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april 2009 rev 1 1/24 AN2818 application note nimh battery charger demonstration board through usb based on the st7flit15by0 introduction this application note describes a nimh battery charger demonstration board used with a usb microcontroller. basically, the battery charger demonstration board consists of two identified sections: a very low cost nimh charger based on the st7flit15by0 microcontroller. a usb microcontroller ? st72f60e2m1 ? used to control the charger. chapter 1 describes the basic theory to connect a usb to the nimh battery charger, and also introduces the demonstration board steval-isb003v1. chapter 2 describes how to use the steval-isb003v1 demonstration board (usb to li-ion demonstration board) as nimh battery charger. refer to um0497 for more information on the steval-isb003v1. www.st.com
contents AN2818 2/24 contents 1 theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 nimh battery charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.1 negative delta v method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.2 zero delta voltage method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.3 maximum temperature detection method . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.4 dt/dt detection method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 implementation of the u sb-based nimh battery charge r . . . . . . . . . . . 7 2.1 introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 flow chart of the nimh charging algorithm . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3 nimh battery detection algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3 example of usb nimh charge r implementation using the steval-isb003v1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1 introduction to the steval-isb003v1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2 customizing the steval-isb003v1 for nimh charger . . . . . . . . . . . . . . 10 3.2.1 negative temperature coefficient (ntc) resistor selection . . . . . . . . . . 11 3.2.2 man-machine interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4 working with the demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1 hand-shaking with the pc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2 connecting the nimh battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.3 monitoring the battery charging status . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.4 practical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5 using the external usb controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6 warnings/restraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 8 board schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
AN2818 contents 3/24 8.1 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8.2 controlling/changing the charging parameters . . . . . . . . . . . . . . . . . . . . . 22 9 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
list of figures AN2818 4/24 list of figures figure 1. nimh battery charging profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 2. block diagram: from usb enumeration to start of charge. . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 3. nimh charging algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 4. nimh battery detection algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 5. usb-based battery charger demonstration board (steval-isb003v1) . . . . . . . . . . . . . . 10 figure 6. enumeration result of the device manager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 7. usb li-ion demonstration board hid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 8. nimh battery and ntc connection to the 3-pin connector . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 9. schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
AN2818 theory of operation 5/24 1 theory of operation 1.1 nimh battery charging the nimh batteries follow a constant current-charging algorithm. figure 1 shows the nimh battery voltage, temperature and current profile during charging. there are three main phases: the pre-charging phase, the charging phase and the saturation mode (shown in figure 1 ). the end of the nimh battery charging is generally detected using the methods described in the following paragraphs. after the end of charging is detected, the charger switches to saturation mode during which a trickle-charging current is provided to the battery to compensate for the self-leakage. 1.1.1 negative delta v method as shown in figure 1 , the voltage of the nimh battery drops a little at the end of the charge time. as a consequence, when the voltage curve versus time becomes negative, charging is stopped and trickle-charging is started. 1.1.2 zero delta voltage method this method is a variant of the negative delta v method. with a nimh battery, there is a very slight voltage drop (5-10 mv/cell) at the end of the charge time which is very difficult to detect with a microcontroller?s 10-bit adc. there is also a risk of detecting the wrong end of charging due to noise. therefore, instead of the negative delta voltage, it is possible to use a dv = 0 condition for a certain time duration, which gives a very good approximation for detecting the end of charging. for this reason, this is the method used in the present demonstration board instead of the negative delta method or the other ones described in the next paragraphs. figure 1. nimh battery charging profile am010 3 9v1
theory of operation AN2818 6/24 1.1.3 maximum temperature detection method with this method, if the temperature rises above a threshold, charging is stopped and trickle charging is started. 1.1.4 dt/dt detection method with an nimh battery, as shown in figure 1 , there is an increase in the temperature vs. time curve (approximately 1 to 2 c/min) towards the end of charging. when a rise in the temperature is detected by the thermistor or any other temperature sensor (ntc in this case) during rapid charging, and the prescribed temperature increase is sensed, rapid charging is stopped and trickle charging is started. in the example implementation mentioned in chapter 4 , the zero delta voltage method is used as the primary technique for terminating the charge. time out, max voltage and max temperature are used as the secondary or backup methods for ending the charge. once the battery is saturated, its volt age is still monitored to preven t the battery fr om discharging completely and a saturation current is provided to compensate for the self-leakage.
AN2818 implementation of the usb-based nimh battery charger 7/24 2 implementation of the usb-based nimh battery charger 2.1 introduction the demonstration board is powered directly by a usb bus. only a 100-ma current is available by default from the usb bus. as a co nsequence, the charger is initially kept in shutdown mode. this shutdown operation is controlled by the usb controller (st72f60e2m1) of the charger (pin 13 of the usb controller configured in output mode connected to pin 14 of the charger controller in input mode via a 100-k resistor and 2str1215). once the enumeration of the usb bus has been done by the usb controller to increase the current limit to 500 ma, the charger turns on and the battery starts charging (as shown in figure 2 ). in addition, a current limiter (st890b) is provided in series with the usb supply. the demonstration board has a status led that is connected to the fault pin of this current limiter. the led switches to on when the current goes above 500 ma. a potentiometer (r43) is used on the demonstration board to reduce this current. figure 2. block diagram: from usb enumeration to start of charge !-v 0owersupply/. $efaultcurrentm! %numerationdoneby53" #ontroller #hargerisin/.stateand startscharging #urrentincreasedtom!
implementation of the usb-based nimh battery charger AN2818 8/24 2.2 flow chart of the nimh charging algorithm figure 3 shows the flow chart of the nimh charging algorithm. 2.3 nimh battery detection algorithm if the battery current is below a predefined threshold, the battery voltage is measured. if the battery voltage is less than a certain threshold (0.9 v/cell for nimh), the battery is considered connected, otherwise it is co nsidered to be disconnected from the demonstration board (see figure 4 ). figure 3. nimh charging algorithm am01042v1
AN2818 implementation of the usb-based nimh battery charger 9/24 vmax = 3.8 v and vmin = 1.8 v (for 2-cell nimh battery). figure 4. nimh battery detection algorithm am0104 3 v1
example of usb nimh charger implementation using the steval-isb003v1 AN2818 10/24 3 example of usb nimh charger implementation using the steval-isb003v1 3.1 introduction to the steval-isb003v1 this board consists of a low-speed usb controller based on the st72f60e2m1 and a battery charger based on the st7flit15by0. the board includes a power selector circuit to select the appropriate power supply source and a step-up converter circuit based on a synchronous step-up converter (l6920) device to provide a fixed o/p voltage to the usb controller. the power supply for the battery charger controller is generated from an adjustable voltage reference (tl1431) keeping in mind the accuracy requirement for charging. an additional current limiter is also located in series with the usb power supply to show any incorrect behavior through a status led. any external low-speed usb controller can be used with this demonstration board to control the charger operation. the charger used in the present demonstration bo ard uses a modified form of non-inverting buck-boost converter to support the charging voltage requirement for single-cell li-ion batteries. in-circuit programming (icp) connectors are also available with the demonstration board to program the usb controlle r and charger system if necessary. 3.2 customizing the steval-isb003v1 for nimh charger the steval-isb003v1 is a platform used to implement the usb-based li-ion battery charger application. refer to um0497 for more information on the steval-isb003v1 demonstration board. figure 5. usb-based battery charger demonstration board (steval-isb003v1) am01044v1
AN2818 example of usb nimh charger implementation using the steval-isb003v1 11/24 3.2.1 negative temperature coeffi cient (ntc) resistor selection unlike li-ion batteries which have an in-built ntc, there is no such internal ntc in nimh batteries. therefore, an external ntc (+/- 5% accuracy) is used in close proximity to the battery. 3.2.2 man-machine interface the charger (st7flit15by0) periodically checks for the presence of a battery, therefore no buttons are needed to start or stop charging. the demonstration board has two leds that show the charging status, as mentioned in section 4.3 . a reset button is also included on the demonstration board to manually reset the application if necessary.
working with the demonstration board AN2818 12/24 4 working with the demonstration board except for connecting the ntc and nimh battery, the settings are the same as for the usb for the li-ion demonstration board described in um0497. refer to section 1.2.7 of um0497 for the jumper settings. once all the jumpers are connected, the demonstration board is ready to be used. 4.1 hand-shaking with the pc as soon as you connect the usb connector to th e demonstration board, you should find the demonstration board enumerated as a hid device, as shown in figure 6 (device manager). if you check the properties of all the hid devices in the device manger, you should find the hid device depicted in figure 7 . figure 6. enumeration result of the device manager am01045v1
AN2818 working with the demonstration board 13/24 the device status ensures that the demonstrat ion board is correctly connected to the usb and ready to be used. 4.2 connecting the nimh battery the demonstration board is not provided with any specific battery slot in order to avoid making it package-specific. instead, a 3-pin connector is provided on the demonstration board as shown in figure 8 . figure 7. usb li-ion demonstration board hid !-v
working with the demonstration board AN2818 14/24 to properly connect the battery to the slot, it is good practice to start by soldering a particular slot to this connector and then co nnect the battery to this slot. you can connect the battery directly to the connector, but note that improper connection can affect the accuracy of the analog measurement, which in turn can affect the overall accuracy of the charge. the battery terminals are connected to pins 1 and 3. one end of the ntc is connected to pin 2, while the other end is connected to ground ( figure 8 ). additionally, the ntc is placed close to the battery so as to have an accurate temperature reading of the battery. 4.3 monitoring the battery charging status the demonstration board has two leds: d8 and d9. the functions of these leds are explained in ta b l e 1 and show the charging status. note: a charging error occurs when: there is a hot or cold condition there is a short-circuit condition the impedance of the battery is very low. note: when the impedance of the battery is very high, the current ceases to flow through the battery. in this case, charging is stopped and the status led shows "charging done" (case 3 in ta b l e 1 ). figure 8. nimh battery and ntc connection to the 3-pin connector !-v table 1. charging status leds case charging status red led (d8) green led (d9) 1 battery not present/idle off off 2 charging ongoing on off 3 charging done off on 4 charging error on on
AN2818 working with the demonstration board 15/24 4.4 practical results some practical tests have been performed on the charging performances of the 2-cell nimh battery. before the tests were started, the battery was completely discharged. ta bl e 2 shows the test results. table 2. charging results time for complete charging peak voltage attained (in volt) current in charging state (in ma) current at end of charging (in ma) 9hrs 40 mins 3.66 350 102 9 hrs 20 mins 3.65 352 100
using the external usb controller AN2818 16/24 5 using the external usb controller before using the external usb controller, you must remove all the jumpers (jp1, jp3, jp12, jp13, jp14 and jp15) and make all the necessary connections. follow these steps to control the charging operation using the external usb controller. 1. disable the shdn_chg pin by switching it to low to switch off the charger. 2. do the proper enumerations of the usb controller to increase the current limit up to 500 ma. 3. since the usb controller is also powered by the usb, during battery charging the power consumption of the system should be kept under 150 ma, otherwise you may not achieve the targeted 250-ma charging current. 4. after completing steps 1, 2 and 3, enable the shdn_chg pin by switching it to high to enable the charging operation. 5. connect the battery. 6. start monitoring the status of the battery charging by monitoring the status pins st1 and st2. these status pins can be used to control the activity of the external usb controller to minimize the current consum ption while the battery is being charged. 7. disable the shdn_chg pin again by swit ching it to low to reduce any charger consumption. 8. follow steps 1 to 7 again to charge another battery.
AN2818 warnings/restraints 17/24 6 warnings/restraints there is no protection for reverse battery polarity connection. however, this can be provided as per customer requirements. the charging current is limited to 300 ma using the usb supply. you can use an external supply to increase the charging current by using it in standalone mode.
references AN2818 18/24 7 references 1. an2390: a flexible universal battery charger 2. um0497: usb li-ion battery charger evaluation board.
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board schematics AN2818 20/24 8.1 bill of materials table 3. bill of materials index qty reference value/generic part number package manufact. manufacturer?s ordering code/ orderable part number 1 2 c1,c2 47 f/16 v electrolytic cylindrical any 2 1 c3 4.7 f/25 v electrolytic cylindrical any 312 c4,c5,c7,c8,c9,c10, c11,c13,c14,c15, c17,c24 100 nf 805 any 4 1 c6 1 f/16 v electrolytic cylindrical 5 1 c12 470 f/16 v electrolytic cylindrical any 6 4 c16,c18,c20,c21 33 pf 805 any 7 2 c19,c22 10 nf 805 any 8 1 c23 200 f electrolytic cylindrical any 9 2 d1,d2 1n5817 do41 st 1n5817 10 4 d3,d4,d8,d9 led axial led any 11 1 d5 tl1431aiz to-92 st tl1431aiz 12 2 d6,d7 1n5819 do41 st 1n5819 13 7 jp1,jp3, jp12, jp13, jp14, jp15, jp16 jumper sip-2 any 14 1 j1 con4 sip-4 any 15 1 j2 con8 sip-8 any 16 2 j3,j4 con12 sip-12 any 17 2 j5,j7 con3 sip-3 any 18 2 j6,j8 con10a box header any 19 4 j9,j10, j11, j12 con1 mounting holes any 20 1 l1 10 h axial inductor any 21 1 l2 33 h axial inductor any 22 1 m1 stt3pf30l sot23-6l st stt3pf30l 23 3 q1,q2,q3 2str2215 sot-23 st 2str2215 24 6 q4,q5,q6,q7,q8, q9 2str1215 sot-24 st 2str1215 25 4 r2,r8, r31,r41 100 k 805 any 26 1 r3 18 k 805 any
AN2818 board schematics 21/24 27 2 r4,r9 0 (optional) 805 any 28 1 r5 200 k 805 any 29 1 r6 16 k 805 any 30 8 r7,r10, r13,r28,r33,r36,r3 8,r42 10 k 805 any 31 6 r11,r14,r15,r27,r3 2,r34 1 k 805 any 32 1 r16 50 805 any 33 1 r17 4k4(0.1%) 805 rs components 215-3112 34 1 r18 330 805 any 35 1 r19 10 k (0.1%) 805 rs components 215-3493 36 1 r20 100 805 any 37 1 r21 1k5 805 any 38 2 r22,r25 5 k (0.5%) 805 rs components 215-3162 39 1 r23 470 805 any 40 1 r24 82 805 any 41 1 r26 0.5/1 w(1%) res axial vishay cpf1r500000fl 42 1 r29 56 805 any 43 4 r30,r35, r39,r40 4k7 805 any 44 1 r37 1m 805 any 45 1 r43 pot top notch 3296 any 46 2 sw1,sw2 reset push button any 47 1 tp1 test point single berg pin any 48 1 u1 st890bdr so-8 st st890bdr 49 1 u2 l6920d tssop8 st l6920d 50 1 u3 stm1061n25wx6f sot23-3 st stm1061n25wx6f 51 1 u4 st72f60e2m1 so24 st st72f60e2m1 52 1 u5 st7flit15by0m6 so16 st st7flit15by0m 6 53 1 y1 16 mhz crystal oscillator any 54 1 y2 12 mhz crystal oscillator any table 3. bill of materials (continued) index qty reference value/generic part number package manufact. manufacturer?s ordering code/ orderable part number
board schematics AN2818 22/24 8.2 controlling/changing the charging parameters ta bl e 4 provides methods to configure various nimh battery-charging parameters. table 4. nimh parameter configuration signal no. parameter name function formulae comments 1 nimh_vf_h to define constant voltage threshold or to fix the constant voltage level x = [(threshold / {2 * 3.6})] * 1024 example: x = 597 for 4.2 v threshold 2 nimh _vfast voltage to switch from pre-charge level x = [(threshold / {2 * 3.6})] * 1024 example: x = 469 for 3.3 v pre-charge threshold 3 nimh _vfail not used 4 nimh _vf_l not used 5 nimh _vsc used along with lion_tfail to define bad battery condition x = [(threshold / {2* 3.6})] * 1024 example: x = 213 for 1.5 v threshold 6 nimh _vsat voltage to switch from pre-charge level x = [(threshold / {2 * 3.6})] * 1024 example: x = 469 for 3.3 v pre-charge threshold 7 nimh _iconst current level during constant current charging y = current_thre / 2.5 example: y = 120 for 300 ma charging current 8 nimh _itri_1 constant current level during pre ? charging phase y = current_thre / 2.5 example: y = 20 for 50 ma pre - charging current 9 nimh _itri_2 not used 10 nimh _ifail short circuit current threshold y = current_thre / 2.5 example: y = 140 for 350 ma short circuit current 11 nimh _isat current threshold to end the charging y = current_thre / 2.5 example: y = 18 for 45 ma short circuit current 12 nimh _vheat_up heat condition indicator threshold z = {r1 / (10 k + r1)} * 256 example: z = 80 for 45 deg temperature (here r1 is in k ) 13 nimh _vheat_do wn cold condition indicator threshold z = {r1 / (10 k + r1)} * 256 example: z = 195 for 0 deg temperature (here r1 is in k ) 14 nimh _tfail used with lion_vsc 15 nimh _texp expiry condition in minutes w = (expected time expiry value in minutes) / 2 example: w = 150 for a 5 hour time expiry condition
AN2818 revision history 23/24 9 revision history table 5. document revision history date revision changes 02-apr-2009 1 initial release.
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