage is applied, but not vice versa.
Before charging the cell, it is desirable to completely discharge it. A sign of a complete discharge of the rechargeable cell is the voltage drop on it to the level of 1V and below when a load is connected to it. This threshold is set by a voltage divider on R5 and R6 resistors, to which the inverting input 6 of comparator D2:2 is connected. The voltage of the rechargeable cell is applied to the non-inverting input 5 of this comparator through resistor R8. The rechargeable cell discharge is managed as follows. So, at the time the supply voltage is applied, the cell is already connected to the charger. If the rechargeable cell is not fully discharged, then the voltage on it, and therefore on the non-inverting input 5 of the comparator D2:2, exceeds the voltage set by the divider R5:R6 on the inverting input 6 of this comparator. The voltage level at the output 7 of the comparator in this case will be high, therefore, the transistor switches VT1 and VT6, to the gates of which this voltage is applied, will close. The rechargeable cell will begin to discharge through the load resistor R7, and until it ends, the red LED VD4 will light up. Transistor switches VT2 and VT4 are opened and do not affect the operation of the device. As soon as the rechargeable cell voltage drops below the threshold set by the divider R5:R6, the comparator will toggle to opposite state, and the voltage level at its output 7 will become low, the transistor switches VT1 and VT6 will open, respectively, the cell discharge through the resistor R7 will stop and the red LED VD4 will go out. When the load resistor is disconnected, the voltage on the battery will increase slightly and may again exceed the voltage at the inverting input 6 of the comparator. Therefore, the transistor switch VT2, closed just by a low voltage level at the output 7 of the comparator D2:2, connects an additional resistor R13 to the divider R5: R6, which doubles the threshold voltage of the comparator, and no re-switching occurs. The D2:2 comparator thus works like a Schmitt trigger. The discharge current is set by resistor R7.
So, the full discharge of the rechargeable cell is completed, and the output 7 of the comparator D2:2 is set to a low voltage level. In this case, the yellow LED VD2 lights up, and through the voltage divider on the resistors R15 and R16 the voltage is applied to the base of the transistor VT4 and the transistor switch VT4 will close. Further, the cell charge is controlled by the comparator D2:1. The rechargeable cell voltage is applied to non-inverting input 3 of this comparator through the resistor R2, and the voltage from the adjustable divider, consisting of R1, R3 and R4 resistors, is applied to the inverting input 2. While the voltage on the cell being charged is less than the level set by this divider, the p-channel MOSFET switch VT3 is closed by a low voltage level at output 1 of the comparator D2:1. Resistor R18 then turns out to be shunted by a small (no more than 0.6 Ohm) channel resistance of this transistor, therefore the transistor VT4 works like switch that is closed completely. The cell charge current in this case flows through the circuit:
+5В -> VT3 -> VT4 -> VD1 -> R9||R10 -> R7 -> U1 -> GND
The charge current is set by resistors R9 and R10 and is equal to:
US — supply voltage;
UVT4 — saturation voltage (collector-emitter) of the transistor VT4;
UVD1 — diode VD1 voltage;
UCELL — approximate value of the average rechargeable cell voltage during charging;
R9||R10 — resistance of a circuit of two resistors R9 and R10 connected in parallel.
The supply voltage US of the charger is 5V. The voltage values UVT4 and UVD1 at a current of up to 250mA for the elements in the schematic diagram do not exceed 0,35V, and the average rechargeable cell voltage for the entire charging period UCELL can be taken equal to 1.36V. Substituting these values into the formula , we get:
The voltage on the rechargeable cell in the process of charging it gradually increases, respectively, the voltage at the non-inverting input 3 of the comparator D2:1 also increases. As soon as this voltage crosses the threshold set by the voltage divider of resistors R1, R3 and R4, output 1 of the comparator will change its state, the voltage level on it will become high and the transistor switch VT3 will open. From this point on, since the resistor R18 is no longer shunted by the channel of MOSFET VT3, the transistor VT4 begins to work as a cell charging current stabilizer. The value of this current does not depend on the resistance of resistors R7, R9 and R10 and is determined by the voltage across the resistor R15 and the resistance of the resistor R18 in accordance with the following expression:
where 0.78V in the numerator is the voltage at the base-emitter junction of the VT4 transistor. The booster charging current is always chosen several times lower than the nominal charge current of the rechargeable cell. But since when the charge current decreases, the voltage on the cell being charged also decreases, the comparator D2:1 can switch back. To prevent this from happening, positive feedback is introduced by the resistor R12, and the comparator D2:1 also works like a Schmitt trigger, that is, it has some hysteresis of the switching thresholds.
The changing of the charger to the booster charging mode is also indicates by the LEDs, now two LEDs are lit: yellow VD2 and green VD3. LED VD3 is drived by transistor switch VT5, which is closed by a high level at output 1 of comparator D2:1.
Since the adjustable divider, consisting of R1, R3 and R4 resistors, is connected to a reference voltage source made on the chip D1 and resistor R11, there are no excessive requirements for the accuracy and stability of the charger supply voltage. Resistors R14, R19 and R20 limit the current through the LEDs VD2, VD3 and VD4, respectively. Diode VD1 prevents unwanted discharge of the battery through the collector junction of the transistor VT4 when the power is turned off. And by pressing the SB1 microbutton, you can discard the pre-discharge of the rechargeable cell and its charge will begin immediately.
The nominals of resistors R7, R9 and R10 is selected based on the magnitude of the discharge and charge currents recommended for a specified type of a rechargeable cell. Let us give an example of calculating these resistances for a inexpensive rechargeable cell of the GP130AAHC type, manufactured by GP Batteries.
First, we calculate the resistor R7, which determines the cell discharge current. According to the discharge characteristic given in the technical documentation, the voltage on an almost completely discharged cell is about 1,18V. This value is common for most nickel-metal hydride rechargeable cells. Neglecting the low resistance of the channel of MOSFET VT1, we calculate the resistance of the resistor R7 for a current of 0,25A. Such current will allow the cell to be completely discharged in less than an hour. As a result, we get:
From a number of standard values of E96, we find the closest to this value and take R7 = 4.7 Ohm. The type of this resistor should be chosen according to the power dissipation:
Now that the resistance of the resistor R7 has defined, we calculate the resistance of the circuit of two resistors R9 and R10 connected in parallel based on the required value of the average cell charge current. On surface of the GP130AAHC cell case itself and in the technical documentation for it, the nominal charge current 130 mA is specified. Let’s take this value for calculation and on the basis of equality  we will make calculations:
Parallel connection of two identical resistors with a nominal resistance of 36 Ohms gives exactly this value with sufficient accuracy. The power dissipated by each of these resistors can be defined from the expression:
The type and power rating of resistors R9 and R10 should be selected based on this value of the maximum power dissipation.
Now you can proceed to the calculation of the resistor R18. To do this, we set the booster charging current equal to about two-fifths of the rated charge current, for example, 50 mA, and use the expression . With the values of resistors R15 and R16 are designated in the schematic diagram, the voltage on the resistor R15 used in the expression  will be about 1,38V. Therefore:
and the power dissipated by the resistor R18 is respectively equal to:
Let’s check the power dissipated by the transistor VT4:
where UVT4 is the voltage between the collector and emitter of transistor VT4:
substituting the obtained value into the expression , we find:
So this value of dissipated power does not exceed the maximum allowable for the type of transistor designated in the diagram.
The device can be assembled on a small printed circuit board with 16×67 mm dimention. The tracing of the board conductors on a scale of 1:1 is shown in Fig.2. Such small dimensions were achieved by using surface-mounted elements and installing them on both sides of the board. The exceptions are chips D1 and D2, button SB2, rectifier VD1 and LEDs VD2..VD4, as well as resistors R1, R7, R9 and R10. The remaining resistors, as well as ceramic capacitors C1 and C2, are of size 0805. All transistors are in a SOT-23 package.
The placement of elements on the printed circuit board is shown in Fig.3.
Setup and adjustment of the charger comes down to setting the threshold for comparator operation D2:1 with a trimming resistor R1 as follows:
With the method of charging the cell described above, the total charging time indicated on it or in its technical description should be increased by one and a half times. It should also be noted that the danger of “overcharging” the cell with the booster charging with a low current appears only with a significant increase in ambient temperature.
To power the charger, any stabilized power source with an output voltage of 5V±10% and a load capacity sufficient to provide the required charging current is suitable.
Copyright © Sergii Zadorozhnyi, 2009
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