Extremely simple, yet effective, NiMH/NiCd intelligent battery chargerAuthor: Glauco Masotti

Note: This document is only a draft and is presented “as is”! Nevertheless it’s quite accurate and you may find useful information in it. It should have served for applying for a patent in 2006, which then I never applied for.You can find further details, and a discussion of this subject in a wider context, here: http://www.scribd.com/doc/99630197/About-Some-Good-but-Neglected-Ideas

Abstract:

A method for detecting the end of charge condition by sensing the battery voltage, employing only analog signal processing, and resulting in an extremely simple solution, is presented. Nevertheless the proposed circuit is suitable for controlling the end of rapid charge, either for NiMH or NiCd batteries, implementing a termination criteria very close to the optimum indicated by most battery manufactures. The proposed solution, for its simplicity, could be very cheap, as it can be realized with common analog components, or be integrated in a chip.

Claims:

What is claimed is:

1. A method for detecting the end of charge condition, by sensing and analyzing the voltage battery, during rapid charge of Nickel based battery, or any other kind of battery which voltage vs. time characteristics shows a peak or a plateau in proximity of full charge, i.e. full charge of battery can be recognized in correspondence of the voltage top of the characteristic or slightly after or slightly before. This method is implemented by means of simple analog circuits.

2. The method of claim 1 is basically implemented employing a resistor, a capacitor and a comparator in a specified arrangement, such that the voltage across the capacitor follows with a certain delay the battery voltage. The comparator senses when the difference between the battery voltage and the capacitor voltage falls within a few millivolts to stop rapid charge (larger differences are also allowed, but are not advisable). The exact trip point depends on comparator offset and can be when sensing a slightly positive, practically zero or slightly negative difference, accommodating for various end of charge conditions.

3. Further improvement of the basic implementation, to cope with a wide range of battery voltages, is accomplished by adding a diode and a resistor in parallel with the resistor named at claim 2.

4. Further improvement of the implementation of claim 3, is accomplished by feeding the capacitor voltage to one of the comparator inputs through a voltage follower, in order to sense the capacitor voltage with a high input impedance and decouple the capacitor from the comparator circuits.

5. Further improvement of the implementation of claim 4 is done by including circuits for hysteresis of the trip point, in order to establish a stable condition of the comparator at end of charge, and circuits for offset adjustement of the comparator, in order to accurately set the trip point and thus the end of charge condition.

6. Further improvement of the implementation of claim 5 is done by including circuits for noise filtering of the voltages feed to the comparator.

Description:

BACKGROUND OF THE INVENTION 1. Field of the Invention

Rechargeable batteries are now used in many appliances, the wide diffusion of cellular phones, portable computers, digital cameras, tools and games have contributed a lot to their diffusion. Moreover they are now replacing non-rechargeable batteries in almost any application which require a significant amount of power on a regular basis. Although Li-Ion cells are gaining place, NiCd and NiMH batteries are still the most widely used, in particular NiMH appear at present, in most cases, the best compromise among cost, energy density, number of charge/discharge cycles, and low self-discharge properties. NiMH cells have a higher energy density, lower environmental impact and also exhibit a weaker memory-effect with respect to NiCd, however the latter can sustain a higher number of cycles and given also the lower initial cost, NiCd are still

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the most cost effective solution in many cases.Unlike lead-acid batteries simple constant-voltage and trickle charging cannot be employed for Ni based batteries, as this would cause overcharging, with degradation of battery properties and thus reduction of battery life. Constant current chargers are thus used, the charging process should then be properly stopped when the battery has reached its full capacity, to avoid overcharging damage. When high current rates are used for fast charging , usually ranging from C/3 to 2C (given C is the nominal capacity of the battery), these damages can be greater and made quicker than with using smaller charging currents, like C/10. The end of charge condition should thus be detected with accuracy. Dumb chargers usually employ a C/10 current rate, and place on the user the responsibility of interrupting the charging process after a certain time, which should last nearly 15-16 hours, if the battery is initially almost exhausted, but still in good shape to maintain its nominal capacity. Smart chargers used for fast charging employ a number of techniques for detecting when the battery is full charged. The invention is concerned with a particular system for detecting the end of charge condition for Nicked based cells as part of an intelligent battery charger. The proposed solution is simple and cheap, yet even more accurate and effective than currently employed methods.

2. Description of the Related Art

Currently intelligent chargers are based on analyzing the evolution of battery voltage (VB) and/or battery temperature (TB) over time during the charging process. Fig. 1 shows their typical behavior for NiCd and NiMH batteries.

To sense the battery voltage a charge controller typically employs an A/D converter to sample VB at certain intervals and then applies digital signal processing. If a temperature sensor is available, in touch or integrated with the battery, TB can be analyzed instead of VB, or both can be taken into account. Charge controllers are thus currently relatively complex devices, which can be realized with general purpose microprocessor or with dedicated LSI chips.

By examining a typical charge characteristic of Nickel based cells the various charge termination criteria devised insofar can be understood. The most popular method relies on the detection of a negative voltage variation after the voltage peak

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VP has been reached. This is called the -∆V method. NiCd cells show a pronounced top, thus negative variation in excess of 40 mv per cell can be registered at a continuos charge rate of 0.5C or more (Fig. 2).

However charger which are less sensitive and that stop fast charge only after a negative variation of about 20 mV per cell or more, like most old chargers, tend to overcharge batteries excessively, thus reducing battery performance and life. A certain amount of overcharge is encouraged or at least allowed by the recommendations of most manufacturers [HAR1, PAN1, GP1], in fact a new battery can be exploited at 110% of its nominal capacity for a considerable number of cycles [GP1]. Manufacturers recommendations are not univocal, also not all batteries are the same, and the optimal trip point to stop fast charging depends also on the rate of charge, however they tend to specify smaller limits for -∆V than in the past, when a value of 45 mV drop was considered typical [NAT1]. Usually they recommend a value between 10 to 20 mV [HAR1, PAN1], but someone recommends no more than 10 mV drop per cell [GP1]. This means that charge should terminate immediately after the voltage peak VP has been reached or even in correspondence of VP in certain conditions (see Fig. 2). The requested resolution for detecting differences below 10 mV by digital processing would be of at least 8 bits, but preferably more for optimum performance. Care should also be placed in keeping noise to a negligible influence.

NiMH cells pose even higher requirements, in fact the voltage peak is not as marked as for NiCd cells and battery voltage tend to plateau for some minutes at the end of charge, before declining a few millivolts. Some manufacture [GP1] thus recommend to stop charge after recognizing a negative variations of 2 mV per cell! This should represent the optimum compromise for achieving full charge with a reasonable termination criteria, without compromising cell life duration. A cell charged at –2 mV drop can typically sustain about 700 cycles and still retain 80% of nominal capacity, while one charged at –30 mV only 400 [GP1].

Voltage sensing has been so popular so far because it is easy, the voltage leads are accessible and no special assembly is required in the battery pack. Nevertheless the cell temperature gives the most accurate information about what is happening within the cell and fast charging without temperature sensing is advisable only at ambient temperature. Temperature sensing would be thus preferable to voltage sensing for these reasons, however, if the cell temperature is to be accurately measured,

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the temperature sensor must be built into the battery pack, which increases the manufactured cost of the battery. If the sensor is only in contact would not be the same, moreover if it is in contact only with one cell of battery pack it would not give accurate information about every cell status. A glance at Fig. 1 shows that as full charge is reached, the rate of temperature rise increases very sharply. Two termination criteria can be used: one measure the rise of temperature battery TB with respect to ambient temperature, the other calculates the rate of temperature change (i.e. the derivative of TB(t)). In either case if an opportune threshold is exceeded charge is terminated. Charge should be stopped when a rise of about 10° above ambient temperature is sensed, but this does not give an accurate criteria. Manufacturers suggest to evaluate the rate of change in temperature. For a charge rate of 1C, charge should be stopped at approximately 1°C/min. rate of change in battery temperature. However considerable differences in manufacturers specifications arise. Temperature behavior is also greatly affected by charging rates and battery type. Settings of charge parameters is thus critical, a wrong setting may result in premature or delayed termination of charge, thus this approach can be used effectively only in well specified conditions, with a particular type of battery. In a general case it is often adopted as a backup termination criteria: with opportune parameter settings temperature information it can be used to stop fast charge safely, if the first criteria, based on voltage analysis fails.

Relying solely or mainly on voltage battery is thus more convenient in most cases. Due to the above mentioned difficulties with the -∆V method, the detection of the inflection point of the battery voltage versus time function VB(t), has been proposed as a method for approaching full charge avoiding the risk of overcharging [STM1]. This method however should usually lead to a premature end of charge, thus batteries will last longer but a full charge should not be achieved, in fact the inflection point IP (as shown in Fig. 3) is located substantially before the end of charge zone which could be traced by summarizing manufacturers’ specifications.

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This method also involves more complex data processing, as the derivative of VB(t) has to be computed, moreover the determination of IP, is not so easy for a function which changes slowly and almost linearly for a relatively long time, it can be detected with a reasonable confidence only with a certain delay, in dependence of the software threshold fixed for its detection. If this threshold is too low false detection could arise, giving for sure a premature end of charge, if the threshold is too high the delay of detection will be large, in this case the end of charge could came close to the incoming voltage peak VP. Performance of the system could then be not much different from that of other systems, perhaps much simpler, which would detect the end of charge condition in proximity of VP.

SUMMARY OF THE INVENTION

This invention consists of a particular method for detecting the end of charge condition by sensing the battery voltage and employing only analog signal processing, resulting in an extremely simple solution. Nevertheless the proposed circuit may detect the end of charge condition in proximity of VP (Fig. 1), sensing a stationary battery voltage condition, or immediately after, i.e. sensing a voltage drop as low as 1 mV! It is thus suitable for effectively controlling the end of rapid charge either for NiMH or NiCd batteries, implementing a termination criteria very close to the optimum indicated by most battery manufactures. Moreover the proposed solution, for its simplicity, will be very cheap, as it can be realized with common analog components, or be integrated in an 8 pin chip!The basic circuit , for understanding how the proposed solution works, is of disarming simplicity, it is shown in Fig. 4.

Obviously a symmetric version with the battery connected to ground rather than V+, can be realized. Assuming that the battery B1 is connected to the circuit, at the same time it is connected to V+, the capacitor C1 is charged towards VB though R1, the voltage VC is feed to one of the comparator inputs, while the other is connected to the negative battery lead and thus senses VB, which varies under the effect of the current Ichrg, which can be assumed equal to 1C. The potential difference across C1 follows the battery voltage with a certain delay. The network R1, C1 acts as a low-pass filter smoothing and delaying variations of VB, but, as VB(t) is already a smooth function, the delay effect is prevalent. A typical course for VB and VC (taken as positive values) is represented in Fig. 5.

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VC follows VB as it raises under charge, VC also tends to approach VB when the rate of change of VB gets smaller. Assuming that the time constant R1C1 is adequate to cope with the duration of the plateau at the peak voltage of VB, VC can approach VB within a few millivolts during this plateau. If the comparator offset is slightly positive it can sense this condition, and rapid charge can be stopped at this point in proximity of the voltage top of VB. If otherwise the comparator offset is truly 0 or negative, the comparator will switch only if VC exactly matches VB or VC surpasses VB, but this is just what is going to happen after the peak of VB is passed! In fact VC will always follow VB with a certain delay, because the capacitor will retain its charge for a while during the decline of VB, thus there should be an instant in which VB = VC and then VB < VC, thus the curve VC(t) crosses VB(t), at a certain point during the decline of VB, as is shown in Fig. 5. As the decline of VB is generally very slow, the difference VC - VB will only be of few millivolts, but it should be enough to make switch, even comparators with a slight negative offset. The greater the negative offset the later will be terminated the fast charge, giving results very similar to those obtained with the -∆V method, but this is the worst case. This means that proper functioning of the method is not critical. If the comparator offset is not set or trimmed within tight limits or towards positive values we will get at worst a -∆V termination, with ∆V increasing with the negative offset of the comparator.It should be noted that also a peak voltage detector could be used, in place of the basic circuit of Fig. 4, to detect the peak voltage of VB and the beginning of its decline. However this alternative, of which a basic circuit is shown in Fig. 6, it is not advisable, as we will see, but it is also covered by this patent.

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With ideal components VP will copy VB during all its ascent and then remain stable at its peak level. In practice the decline of VB is typically really slow, thus leakage of the capacitor as well as input current of the op-amp should drive VP to decline even faster than VB, thus we will be unable to detect any real positive difference VP – VB during the decline of VB, as shown in Fig. 7, unless we use special components, a very low leakage capacitor and a mosfet input op-amp will be necessary, this solution will then be much more expensive than the previous one, and also less functional, as we will be unable to react at VB summit, but only during its decline, thus only a -∆V detection can be realized.

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DETAILED DESCRIPTION OF A POSSIBLE IMPLEMENTATION

A possible implementation of the proposed invention is shown in the schematic of Fig. 8. The positive electrode of the battery is wired to the positive power supply V+, thus the potential difference VB = VB+ - VB- is sensed at the negative electrode of the battery. Obviously a symmetric configuration with the battery negative connected to ground can also be implemented. It is also shown a current generator, realized with a linear IC like the LM317, providing a constant charge current Icrgh, as well as the trickle current after a full charge is detected. Linear solutions like this are simple and noise free, but less efficient, however also a more efficient solution, based on a charge pump switched at high frequency, can be adopted, given an adequate filtering of the charge current pulses is provided. As we said, the present invention is not concerned with how the charge current is generated, but rather with the method for detecting when the cells are fully charged and thus a fast charge should be stopped.Let us assume that SW4 is open and the battery is connected to the circuit. The diode D4 block reverse polarization of some parts of the circuit to a safe value, in this condition the inputs of all op-amps are also protected by series resistors.The capacitor C1 thus charges itself towards VB. Given R2 << R1 it charges at first quickly, trough the path D1-R2. This allows the circuit to cope with a wide range of battery voltages, as in this range the initial charging time can be made negligible with respect to the second phase, when C1 is charged though R1 only. This happens when the difference between VB and the voltage across C1 (VC) falls below 100 mV. In fact if the diode D1 is forward biased with 175 mV its forward current is still typically of 0.1 µA, but below 100 mV it should be negligible with respect to the current flowing though R1, even for large values of R1 (up to some MΩ’s). Thus when VC is within 100 mV from VB we can assume that the time constant of the network is given by R1C1. With R1 = 1 MΩ and C1 = 100 µF, and VB constant, VC can approach VB within 1 mV in about 8’, a time which is close to the typical duration of the plateau at top voltage of the charge characteristics we have seen. Obviously this time constant could be adjusted for each particular case for optimum performance, but the value indicated behave well in a general case.

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When we close SW4 the circuit is powered and the voltage VC is copied by the voltage follower A1 and feed to the comparator A2 positive input, which senses on the negative input the voltage VB. Since at this point we still have VC < VB the output of the comparator will go high turning Q1 on and letting the charge current flow, which magnitude Icrgh is set by the total resistance resulting between the VOUT terminal of IC2 and ground. The charge current will boost immediately the battery voltage VB, while VC will follow with a certain delay, as shown in Fig. 5. The capacitor C2 is added to filter out noise which may reach the comparator inputs, as it forms a low-pass filter with R3 and R5. As VC approaches VB within a few millivolts the trip point of the comparator A2 may be reached, depending on its offset. As we have seen, discussing the basic circuit, by trimming this offset the termination criteria of fast charge can be adjusted appropriately, according to manufacturer’s specifications. In Fig. 8 the offset is trimmed by means of R11. The diode D3 is a low voltage zener, it may also be replaced by a forward biased diode as a high stability of the voltage across D3 (VZ) would be desirable but it is not necessary. The zener voltage VZ is mirrored with respect to VB by the unity gain inverting amplifier A3, therefore R11 can feed, through the voltage divider R4-R3, a voltage varying from VB+VZ and VB-VZ, thus allowing for adjustment of the trip point of the comparator. When the comparator switches the diode D2 will conduct, thus A2 will now act as a Schmidt trigger and gaining a stable low output state (given R6 is not too high), Q1 is switched off and IC2 will sink now only a trickle current set mainly by R13.It should be noted that the circuit inside the dotted line rectangle of Fig. 8 could be integrated in a 10-pin package IC. If external offset trimming or noise filtering is not necessary an 8 or even a 5 lead package can be adopted for the entire charge controller! If the proposed solution is integrated, the relatively complex offset adjustment circuit, as realized with discrete components, can become much simpler, by acting directly on comparator A2 offset adjustment, thus the entire circuit could be realized

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entirely around a single comparator, or with a voltage follower and a comparator, plus a few additional components1.

References

1. Yuen Tang K., Battery charging circuit for charging NIMH and NICD batteries, United States Patent 5489836 (US5489836), http://www.freepatentsonline.com/5489836.html

2. Nicolai Jean, Method for the fast charging of a battery and integrated circuit for the implementation of this method, United States Patent 5612607 (US5612607), http://www.freepatentsonline.com/5612607.html 3. Yeon Sang-Heum, Charge mode control in a battery charger, United States Patent 5821736 (US5821736), http://www.freepatentsonline.com/5821736.html

[NAT1] Chester Simpson, BATTERY CHARGING, 1995, National Semiconductor, http://www.national.com/appinfo/power/files/f7.pdf

5. Chester Simpson, Characteristics of Rechargeable Batteries, National Semiconductor, 1995, http://www.national.com/appinfo/power/files/f19.pdf

[STM1] J. NICOLAI, L. WUIDART, From Nickel-Cadmium To Nickel-Hydride Fast Battery Charger, STMicroelectronics, 1994, http://www.st.com/stonline/products/literature/an/2074.pdf

J-M. Ravon and L. Wuidart, AN INTELLIGENT ONE HOUR MULTICHARGER FOR Li-Ion, NiMH and NiCd BATTERIES, STMicroelectronics, 1998, http://www.st.com/stonline/products/literature/an/4391.htm

[HAR1] Harding Technical Handbook, Harding Energy Inc., http://www.hardingenergy.com/techmanual.htm

[PAN1] Charge methods for ni-cd batteries, Panasonic 2005, http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_NiCd_ChargeMethods.pdf

[PAN2] Charge methods for Nickel Metal Hydride batteries, Panasonic 2005, http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_NiMH_ChargeMethods.pdf

How to Design Battery Charger Applications that Require External Microcontrollers and Related System-Level Issues, Maxim Integrated Products, Application Note 680, 2005, http://www.maxim-ic.com/an680

MAX712/MAX713 NiCd/NiMH Battery Fast-Charge Controllers, Maxim Integrated Products, 2002, http://datasheets.maxim-ic.com/en/ds/MAX712-MAX713.pdf

Andre Vilas, Boas Marcus, Espindola Alfredo Olmos, Smart NiCd/NiMH Battery Charger Using MC68HC908QY4,

1 The following table lists the values of the components which are not specified in the schematic and which have been used for realizing the prototype, which is still in use (with minor modifications, reported below).

R1 = 1M, R2 = 100K, R3 = 10K, R4 = 200K, R5 = 10K, R6 =240K, R7 = 2.2K, R8 = 20K, R9 = 20K, R10 = 20K,R11 = 20K, R12 = 1.5K, R13 = 100, R14 = R15, = R16 = R17 = 2.2; C1 = 100uF, C2 = 0.1uF, C3 = 220uF.

Q1 = IRF630

D3 = 1N4148 (forward biased! See text)

For greater noise immunity C2 can be substituted with a capacitor up to 100uF connected to the non-inverting input and V+, similar capacitors may shunt D3 and also the battery. The circuit has been tested with V+ ranging from 8 to 16 V, and 2 to 4 cells in series have been successfully charged.

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Freescale Semiconductor, Inc., 2004, http://www.freescale.com/files/microcontrollers/doc/app_note/AN2679.pdf

NiMH Battery Charger Designer Reference Manual — DRM051, Freescale Semiconductor, Inc., 2003, http://www.freescale.com/files/microcontrollers/doc/ref_manual/DRM051.pdf

Z8 Encore!® Based AA Type NiMH and NiCd Battery Charger Reference Design, ZiLOG, Inc, 2005, http://www.zilog.com/docs/z8encore/appnotes/an0229.pdf

LTC4060 Standalone Linear NiMH/NiCd Fast Battery Charger, 2004, Linear Technology Corporation, http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1037,C1078,C1088,P7601

LTC4011 High Efficiency Standalone Nickel Battery Charger, 2005, Linear Technology Corporation, http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1037,C1078,C1089,P9615,D6918

[GP1] GP Batteries Product Brochures, http://www.gpbatteries.com/Product_Brochures.html http://www.gpbatteries.com/pdf/NiCd_Rechargeable.pdf http://www.gpbatteries.com/pdf/NiMH__Rechargeable.pdf http://www.gpbatteries.com/pdf/NiMH_Technical.pdf http://www.gpbatteries.com/pdf/NiCd.pdf

L. Wuidart, J.M. Ravon, A COST EFFECTIVE ULTRA NI-CD FAST BATTERY CHARGER, STMicroelectronics, 1999, http://www.st.com/stonline/products/literature/an/3723.pdf

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