[Engineering Essentials]
Battery ICs Charge, Gauge, And Authenticate
OEMs and battery pirates are constantly in a quality-control tug of war. So how do companes ward off deviants and dodge the lemons?
Subsequently, the controller in the end product uses the polynomial coefficients, seed, and device ID that it decrypted, along with the 32-bit random challenge that it sent to the battery, to calculate the authentication CRC value. Then, it compares its results with what the battery returned.
Rather than giving a lesson in bomb-making, let's just say that this approach can also be beat. The highest-level (and more expensive) approach presently preferred employs the SHA-1/HMAC secure hash algorithm. This algorithm is used on the Internet to authenticate transactions on VPNs and for digital certificates.
It works in a fashion similar to the CRC scheme, but with a different algorithm. With a SHA, the controller in the end product reads the battery's 128-bit encrypted device ID from the battery's public memory. It then decrypts those values using its secret key and generates a 160-bit random challenge and sends it to the authentication chip on the battery. That chip uses the plain-text version of its ID (stored in its private memory) and the 160-bit random challenge to calculate an authentication "digest" value, which refers to the result of the hash algorithm processing the message to produce a condensed representation of the message, called a digest.
If the message was altered (about a 1-in-2000 chance), it's virtually certain that the algorithm will produce a different digest. As in the case of the CRC, the controller in the end product compares the digest it produced with the digest returned by the battery and acts accordingly.
Gas gauging Older laptops and handhelds had primitive battery-life gauges that provided only a rough approximation of time left before the system shut down. Next-generation systems will be able to tell users exactly how different kinds of applications-watching movies, listening to music, phone calls, etc.-will affect time remaining on the battery.
Gas gauging starts with coulomb counting. Current into and out of a battery is measured with an analog-to-digital converter (ADC) across a sense resistor, and the value in an accumulator is incremented and decremented accordingly.
One could say this is effective but crude, except it isn't terribly effective. The reason is that guard-banding for the inaccuracies of simple coulomb counting gives end users less accurate battery-life predictions than they could have had with more effective gas gauging. That's not good because longer perceived battery life is a key selling point for cell phones and other handheld gadgets.
The limitation of simple coulomb-counting concerns its environmental effects on batteries. For example, lithium-ion (Li-ion) cell capacity varies with temperature and discharge rate (Fig. 2). The plot shows a particular cell's charge capacity in milliamp-hours as temperature and discharge rate are varied. The "Full" line on the chart is the point at which the cell is considered fully charged. The "High Current Empty" line is the point at which the cell is considered fully discharged by a 1-C rate at each temperature.
Charging rates are defined in terms of a parameter called C, which is the same as the ampere-hour capacity rating of the battery. The "Low Current Empty" line was plotted for a discharge rate of 0.2 C. The capacity of the cell at a given rate and temperature is the difference from the "Full" line to the corresponding "Empty" line.
Problems arise when the cell is charged and discharged at different temperatures and different rates. Hence, more sophisticated coulomb counters account for cell temperature and charge/discharge rates in their algorithms.
Aging is another challenge. The older the Li-ion cell, the less able it is to store charge. Empirical data shows that aging affects the "Full" characteristic only. The "Empty" line remains unchanged. Thus, keeping track of the number of charge/discharge cycles adds additional complexity to gas-gauging algorithms. The smarter algorithms work by comparing the values in the coulomb counter's register with pre-stored standard "Empty" and "Full" values for that cell type. (For a detailed explanation, see Maxim's application note at www.maxim-ic.com/appnotes.cfm/appnote_number/131.)
Recently, Texas Instruments introduced a new level of gas-gauging sophistication that it calls impedance tracking. To simplify the concept a little, another way to look at the phenomenon in Figure 2 is to consider the discharge curves of the battery in terms of voltage versus time for different temperatures (Fig. 3).
Texas Instruments' engineers noted that "The key variable in discharge capacity variation is the internal impedance of the battery cells, which shifts the discharge curve by the IR drop." TI's impedance-based fuel-gauge chips incorporate the measured impedance of the battery's cells in their capacity prediction algorithms, measuring and storing the battery pack's resistance as a function of state-of-charge in real time. These real-time resistance profiles, along with stored tables of battery open-circuit voltage versus state-of-charge, are used to predict the battery pack's discharge curve under any conditions of system-use and temperature.
sir, can you help me to make a triac based variable dc regulated power supply
sumesh -September 19, 2009
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