nt } \R Application Note 37 TECHNOLOGY February 1990 Fast Charge Circuits for NiCad Batteries Jim Williams Safe, fast charging of NiCad batteries is attractive in many applications. Short charge time requires high current. A potential difficulty with high current charging is battery heating. Excessive internal heating degrades the battery and can cause gas venting to the outside atmosphere. Fast charge schemes based on monitoring cell voltage during charge suffer because cell voltage is not necessar- ily indicitive of the batterys charge state. Additionally, the batterys charge-voltage relationship may alter over life and temperature. Similarly, open loop charging techniques involving high charge rates for a fixed time do not account for battery charge state or characteristic shifts over life and temperature. One way to charge batteries without abuse is to measure cell temperature and taper the charge accordingly. This method is based on the fact that a discharged battery con- verts charging current to stored electrochemical energy, with relatively little heat produced. When the battery ar- rives at full charge the cell is saturated and cannot hold any more energy. As such, heat is produced, raising bat- tery temperature. One way to detect this point is to measure cell surface temperature referred to ambient. An absolute temperature measurement is undesirable THERMAL RESISTANCE TO DUMMY AMBIENT TEMPERATURE f THERMAL = RESISTANCE TQ BATTERY and aS AMBIENT MASS a TEMPERATURE DUMMY MASS THERMAL CAPACITANCE BATTERY a TEMPERATURE because cell temperature represents the summation of ex- cess charging energy and ambient temperature. Addi- tionally, the ambient and battery temperatures must be measured in phase. The thermal time constant of a battery pack can easily exceed one hour. If battery temperature is referred to a quickly responding ambient temperature poor charging characteristics can result. Consider the case of a portable computer retrieved from a locked automobile on a summer day. Passenger compartment temperature can exceed 120F. The computer is brought inside, where the ambient temperature sensor quickly settles to 73F. The battery pack temperature is sitting at 120F looking through a one hour thermal time constant. Under these conditions the system is fooled into believing the battery has just received a full charge, and no charge is delivered. The opposite effect occurs if the computer is in a car parked overnight in Minneapolis in January. These effects are avoidable by lagging the ambient temperature in- formation with a time constant similar to the battery packs. Figure 1 shows a simple analog. The resistors represent thermal resistance while the capacitors corre- spond to thermal capacitance. Ambient temperature ap- pears as a common mode term, while charger energy affects the battery only. Note that the ambient and battery DIFFERENCE AMPLIFIER BATTERY TEMPERATURE RISE OVER AMBIENT CHARGER CIRCUIT one ENERGY + | BATTERY 7 THERMAL CAPACITANCE Figure 1. Simplified Thermal Analog. Matched Thermal RC Terms Provide Immunity to Ambient Temperature Shifts. LI NY AN37-1Application Note 37 temperatures do not require the same individual R-C val- ues to present phased information to the difference am- plifier. Rather, their RC products must be matched. A massive battery pack with relatively low thermal re- sistance to ambient can be matched by the time constant of a well insulated (e.g., high thermal resistance) small thermal mass. Practical Thermally Based NiCad Charger Figure 2 shows a practical circuit. Thermocouples sense cell and ambient temperatures. The LT1006 amplifier fur- nishes the low level capability necessary to work with the microvolt level thermocouple signals. To understand the circuits operation, assume a discharged battery pack in the transistor collector line. The battery and ambient thermocouples are at the same temperature. The battery thermocouple is directly mounted to one of the cells in the pack. The ambient thermocouple is thermally insulated and mounted to a mass, perhaps a frame member of the equipment. Under these conditions the sensors are phase matched, their outputs cancel and A1 sees OV. The offset adjustment deliberately introduces enough input offset for A1 to swing positively, turning on the transistor, Cur- rent flows from the supply, through the battery pack and to ground via the 250.0 shunt. The low impedance shunt minimizes losses, cost, and complexity, The voltage across the shunt rises to about 625pV (the amount of off- set forced by the potentiometer), and the amplifier servo controls about 2.54 through the battery pack. As the bat- tery charges, it heats. This heat is picked up by the *See Appendix A for construction information on low resistance shunts. va eee eee eee eee eee ee ir 6V NICAD . BATTERY PACK ' + BATTERY AMBIENT > THERMOCOUPLES ARE TYPE K ~ =40,ViC 25040 TRIMPOT MAY BE ELIMINATED SEE TEXT CONNECT ALL GROUNDED POINTS DIRECTLY TO SUPPLY SEE TEXT - Figure 2. Thermally Controlled NiCad Battery Charger AN37-2 (=1.8" OF #12 WIRE) -) battery-mounted thermocouple. The temperature differ- ence between the two thermocouples determines the volt- age which appears at the amplifiers positive input. As battery temperature rises, this small negative voltage (1C difference between the thermocouples equals 40,V) be- comes larger. The amplifier gradually reduces the current through the battery to maintain its inputs at balance. The effect of this action is shown in Figure 3. The battery charges at a high rate until heating occurs and the circuit then tapers the charge. The values given in the circuit limit the battery surface temperature rise over ambient to about 15C. Figure 4s circuit is arranged for use with batteries which are committed to ground. The common emitter output ne- cessitates exchanging amplifier input assignments, but circuit operation is identical to Figure 2. In both circuits the trimpot may be eliminated by specifying an LT1006 set at manufacture to the desired offset value. 26 24 22 2.0 18 16 14 1.2 4.0 08 06 04 0.2 BATTERY CURRENT IN AMPERES. 0 0 0 40 @ 80 TIMEIN MINUTES 100120140 Figure 3. Figure 2s Charge Characteristics +V BATTERY AMBIENT =e PACK ~ TRIMPOT MAY BE ELIMINATED SEE TEXT 26040 CONNECT ALL GROUNDED POINTS (= 18" OF #12 WIRE) DIRECTLY TO SUPPLY SEE TEXT THERMOCOUPLES ARE TYPE K = 40,/C = Figure 4. Figure 2s Circuit Arranged for a Grounded Battery LT WG -The small shunt sense voltage requires a high quality ground for accurate results. This ensures that the large current flow through the transistor does not combine with ground return impedances to create errors. In practice, all returns should be brought directly back to the supply com- mon terminal. Similarly, parasitic thermocouple effects should be avoided (see LTC Application Note 9 for a dis- cussion on minimizing parasitic thermocouple effects). Both circuits force the transistor to dissipate some power, particularly in the middle of the charge curve. The heat produced may be a problem in a very small enclosure. Figure 5s circuit eliminates this problem. This design is similar to the others, except that the A2 duty cycle modulator configuration is interposed between A1 and the output transistor. The transistor, in this case a power FET, operates in switched mode, delivering duty cycle modu- : AC LINE S | i Application Note 37 lated current pulses to the battery pack. R7-C4 filters the switching waveform to DC. R6 and R7 present a balanced source impedance to A1. C2 sets gain roll-off. This design relies on the source impedance of the wall transformer to limit the current through Q1 and the battery pack. This parameter may be set when specifying the transformer. Figure 6 should be used in cases where the charging source has low impedance. Here, the circuits output is recontigured as a simple step down switching regulator (basic operation of step down switching regulators is de- scribed in LTC Application Note 35). The 74C04s provide phase inversion and drive for Q1, a P-channel MOSFET. Figure 7 shows waveforms. Trace A is A2s output with trace B showing Q1s gate drive. Trace C is Q1s drain volt- age and trace D its current. Trace E is the MR850 catch diode current. Trace F is L1s current. L1 smooths current flow, resulting in low loss operation. Zsource SEE TEXT ren Te ry IRF540 TRIMPOT MAY BE ELIMINATED SEE TEXT CONNECT ALL GROUNDED POINTS DIRECTLY TO SUPPLY SEE TEXT THERMOCOUPLES ARE TYPE K = 404V/C ee tottenham RA RR A pee et Wen Figure 5, Switched Mode Thermal NiCad Charger AN37-3Application Note 37 = ' SEE TEXT ! A=10V/DV 7 = ' A AY ! B= 10V/DIV RA RS 1 45a 10k 51k 1 C= i0VIDIV i] t 1 tL. D=4ADIV a6 a7 E=AAIDIV C2 F=0.5A/DIV 0 wt tok ON2.5A LEVEL TRIMPOT MAY BE ELIMINATED SEE TEXT [+ cA HORIZ = Sps/OV ~+ CONNECT ALL GROUNDED POINTS DAE DIRECTLY TO SUPPLY SEE TEXT - I 22 ~+ CONNECT ALL SIX DEVICES IN THE = 74CO4 PACKAGE IN PARALLEL L1 = PULSE ENGINEERING #PE-92105 + THERMOCOUPLES ARE TYPE K = 40xViC Figure 6. Switched Mode Thermal NiCad Charger Figure 7. Figure 6s Switching Waveforms Note This application note was derived from a manuscript originally prepared for publication in EDN magazine. Acknowledgement The contributions of William Cho towards the completion of this Application Note are gratefully acknowledged. APPENDIX A Construction of Low Resistance Shunts A simple, inexpensive way to construct low resistance _ gives resistance vs length characteristics for various wire shunts is to use a small length of wire or PC trace. The _ sizes. The shunt should have separate connections for type and length of wire determine shunt resistance, which sensing (Kelvin style) so that the high current does not cor- will vary with desired charging characteristics. Figure A1 _ rupt readings. Figure A2 shows a typical! configuration. WIRE GAUGE pONNCH 4 . LENGTH DETERMINES ETE! " 100 MAJOR SHUNT VALUE MAJOR 2 130 CURRENT CURRENT > 13 160 FLOW FLOW 14 210 LOW RESISTANCE WIRE 15 265 "6 335 SENSE SENSE 17 421 POINT POINT 18 530 WIRE WIRE 19 670 20 890 SENSE SENSE POINT POINT 1300 PRINTED | TRACE TRACE CIRCUIT & 3) 23 1700 VERSION mp ee 24 2100 MAJOR CURRENT FLOW 25 2700 Figure A1. Resistance ys Size for Various Copper Wire Types Figure A2. Detail of a Low Resistance Current Shunt AN37-4 LY Wee