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Build this multi-purpose Fast Battery Charger For tools, camcorders, R/C equipment & car batteries This truly versatile Multi-Purpose Fast Battery Charger will charge your NiCd and NiMH power tool batteries in less than 15 minutes for a 1.2Ah pack. It includes full battery protection & employs well proven end of charge detection methods to ensure that the cells are not damaged. You can also charge 6V & 12V sealed lead acid (SLA) packs and lead acid car and motorcycle batteries. By JOHN CLARKE So you got a new battery power tool for Christmas? Great, isn’t it? You can use it anywhere, any time and there’s no power cord to get in your way. Not so great is when the battery runs down. Unless the tool is a high-priced model with a fast charger, it can take three hours or more to charge the battery. Three hours is a long time when you want to get on with the job. So fast charging for power tools is the main reason for this new design. But in our never-ending quest for getting more and more performance out 18  Silicon Chip of less and less circuitry, we were not going to be content with a design that just did Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) batteries. We wanted to use the basic charger components to cater for Sealed Lead Acid (SLA) and ordinary Lead-Acid batteries as in cars and motor bikes. Could we do it? As luck would have it (“There is a tide in the affairs of men which taken . . .”), Philips have recently introduced a new battery management chip which takes care of NiCd, NiMH, SLA and Lithium-Ion batteries. So that would take care of most of what we wanted. Could we make it do ordinary Lead-Acid batteries as well? We could, and did, and you see the result here. Features of the new charger It is crucial when fast charging batteries that they are not overcharged. If NiCd and NiMH types are given too much charge, they will overheat and be permanently damaged. Nor should SLA and Lead-Acid types be charged beyond a certain voltage or they too will be damaged and their life reduced. The same applies if they are consistently undercharged. NiCd batteries should also be discharged before recharging. If they are recharged before being discharged they will exhibit the dreaded “mem­ ory” effect whereby they will not provide their full discharge capacity. And nor should NiMH batteries be contin­ uously trickle charged since they form dendrites which will eventually short out the cell. That’s a lot of “shoulds” and “should nots” to be catered for but our new charger design takes care of all these points and a lot more. The new SILICON CHIP Multi-Purpose Fast Battery Charger provides accurate detection of full charge for NiCd and NiMH batteries and precise end point voltage regulation for SLA and Lead-Acid types. It also has various protection features to prevent fast charge when the battery temperature is too high or low for NiCd and NiMH types and if the battery voltage is ini­ tially low for all battery types. An added feature is the Refresh cycle for NiCd batteries. This discharges the battery so that each cell reaches a nominal 1V before the charger begins to fast charge. Fast charging stops when the cell voltage begins to drop off from a maximum value. There is provision for temperature monitoring as well. Some battery packs have inbuilt thermistors and the charger uses this to detect when the cell temperature begins to rise at a rapid rate. When fast charging ceases, NiCd & NiMH batteries are topped up at 200mA for about 90 minutes and then trickle charged at 62mA to maintain their capacity before use. This trickle charge com­prises short bursts of current which averages to 62mA. These bursts of current prevents dendritic growth within NiMH and NiCd cells. SLA and Lead-Acid batteries are initially fast charged, tapering off to zero as the battery voltage approaches 2.4V per cell. This corresponds to 14.4V for a 12V battery. Charging automatically starts again when the cell voltage drops to 2.2V or 13.2V for a 12V battery. Timer & LED indicators The charger incorporates a timer which stops fast charging after a set period. This prevents overcharging should the end of charge detection methods fail. Normally the timeout is about 1.6 times the expected charge time of the battery, as determined by the capacity and charge current. When charging Lead-Acid batter­ies, the timer is reset at regular intervals to disable this function. This is because large Lead-Acid batteries require a much longer time to charge than the timer can accommodate. The Multi-Purpose Charger is hous­ ed in a plastic instrument case with a front panel which looks fairly complicated. However, it only has two knobs and a couple of switches and these Specifications • • • • • • • • • • • • • • • • • • Fast Charge Current ............................................................nominally 6A Topoff current (NiCd & NiMH) ....................................................... 200mA Trickle current (NiCd & NiMH) ......................................................... 62mA Refresh current (NiCd) ......................................................................... 2A Refresh discharge end point.................................................... 1V per cell Battery low detect (NiCd & NiMH)........................................ 0.3V per cell Battery low detect (SLA & Lead-Acid)................................ 0.45V per cell Battery high detect (NiCd & NiMH).......................................... 2V per cell Battery high detect (SLA & Lead-Acid)............................... 2.97V per cell Charge end point (SLA & Lead-Acid)................................... 2.4V per cell Recharge after end point (SLA & Lead-Acid)....................... 2.2V per cell Voltage peak detection (NiCd & NiMH)................0.25% drop in top value Temperature rate detection level (NiCd & NiMH)............................ 0.25% Under-temperature cutout (NiCd & NiMH)........................................ 12°C Over-temperature cutout (NiCd & NiMH).......................................... 50°C Charger over-temperature cutout...................................................... 80°C Fast charge timeout..................................15, 30 or 60 minutes (nominal) Top-off charge time (NiCd & NiMH)...............................about 90 minutes Features • • • • • • • • • • • • • • Fast charges NiCd, NiMH, SLA and Lead-Acid (car) batter­ies Suitable for 6, 7.2, 9.6, 12 & 14.4V NiCd & NiMH batteries from 1.2Ah to 4Ah. Suitable for 6V or 12V SLA batteries from 1.2Ah to 4Ah Suitable for 6V or 12V Lead-Acid (vehicle) batteries of more than 1.2Ah Includes a discharger for NiCd batteries Top-off charging at end of fast charge plus pulsed trickle for NiCd & NiMH batteries Voltage limited charge for SLA & Lead-Acid batteries Voltage drop (dV/dt) & temperature rise (dT/dt) full charge detection for NiCd & NiMH Under and over-temperature cutout for battery Over-temperature cutout for charger Short circuit battery protection Timeout protection Fuse protection Multi-LED charge indicators are used to select the type of battery to be charged, the battery voltage and charge time. It might look complicated but it is quite simple to operate. Six LEDs are provided on the front panel to indicate the status of the charger. The first of these is the REFRESH LED which indicates when a NiCd battery is being discharged. The discharge cycle is activated by the Refresh pushbutton immediate­ly above the LED. The FAST LED shows that the charger is delivering maximum current, 6A, to the battery. When the charger deems the battery to be charg­ed, it shows the 100% LED. While this LED is alight, the charger is in “Top off” mode; ie, 200mA charge. At the end of the “Top Off” mode, the charger goes into trickle mode and all LEDs are off. The PROTECT LED shows when the battery is shorted or has low voltage February 1998  19 Fig.1: this schematic diagram shows the various functions of the Philips TEA1102 battery management IC. after a certain period of charge. It will also light with over or under temperature, if the thermistor is connected. The NO BATTERY LED only lights when NiCd & NiMH battery types are selected and only if the thermistor is not connected to the charger. It simply indicates that the battery is not connect­ed or has a high impedance. Battery management IC As noted above, all of the charging features described so far are provided by virtue of a battery management IC made by Philips Components. It is designated the TEA1102. Its block diagram is shown in Fig.1. We downloaded this diagram and the data sheet from the Philips web site at WWW. SEMICONDUCTORS.PHIL­IPS.COM The operation of the TEA1102 is rather complex and compris­es analog and digital circuitry which can be divided into six separate sub sections as shown on the block diagram. Starting at the top righthand corner of Fig.1, the charge control and output driver section comprises a current source, battery type selection, oscillator, comparators, amplifiers and a pulse width modulation (PWM) and analog control output. 20  Silicon Chip Battery voltage is monitored at the Vbat input (pin 19, top of diagram) and this is compared against Vreg which sets the endpoint voltage for charging the selected battery type. Options are NiCd (Nickel Cadmium) & NiMH (Nickel Metal Hydride), Lithium-Ion and SLA (Sealed Lead Acid). Note that we have not used the Lithium-Ion facility as these batteries are comparatively rare in consumer equipment, apart from computer backup batteries. There is a different Vreg selection for each type of bat­tery but these do not necessarily correspond to the “end-point” voltage for each cell type. The comparator monitoring Vbat and Vreg controls the con­stant current source transistor which is supplied with one of four currents; fast charge, top off, standby and load. At switch on, the TEA1102 is reset and fast charge mode is selected. This fast charge is set by a resistor at Rref (pin 20) to select the current flow to the IB output (pin 2). The current from the IB output pin flows through an exter­nal resistor to develop a voltage which is monitored by the internal op amps A1 and A4. A1’s output is amplified by A3 to give an analog control output (pin 18) and is compared in A2 against a triangle waveform set by the oscillator at pin 14. A2’s output is a pulse width modulated (PWM) signal which is used to control the charge current. PWM operation The oscilloscope waveform of Fig.2 gives us an idea of how this works. The lower trace triangle waveform is the oscillator output and the horizontal cursor line represents the DC output of A1 (pin 17). The upper trace is the PWM output to drive a switching transistor. This PWM output goes high when the oscillator waveform goes below the A1 output. If the current decreases, the A1 output will rise and produce a wider PWM signal to increase the current. The Vbat input at pin 19 also connects to the battery low, end refresh and no battery comparators in the Protection block. These are to prevent fast charge when the battery is low, cease the refresh at 1V per cell and prevent a high output voltage with no battery connection. The Vbat signal also is applied to the Analog to Digital converter and Digital to Analog converter, shown as the DA/AD converter on the Fig.2: these waveforms show the switchmode operation of the charger. The lower trace triangle waveform is the oscillator output and the horizontal cursor line represents the DC output of A1 (pin 17). The upper trace is the PWM output to drive a switch­ing transistor. This PWM output goes high when the oscillator waveform goes below the A1 output. If the current decreases, the A1 output will rise and produce a wider PWM signal to increase the current. block diagram of the IC. The DA/AD converter monitors battery voltage when charging NiCd & NiMH batteries. As the battery is charging the voltage gradually increases and at a regular interval, the A/D converter samples the voltage and stores it as a digital value if the voltage has increased from the previous reading. When the voltage begins to fall the lower voltage is not stored but compared with the analog voltage resulting from the digital stored value. A fall of 0.25% indicates that the battery is charged and the charger will switch to trickle mode. The DA/AD converter also monitors the thermistor voltage via the NTC input at pin 8. If the thermistor is connected the DA/AD converter switches off fast charge when there is a sudden rise in temperature of the battery which also indicates full charge. Note that fast charge will be switched off if there is a low or high temperature detected by the Tmin and Tmax compara­tors. The “NTC present” comparator detects the connection of the thermistor. The Tcut-off comparator is the detector for the change in battery temperature which switches on for a 0.25% rate of rise in temperature. The MTV input (pin 9) can be used to cali­brate the thermistor temperature at Tmax although we have not used this feature. The Control Logic section monitors and sets the operation of the various blocks within the IC. Voltage on the FCT input (pin 11) selects the type of battery to be charged. The Supply Block takes its supply at the Vp input (pin 12) and produces a reference voltage at the Vs output (pin 16). This reference provides an accurate and stable source for the battery end point voltages. The Vsl output (pin 13) is used to switch on power to the indicating LEDs. This is necessary since the LEDs are driven by dual purpose outputs which also provide programming for the timers. These pins are initially monitored at power on to check what Fig.3: transformer T1 and bridge rectifier BR1 provide an unfil­tered 18V DC supply for the main charger circuit. This is fed through directly (ie, essentially unfiltered) to the switchmode step-down converter comprising transistor Q1, inductor L1 and diodes D1 and D2. In effect, the battery is charged with chopped and unfiltered DC. February 1998  21 22  Silicon Chip Fig.4: As you can see, there is quite a lot of switch circuitry hanging off the TEA1102, emphasising the fact that it does most of the work. IC2 & IC3 provide a timer reset function so that Lead-Acid batteries can be charged. Fig.5: this is the current waveform across the sensing resistor Rx. Its value is 0.05Ω and the RMS voltage reading is 294mV or 5.88A. The mean value (and the reading obtained on a multimeter set on DCV) shows only 212mV or 4.24A. options are set, before the LEDs are powered. Block diagram Fig.3 shows how we have used the TEA1102 battery management IC in our circuit. Transformer T1 and bridge rectifier BR1 pro­vide an unfiltered 18V DC supply for the main charger circuit. This is lightly filtered to provide DC for the control circuitry but is fed through directly (ie, essentially un­filtered) to the switchmode step-down converter comprising transistor Q1, inductor L1 and diodes D1 and D2. In effect, the battery is charged with chopped and unfil­ tered DC. This allows a considerable saving on electrolytic filter capacitors as well as reducing power losses in the main series pass transistor, Q1. Circuit description Fig.4 shows the full circuit for the Multipurpose Fast Battery Charger. It comprises three ICs including the TEA1102, two power transistors and diodes and not a great deal else. As you can see, there is quite a lot of switching circuitry hanging off the TEA1102, which emphasises the fact that it does most of the work. Power for the circuit comes from an 18V 6A transformer which feeds a bridge rectifier and two 10µF poly­ ester capacitors. These capacitors supply the peak switching current to the switchmode supply comprising transistor Q1, diode D1 and inductor L1. The Pulse Width Modulation output at pin 15 of IC1 drives transistor Q3 which operates as a pulsed “current sink” pulling current out of the base of Q1. The 68Ω resistor in the emitter of Q3 sets the current pulses to about 34mA and these ensure that Q1 is turned on hard. The collector current from Q1 flows through inductor L1 and diode D2 into the battery load. Each time Q1 switches off, the fast recovery diode D1 provides a current path so that the energy stored in the inductor can be fed into the battery. Diode D2 prevents the battery from feeding current back into the switchmode circuit when the charger reaches the end of its cycle. The 100µF capacitor connected across the battery is there to filter the supply when no battery is connected so that the “no battery” detection will operate within IC1. The charge current is detected in the 0.05Ω resistance comprising two 0.1Ω resistors connected in parallel to the emit­ter of Q2. This “ground” point is tied to pin 2 of IC1 via a 3.3kΩ resistor and this allows IC1 to monitor the current. Operation is as follows: The Vref output at pin 20 which has a 1.25V supply sets the current flow out of the IB pin so that it is equal to 1.25V/27kΩ = 46µA. This current produces a voltage across the 3.3kΩ resistor and this is used to set the maximum current from the charger. Fig.5 shows the current waveform across the sensing resis­ tor Rx. Its value is 0.05Ω and the RMS voltage reading is 294mV or 5.88A. The mean value (and the reading obtained on a multi­met­er set on DCV) shows only 212mV or 4.24A. The 27kΩ resistor at pin 20 also sets the oscillator fre­quency in conjunction with the 820pF capacitor at pin 14. Fre­quency of oscillation is about 50kHz which sets the PWM switching speed and the timeout periods. The Timeout period is adjusted by the switch setting at pin 7. When pin 7 is pulled low via the 33kΩ resistor at switch S2, the timeout is about 15 minutes. An open setting of S2 increases the timeout by a factor of two and when S2 pulls pin 7 high, the timeout is increased again by a factor two. These last two set­tings give the 30-minute and 60-minute settings respectively. Battery selection Detection of battery type is done with the FCT (Fast Charge Termination) input, pin 11. When pin 1 is grounded via switches S3a and/or S4a, the SLA battery charge procedure is used by IC1. S4a ensures that pin 11 is at ground regardless of the position of S3a when S4 is in position 2 when 6V or 12V Lead-Acid batter­ ies are being charged. This prevents Lead-Acid batteries being charged as NiCd or NiMH types which would lead to overcharging. The NiCd and NiMH charge cycle is selected when pin 11 is connected via S3a to the 4.25V reference at pin 16. The Vstb (pin 1) input selects trickle charging after the NiCd or NiMH batter­ies are charged rather than the voltage regulation option when pin 1 is open circuit. Pin 19, the Vbat input monitors battery voltage via a switched voltage divider connected via a 10kΩ resistor and 0.47µF capacitor filter. The divider for NiCd & NiMH batteries is via S5a, catering for 6V, 7.2V, 9.6V, 12V and 14.4V packs. The divid­er for SLA and Lead-Acid batteries is via S3b and S5b, catering for 6V and 12V. Pin 8, the NTC input, detects the February 1998  23 The new multi-purpose charger will cater for NiCd, NiMH, SLA and Lead-Acid (car) batteries. Intended mainly as a fast charger for power tools and R/C gear, it does double duty with car and SLA batteries. presence of a thermistor in the battery pack. The 100kΩ resistor pulls pin 8 up to +4.25V when the thermistor is disconnected and to about +2V when it is connected, at normal room temperature. As the thermistor heats up, the rise in temperature on the battery should correspond to a voltage reduction; ie, dV/dt detection. If this is not detected before the thermistor voltage reaches 1V, the fast charge will cease because of over temperature. LED indication is provided on the LED, POD, PTD and PSD pins and controlled via the Vsl output. At power up, all LEDs are off and the IC looks at the POD, PTD and PSD pins to check the division ratio programming set on these pins. After this, the LEDs can be lit when Vsl goes high to turn on transistor Q4 which feeds them via the 680Ω resistor. If LEDs 1-4 are off then the “No battery” indicator, LED5, can light. However, if any of the other LEDs are alight, LED5 will extinguish. This is because LED5 requires more 24  Silicon Chip voltage than the other LEDs due to the series diode, D4. Refresh cycle Transistor Q2 turns on to discharge NiCd batteries when pin 10 of IC1 is momentarily shorted to ground via pushbutton S6. Note that the switchmode output at pin 15 is low while Q2 is turned on. Current flow through Q2 and the battery is also via the 0.05Ω resistor and is detected at the IB input at pin 2. This discharge current is regulated to 2A. Power for IC1 comes from the positive side of the bridge rectifier which charges a 1000µF capacitor via diode D3. The diode reduces the ripple on the capacitor and also prevents the charging current for the battery being drawn from this capacitor. A 470Ω resistor supplies current to pin 12 of IC1 which has an internal 12V zener diode regulator. A 10µF capacitor decouples the supply. A 1kΩ resistor supplies current to a separate 12V zener diode, ZD1, to power IC2 and IC3. These two ICs form the reset timer. The AC side of bridge rectifier BR1 supplies an 11V zener diode, ZD2, via a 2.2kΩ resistor. The zener diode limits the resulting 50Hz signal to +11V and -0.7V and this is fed via an RC filter to Schmitt trigger IC2a which squares up the waveform. This signal is then applied to the clock input of IC3, a 14-stage binary counter. The resulting output at pin 3 goes high once every 5.5 minutes. The high output is fed to inverter IC2b via the 3.3µF capacitor and then to inverter IC2c. IC2c then drives transistor Q5 which switches the supply of IC1 to ground via a 10Ω resistor. This action resets the internal timer of IC1. This cycle repeats while ever S4d is in position 2 which corresponds to charging for Lead-Acid batteries. Hence, the only reason why IC2 & IC3 and the associated circuit have been included is to allow lead-acid batteries to be charged. Next month, we will present the full con­struction details for the MulSC ti-Purpose Fast Charger.