MAX712 - Controller for fast charging of NiCd / NiMH batteries

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The purpose of the MAX712 / MAX713 specialized integrated circuit

MAX712 (and MAX713, with very small differences) is a specialized integrated circuit for fast charging of NiCd or NiMH batteries from a direct current voltage source.

1 to 16 elements can be charged (1.2V) connected in series, their number being programmed by hardware, by configuring the potentials at the PGM_ pins. Fast charging is done from C / 2 to 4C (C = battery capacity). The protected (maintenance) load with C / 16 is also available, the switching being done automatically, by the internal elements that determine the working logic of the circuit.

The detection mode of the end of charging is done by noticing the voltage slope, the battery temperature or by exceeding the required charging time. Charging is performed at a constant current. The element that provides information about the current state of the battery is the resistor RSENSE. After disconnecting from the power supply, MAX712 consumes from charged battery max. 5uA.

MAX712 - The significance of the pins on the DIP16 capsule (fig.1)

supply voltage must be at least 6V higher than the combined voltage of the batteries connected in series, subject to charging. Figure 1 shows the drawing of the DIP16 capsule, and the significance of the pines is as follows:

• Pin 1 "VLIMIT" - notices the number of inserted cells; the voltage between BATT + and BATT- terminals must not exceed "VLIMIT x number of cells";
• Pins 2 and 15, "BATT + 2 and" BATT- " connects to the charged battery;
• Pins 3 and 4, "PGMO" and "PGM1" - by connecting them to "V +", "REF", "BATT-" or left in the air, the number of inserted elements subjected to loading is determined (see table 1);

Configuring PGM1,0 pins (Table 1)

Number of cells	PGM1	PGM0
1	V+	V+
2	Open	V+
3	REF	V+
4	BATT-	V+
5	V+	Open
6	Open	Open
7	REF	Open
8	BATT-	Open
9	V+	REF
10	Open	REF
11	REF	REF
12	BATT-	REF
13	V+	BATT
14	Open	BATT-
15	REF	BATT-
16	BATT-	BATT-

• Pin 5 - "HI" - threshold for battery over-temperature; above the set value the loading ends;
• Pin 6 - "TLO" - sub-temperature threshold;
• Pin 7 - "TEMP" - detection of temperature dependence on a thermistor;
• Pin 8 - "/ FASTCHG" - load status signaling;
• Pins 9 and 10, "PGM2" and "PGM3" - by connecting to "V +", "REF", "BATT-" or left in the air, the charging time is reduced from 33 min to 264 min (table 2). "PGM3" determines the load rate in protected mode (trickle, see table 3);

Selecting the loading time (Table 2)

Charging time [min]	PGM3	PGM2
22	V+	Open
22	V+	V+
33	V+	V+
33	V+	BATT-
45	Open	Open
45	Open	REF
66	Open	V+
66	Open	BATT-
90	REF	Open
90	REF	REF
132	REF	V+
132	REF	BATT-
180	BATT-	Open
180	BATT-	REF
264	BATT-	V+
264	BATT-	BATT-

• Pin 11, "CC" - constant current loop compensation;
• Pin 13 "GND" - table. The load current is measured on the resistor connected between "BATT-" and GND;
• Pin 14 - "DRV" - current output for driver with bipolar PNP transistor;
• Pin 15 - "V +"- for current (R1), "V +" is + 5V compared to "BATT-". Power pin;
• Pin 16 - "REF" - 2V voltage reference.

Example of typical application with MAX712 - fast battery charger

How to use the typical MAX712 application

How to use it is simple and involves the following steps:

a) identifying the characteristics of the battery used for charging;
b) determining the rate and the loading time, according to tables 2 and 3. It will be taken into account that the loading efficiency is 80% and the choice of the loading time is increased accordingly. IFAST [mA] = (battery capacity in mAh) / (charging time);

PGM3	Fast charging rate	Protected charging current
V+	4C	IFAST / 64
Open	2C	IFAST / 32
REF	C	IFAST / 16
BATT-	C / 2	IFAST / 8

c) choosing the number of connected cells in series, according to table 1 (maximum 11). When summing up, keep in mind that the internal analog-to-digital converter block limits the voltage on an element between 1.4V and 1.9V, otherwise the slope is negative and charging will be stopped;
d) choose the voltage source so that it is 6V higher than the maximum total voltage of the battery elements;
e) dermatological power dissipation on the PNP transistor ca (max. supply voltage - min. battery voltage) x (charging current);
f) the current on the V + pin is limited between 5mA and 20mA. R1 = (minimum supply voltage - 5V) / 5mA;
g) is calculated RSENSE = 0.25V / IFAST;
h) consult tables 1, 2 and 3 before loading. Example: for fast charging with C / 2, select a time between 1.5x or 2x of the charging time, respectively 3 or 4 hours.

Conclusion for the operation of the electronic circuit

In conclusion, MAX712 can be in two states: fast loading or maintenance (protected, trickle).

The circuit monitors 3 parameters: the slope of the charging curve, the temperature of the battery (if thermistors are used) and the charging time.

Through the resistor RSENSE the presence of the battery and the charging current are determined. If the voltage on the battery element is below 0.4V, protected charging takes place, the fast charging follows, and when the charging is finished (situation notified by one of the three parameters listed above) it switches back to the protected charging mode. The sign of the voltage slope, at load, is determined by measuring the voltage at two successive time intervals; if the slope is negative, fast charging stops.

If you want to charge more than 11 cells or consider that the power dissipated on this transistor is much too high, and the cost for the radiator is exaggerated, you can configure the MAX712 circuit as a switching source.

List of components required to make the typical application

R1 = (minimum wall-cube voltage - 5V) / 5mA;
R2 = 150 Ohms;
R3 = 68 KOhm;
R4 = 22 KOhm;
RSENSE = 0.25V / (IFAST);
C1 = 1uF;
C2 = 0.01uF;
C3 = 10uF;
C4 = 0.01uF;
C5 (DC IN) = 10 μF;
D1 = 1N4001 (or 1N4007);
Q1 = 2N6109 (or TYPE42);
IC1 = MAX712CPE+ (16 Plastic DIP) or MAX712CSE + (6 Narrow SO);
3 x terminal contacts (for power, rechargeable batteries and consumer).

Bibliography:

Conex Club Magazine - no.5 - 2006
https://datasheets.maximintegrated.com/
https://www.qsl.net/eb4eqa/ (see here the developed application and the PCB layout)
https://www.qsl.net/ve3lny/ (see other developed versions of the application here)