ST APPLICATION NOTE AN INTELLIGENT ONE HOUR MULTICHARGER FOR Li-Ion NiMH NiCd BATTERIES
Contents
1. To measure this no charge battery voltage the charging current delivered by the switch mode power supply is interrupted by the microcontroller every 20 seconds as can be seen in Figure 8 The true value of the no charge Li lon cell voltage is measured by the on chip 8 bit Analog to Digital converter some seconds after interrupting the charging current to allow the battery to recover Under charging current conditions the Li lon cell voltage can be higher than 4 2 V reaching for example 4 4 V mainly because of the additional voltage drop due to its internal resist ance This additional voltage drop does not lead to overchargeof the Li lon battery Figure 8 To measure no charge battery voltage the charging current delivered by the switching power supply is interrupted by the microcontroller every 20 seconds Battery voltage 42v fr 2 7V O 30mn th 25 1A Current Battery A voltage 42V f OEE 1A Current 8 12 ky A ONE HOUR MULTICHARGER CONCEPT LI ION VOLTAGE CALIBRATION Whatever the charging method for a Li lon cell a 100 mV cell voltage difference represents in a change of more than 7 in cell capacity Since the absolute value of the Li lon cell voltage is used to stop the charging process battery manufacturers recommend charging methods with an overall voltage accuracy of 1 To acheive such an accuracy level conventional2. be catered for References 1 From NiCd To NiMH Fast Battery Charger AN417 L WUIDART J NICOLAI STMicroelectonics ky 11 12 A ONE HOUR MULTICHARGER CONCEPT The present note which is for guidance only aims at providing customers with information re garding their products in order for them to save time As a result STMicroelectronics shall not be held liable for any direct indirect or consequential damages with respect to any claims arising from the content of such a note and or the use made by customers of the information contained herein in connexion with their products Information furnished is believed to be accurate and reliable However STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics Specifications mentioned in this publication are subject to change without notice This publication supersedes and replaces all information previously supplied STMicroelectronics products are not authorized for use as critical components in life support devices or systems without the express written approval of STMicroelectronics The ST logo is a registered trademark of STMicroelectronics 1998 STMicroelectronics All Rights Reserved Purchase of C Components by STMicroelectronics conveys a
3. discrete solutions require expensive compo nents 0 2 resistors 0 5 voltage references etc This microcontroller based multicharger concept offers a cheaper solution by using a voltage reference software calibration technique as can be seen in Figure 9 The microcontroller memory allows compensation of the voltage reference The voltage reference only needs to be temperature stable For example a standard UC3843 voltage reference circuit provides a temperature stability of 0 2 mV C which is more than suf ficient Calibration and memorisation of the Li lon reference battery voltage is carried out during fac tory testing Figure 9 The microcontroller memory allows Vref calibration Vref only needs to be temperature stable UC3843 Vref has a sufficient temperature stability 0 2 mV C Calibration and memorisation of the Li lon reference battery voltage is carried out during factory testing UC3843 Vref ST6220 calibration STA 9 12 A ONE HOUR MULTICHARGER CONCEPT Figure 10 Electrical diagram of the Li lon NiCd NiMH multicharger DC DC 100 kHz SMPS 10 Watts Vin cs CNA pii sorsi l iuse software b of AILL RAT hi rrr or ia 10 12 A ONE HOUR MULTICHARGER CONCEPT CONCLUSION Li lon is an emerging battery technology because of its higher capacity for a given weight and size compared to the latest NiMH rechargeable bat
4. license under the Philips IC Patent Rights to use these components in an 12C system is granted provided that the system conforms to the I C Standard Specification as defined by Philips STMicroelectronics Group of Companies Australia Brazil Canada China France Germany Italy Japan Korea Malaysia Malta Mexico Morocco The Netherlands Singapore Spain Sweden Switzerland Taiwan Thailand United Kingdom U S A 12 12 x
5. life by avoiding excessive overheating This avoids the user having to prematurely replace costly battery packs Safe charging in one hour or less is possible using this low cost microcontroller design which combines three other back up termination methods worthy of high end dedicated control cir cuits AV detection battery temperature monitoring and timed shutdown Figure 3 Safe charging of NiCd and NiMH batteries is ensured by combining three back up termination methods AV detection battery temperature monitoring and timed shutdown battery battery cell voltage temperature Vv C 1 6 j D y 1 5 inflexion point J cK 40 p30 1 4 voltage 1 3 1 2 temperature 20 charging time 4 12 ka A ONE HOUR MULTICHARGER CONCEPT 2 EXTENDING THE CONCEPT TO Li lon The inherent programmability of such a microcontroller based charger concept allows it to be upgraded from the NiCd NiMH version to cope with the new Li lon batteries Such a multicharger concept is fully Li lon NiCd NiMH compatible charging any of these bat tery types becomes transparent to the user Li lon batteries can be charged using two different methods both of which are easily moni tored by means of an industry standard low cost 8 bit microcontroller In the conventional method charging begins under constant current conditions Once a specified battery voltage 4 2V has been reac
6. Ky APPLICATION NOTE AN INTELLIGENT ONE HOUR MULTICHARGER FOR Li lon NiMH and NiCd BATTERIES by J M Ravon and L Wuidart INTRODUCTION An increasing number of cordless and portable appliances are powered by rechargeable bat teries Products such as portable audio equipment mobile phones and cordless power tools illustrate the enormous contribution which rechargeable batteries make to our comfortable life style As equipment becomes lighter and more compact battery performance must follow suit hence NiMH batteries have overtaken NiCd batteries due to their superior capacity for a given weight and size More recently the new Li lon battery technology has been used on camcorders and will soon spread to other applications Other new battery technologies such as Zinc Air are already ap pearing on the market Rapid charging of NiCd and NiMH batteries in one hour or less is already commonplace Many battery chargers already cope with both these battery types and in future chargers will have to be compatible with emerging battery technologies This paper presents a new multicharger concept which is fully compatible with Li lon NiCd and NiMH Moving from NiCd NiMH to Li lon technology is easy when battery charging is monitored by a flexible industry standard 8 bit microcontroller AN859 0496 1 12 A ONE HOUR MULTICHARGER CONCEPT In this example of an intelligent one hour multicharger the battery is charged by a constant cur
7. hed the charger applies a regulated constant voltage to the Li lon cell C1 phase in Figure 4 Current thus decreases as charging proceeds until current flow ceases when full capacity is reached preventing overcharging C2 phase in Figure 4 Figure 4 Conventional Li lon charging method C1 phase constant current charge C2 phase decreasing current charge Battery voltage 4 2V 2 7V 0 Current 1A stop charge 0 1A ky 5 12 A ONE HOUR MULTICHARGER CONCEPT Although a 1 Ah NiCd NiMH battery pack is fully charged in one hour with a 1 Amp output charger it will take 2 5 hours to complete the charge of the corresponding Li lon 1Ah pack as can be seen in Figure 5 An alternative technique consists in using a constant current source just as in NiCd NiMH bat tery chargers This new Li lon method may be applied because the capacity level is known during the entire charging process Once this capacity level indicates full charge the monitoring microcon troller stops the constant current flow thus avoiding any overcharging Such a constant current source based charging method has an obvious benefit when one hour charging is required To reach full capacity in a 1 Ah Li lon pack in one hour a charger must deliver twice the output power using a conventional method compared to a constant cur rent source method Referring again to our 1 Ah battery example a Conventional one
8. hour multicharger must deliver 2Amps for20 minutesin ordertoreachfullcapacity ofthe Li lonbattery whilstNiIMH NiCdbatteries require only 1 Amp to be fully charged under the same conditions as can be seen in Figure 6 Figure 5 A1 Amp output multicharger will reach full capability of 1Ah pack in in 1 hour for NiCd and NiMH batteries in 2 5 hours for Li lon batteries 1 Ah NiMH or NiCd 1 Ah Li lon 2 5 time hour 6 12 ky A ONE HOUR MULTICHARGER CONCEPT Figure 6 To reach full capacity of a 1 Ah Li lon pack in one hour a multicharger must deliver twice the output power with a conventional method compared to a constant current source method TZN 1 Ah NiMH or NiCd 1 Ah Li lon s 20 40 60 time min Figure 7 New Li lon charging method using a constant current source charger battery voltage and current envelopes Battery voltage ae envelop a 0 Current envelop 14 7 12 A ONE HOUR MULTICHARGER CONCEPT This new charging method is based on measurements of the Li lon cell voltage under no charge conditions i e when charging current is stopped The value of this no charge Li lon cell voltage gives a good indication of the charge capacity level Since a 4 2V no charge voltage corresponds to a fully charged battery charging is stopped as soon as the no charge voltage reaches 4 2V as can be seen in Figure 7
9. rent source while an industry standard 8 bit microcontroller monitors both battery voltage and temperature using its on chip Analog to Digital converter as illustrated in Figure 1 Figure 1 Intelligent multicharger concept Li lon NiCd NiMH compatibility is ensured by an STMicroelectonics low cost 8 bit microcontroller Vin i charge The key to the charging methods used to ensure compatibility with different battery technolo gies is the ability to monitor the charging parameters using a low cost industry standard STMicroelectonics 8 bit microcontroller the ST6220 The inherent programmability of a micro controller based charger concept allows the existing NiCd NiMH design 1 to be upgraded for use with the new Li lon rechargeable batteries simply by modifying the software while re taining the same hardware design Safe charging is ensured by combining several termination methods guaranteeing results on a par with high end dedicated monitoring circuits These back up termination criteria can be optimised by software for any specific application requirement For example the termination criteria can be easily adapted by software to specific cell technologies or to dedicated battery pack shapes The intrinsic adaptability to new battery technologies and charging methods represents a major benefit with respect to dedicated solutions 2 12 TA A ONE HOUR MULTICHARGER CONCEPT 1 NiCd AND NiMH FAST CHARGING Safe fas
10. t charging relies on the charging cycle being monitored intelligently This has already been successfully achieved in a design based on a standard ST6220 low cost 8 bit microcon troller This charger design is capable of charging both NiCd and NiMH batteries using a charge termination method based on detecting the inflexion point in the battery voltage curve 1 However sophisticated termination methods such as inflexion point detection require battery voltage measurement with a resolution of at least 10 bits This can be achieved by using an external operational amplifier to improve the resolution of the on chip 8 bit Analog to Digital converter 1 A cheaper means of obtaining high resolution voltage measurement lies in measuring the charge time of a capacitor using the microcontroller s on chip timer A simple RC circuit using a polyester capacitor for its temperature stability connected to a triggered input port allows battery voltage variations to be converted to time measurements with an equivalent resolution of 11 bits Figure 2 RC charge time voltage measurement technique Vcap T STA 3 12 A ONE HOUR MULTICHARGER CONCEPT The effective resolution achieved using this technique has been measured as better than 11 bits allowing more reliable detection of the inflexion point on the battery voltage curve Charge termination based on inflexion point detection improves NiCd NiMH battery
11. teries For this reason the next generation of battery chargers must be fully Li lon NiCd NiMH compatible This paper describes an example of such an intelligent multicharger concept using a DC DC switch mode power supply providing a constant current to the battery pack as illustrated in Figure 10 Battery type may be determined by means of sensing contacts on the battery pack which are read by I O ports on the microcontroller The internal memory of this 8 bit standard microcontroller provides a cost effective calibration solution to acheive the high level of voltage accuracy required for Li lon charging The intrinsic programmability of such a microcontroller based charger has allowed an easy up grade path from the previous NiCd NiMH version 1 to the new Li lon rechargeable batteries without major hardware changes Other charging and back up termination methods can be easily implemented simply by modi fying the control software while retaining the same hardware design This simplifies the de sign task compared to a fully dedicated control circuit A standard microcontroller based ap proach can thus significantly reduce the overall design cycle time of battery chargers Software adaptability to new battery technologies and charging methods represents a major benefit compared to dedicated solutions The inherent flexibility of the microcontroller approach will allow rapidly evolving battery tech nologies such as Zinc Air or Zinc Silver to
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