Low-Cost NiMH Battery Charger Using the Z8F0830 - Digi-Key
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1. I a Application Note y i PN J Low Cost NiMH Battery Charger Using the Z8F0830 AN025902 0208 Abstract This application note presents a concept for a simple low cost nickel metal hydride NiMH battery charger using the Zilog s 20 pin Z8 Encore F0830 series microcontroller The battery charger design uses the 5 5296 MHz internal clock An internal reference voltage of 2 V is applied to the on chip analog to digital converter ADC which is used to monitor battery voltage cur rent and temperature Note The source code associated with this application note is available under ZS Encore Applications Code Library in Application Sample Libraries on www zilog com Features The key features of the battery charger design include e Minimal hardware required e Constant current charging using the Z8 Encore F0830 pulse width modulation PWM e Fast charge 0 25 C 500 mA for 2000 mAh battery e Ability to charge three serially connected 1 2 V AAA batteries at up to 500 mA e I n circuit programming simplifies firmware upgrades and evaluation e Battery voltage and current monitoring using Z8 Encore F0830 10 bit ADC e Three methods for ending the charge cycle are Detecting negative dv dt Z8 amp Encore Flash Microcontrollers Setting maximum charging time between 4 hrs and 4 5 hrs charging at C 4 of 2000 mAh Over voltage termination e Bad battery detection using2. rather than the drop Consequently this termination mechanism is called zero dv dt termination NiMH batteries do not suffer from memory effect as do NiCd batteries As a result they replace NiCd bat tery types in applications such as cell phones because the increase in price is justified by the reduction in weight and absence of memory effect Page 6 of 7 Low Cost NiMH Battery Charger Using the Z8F0830 DO NOT USE IN LIFE SUPPORT AN Warning LIFE SUPPORT POLICY ZILOG S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF ZILOG CORPORATION As used herein Life support devices or systems are devices which a are intended for surgical implant into the body or b support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness Document Disclaimer 2008 by Zilog Inc All rights reserved Information in this publication concerning the devices applications or technology described is intended to suggest possible uses and may be superseded ZIL
3. OG INC DOES NOT ASSUME LIABILITY FOR OR PROVIDE A REPRESENTATION OF ACCURACY OF THE INFORMATION DEVICES OR TECHNOLOGY DESCRIBED IN THIS DOCUMENT ZILOG ALSO DOES NOT ASSUME LIABILITY FOR INTELLECTUAL PROPERTY INFRINGEMENT RELATED IN ANY MANNER TO USE OF INFORMATION DEVICES OR TECHNOLOGY DESCRIBED HEREIN OR OTHERWISE The information contained within this document has been verified according to the general principles of electrical and mechanical engineering Z8 Z8 Encore and Z8 Encore XP are registered trademarks of Zilog Inc All other product or service names are the property of their respective owners ANO025902 0208 Page 7 of 7
4. battery voltage current measurement e LED indication for battery charging and charge completion Discussion NiMH batteries are an excellent choice for powering high drain devices such as digital cameras They last longer than alkaline batteries and are less expensive The challenge with recharging NiMH batteries 1s overcharging Overcharging an NiMH battery sig nificantly reduces its operating life With traditional overnight chargers they are left un plugged These low cost chargers seldom have a charge complete indicator so that they are too easy to neglect A slow charge is technically more efficient than a fast charge The workaround is create a low cost charger design that charges batteries with a maximum 4 hrs to 4 5 hrs charging cycle The circuit is designed to stop the charging cycle when it detects negative dv dt The circuit continu ously monitors battery voltage and current The advantage of this proposed design is its simplic ity tfewer components reduce the cost of the circuit and implementation is fast and easy Hardware Architecture The hardware circuit is designed using the 20 pin Z8 Encore F0830 microcontroller The design speci Copyright 2008 by Zilog Inc All rights reserved www zilog com Low Cost NiMH Battery Charger Using the Z8F0830 fies generally available components and is simple to build and test The following Z8 Encore F0830 microcontroller features are used
5. e 2 Flowchart for NiMH Battery Charger ANO025902 0208 Page 5 of 7 Low Cost NiMH Battery Charger Using the Z8F0830 Appendix C Battery Technology The four mainstream battery chemistries NiCd NiMH SLA Li Ion have different charging and dis charging characteristics Long term battery life and performance is critically dependent on how the bat tery is charged It is therefore critical to charge the respective batteries with the mechanism they require It is also important to know when to terminate charg ing Battery overcharging causes in poor perfor mance and can damage the battery in extreme cases Different battery types behave differently in full charge condition and thus require specific charge ter mination techniques ANO025902 0208 7 l A Charging a Nickel Metal Hydride NiMH Battery NiMH batteries exhibit higher power density com pared to their NiCd counterparts The per cell voltage of the NiMH battery type is 1 2 V similar to NiCd batteries NiMH batteries must be charged via the constant cur rent charging method While charging as the voltage crosses the full charge point the voltage drop is not as low as it is in NiCd batteries Consequently nega tive dv dt charge termination is usually not recom mended for these batteries Instead of the drop in cell voltage the battery tends to plateau after a small drop This flat region is the pre ferred indication for full battery charging
6. in the design e Internal precision oscillator 5 5296 MHz used to set the system clock e Internal PWM frequency for charging set to 10 kHz The design uses a 7 5 Vpc regulated external power supply to provide 3 3 Vpp for the Z8 Encore F0830 MCU For details on hardware connections see Appendix A Schematics on page 4 Software Implementation The software design consists of the following mod ules e ADC e Timer e Feedback e Charging For information on each module see Appendix B Flowchart on page 5 ADC The Z8 Encore F0830 MCU provides eight ADC channels Three channels are used to read the battery voltage current and temperature using an internal voltage reference of 2 V A set of readings is obtained from each of the channels and stored in an array which is later used for calculating the average volt age and average current The ADC values are read with respect to timed intervals The initialization rou tines are available in adc c file ANO025902 0208 Timer The Z8 Encore F0830 MCU provides two timers Timer0 and Timer1 Timer0 is initialized to generate the PWM output at a frequency of 10 kHz Timer generates regular 50 us interrupts during which Flags are set or reset depending on the task to be completed The initialization routines are available in timer c file Feedback The feedback module provides the logic for calculat ing average voltage and current from the values cap tu
7. ramming Spec ification PRS0010 e Z8 Encore Based AA type NiMH and NiCD Battery Charger Reference Design ANO0229 e Z8 Encore Based SLA Battery Charger Application Note AN0223 e Z8 Encore Based NiMH Battery Charger Application Note AN0222 e Z8 Encore Based NiCD Battery Charger Application Note AN0221 e Z8 Encore XP Based Lithium Ion Battery Charger AN0218 e Z8 Encore Based Battery Charger AN0137 e eZ8 CPU Core User Manual UM0128 Page 3 of 7 Low Cost NiMH Battery Charger Using the Z8F0830 Appendix A Schematics Appendix A displays the schematics of Low Cost NiMH Battery Charger Figure 1 AN025902 0208 1 4 Timer WM hak Sense Timer awa Low Cost NiMH Battery Charger fa ae Document Humber ining all resistors are l 43W 1 tol nce unle J A to a af Frida E ar 2007 4 3 Figure 1 Low Cost NiMH Battery Charger Schematic Zilog Page 4 of 7 Low Cost NiMH Battery Charger Using the Z8F0830 Z fi 0 ra Po jia a i af EE Appendix B Flowchart This Appendix displays the flowchart for the Low Cost NiMH Battery Charger Figure 2 Initialize peripheral e ADC e Timer in continuous mode e Timer in PWM mode ADC sample collected Yes Calculate PI loop parameter No Yes PI loop parameter calculated Check for termination Terminate battery charging Load new PWM No Yes Stop charging End Figur
8. red by the ADC The routines are available in feedback c file Charging After calculating the average battery voltage and bat tery current the loop parameter is calculated using the proportional and integral PI loop The routine returns an error percentage with respect to its previ ous value The error percentage is used to stabilize the current flowing into the battery The Ki and Kp variables of the PI loop can be changed if required The routine is available in charger c file Once the charging current is stabilized the battery is in the charging state The charging routine is designed to prevent overcharging and overheating The charge cycle is ended when any one of the fol lowing occur e Negative dv dt is detected e Maximum charge time as set in the charging routine elapses e Maximum battery voltage is detected e A bad battery is detected The charging current is controlled by increasing or decreasing the PWM duty cycle This process 1s adapted with respect to the error percentage returned by the PI loop thus stabilizing the charging current The routines for the above tasks are available in charger c file Page 2 of 7 Low Cost NiMH Battery Charger Using the Z8F0830 Overcharging Protection Three additional charge termination checks are con sidered in our design Therefore if negative dv dt is not detected for any reason then the application ter minates charging based on over voltage or timeout to p
9. revent overcharging You can set over voltage MAX BATT VOLT cur rent set limit CURRENT MAMP maximum count of negative slope MAX SLOP COUNT and max imum charging time BATTERY TIMEOUT to the desired values in charger h file and timer h file Example Maximum timeout calculation if battery rating is 400 mAh and charging current is 200 mA Maximum Time Hour Rated battery current charging current 400 200 2 hrs Set the macro in timer h file for 2 hrs maximum BATTERY TIMEOUT 2 battery charging time Results The batteries are charged in CONSTANT CUR RENT mode up to 500 mA Charging is terminated when negative dv dt is detected maximum battery voltage is detected or the maximum charge time elapses Summary This application note describes a low cost NiMH bat tery charger implementation using the Z8 Encore F0830 MCU The MCU s internal 10 bit ADC makes accurate recharging possible and the overall design ensure that the NiMH batteries are never over charged The design also makes use of PWM to provide an accurate DC DC converter buck implementation so that the hardware cost 1s low ANO025902 0208 References The documents associated with Z8 Encore F0830 available on www zilog com are provided below e Z8 Encore F0830 Series with Extended Peripherals Product Brief PB0161 e Z8 Encore F0830 Series Product Specifica tion PS0251 Z8 Encore F083x Series Prog
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