[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?
In practice, the TI algorithms use coulomb counting when the system is on and open-circuit voltage measurement when the system is off or in sleep mode to adjust remaining stateof- charge (RSOC) as appropriate. This provides extremely realistic predictions of remaining battery life.
One key side benefit of impedancetracking's real-time updating of actual state of charge is that it allows the battery gas gauge to reside on the system board, rather than on the battery pack. That means one gas-gauge chip per system, rather than one per battery pack, can be used to account for end-user battery swaps.
According to TI, if the battery pack remains in system, an impedancetracking gas-gauge chip uses the created cell profile as the basis for its fuel gauging. When removing/re-inserting or replacing the system's battery, the chip arbitration algorithm compares the measured characteristics of the inserted battery pack with the default profiles and the previously created cell profiles and chooses the profile that matches the characteristics of the battery pack best.
Charging Battery-charger chips range from autonomous drop-in ICs to highly programmable devices. With so many contemporary chargers hiding their algorithms on-chip, it's hard to find resources for explaining what actually goes on in battery charging. Happily, a Maxim Integrated Products applications note (www.maxim-ic.com/appnotes.cfm/an_pk/680) from 2000 explains basic battery charging using the state diagram in Figure 4. Here's an edited and condensed version of what Maxim had to say:
The state machine starts even before the battery is connected, with the charger initializing itself and performing a self-test, including checking whether a battery is present at its output. The point of the test is to catch events in which the charging process has been interrupted, perhaps by a user unplugging the charger before charging is finished.
Actual charging begins with cell qualification, a state in which the charger detects the installation of the battery and determines whether it can be charged. The charger may look for a voltage on its charging terminals or look for the external jumper or thermistor that's present in some battery packs. If an authentication scheme is used, the charger will determine whether the battery is an approved type. The next state, qualification, determines whether the cell is functional. The charger checks for opens, shorts, and (sometimes) temperature.
After qualification, the next state for some batteries, notably nickel-cadmiums (NiCds), is preconditioning, essentially discharging the battery output down to a level of 1 V over the course of several hours. This is done to help prevent the NiCd's notorious (and misnamed) "memory effect." It's not necessary for more modern battery chemistries.
What happens in the fast-charge state depends on battery chemistry. Broadly speaking, for fast-charging NiCd and nickelmetal- hydride (NiMH) batteries, the charger applies a constant current while monitoring battery voltage and other variables to determine when to terminate the charge. The most common fast-charge rate is C/2, so a totally discharged battery with a 2-Ah rating would be fast-charged at 2 A for approximately two hours.
Of course, without preconditioning, a battery may be connected to a charger at any state of charge. Therefore, the charger must be able to avoid overcharging. When a constant charging current is applied in NiCd and NiMH batteries, the cell voltage rises slowly and eventually peaks. NiMH charging should stop when dV/dt hits zero. NiCd charging should stop when dV/dt inflects downward. Chargers that use faster charging rates than C/2 monitor temperature as well as voltage and terminate fast charging based on the rate of increase in cell temperature.
Li-ion battery chargers need more precise control of charging voltage than nickel chemistry, and their maximum charging rate is set by current limiting. They also add a top-off charging state after reaching the nominal float-voltage.
Different battery chemistries possess different self-discharge rates, which affect the trickle-charge state. Li-ion batteries self-discharge very slowly, and chargers designed for them rarely include trickle-charging. The Maxim note observes that "NiCds, however, can usually accept a C/16 trickle charge indefinitely. For NiMH cells, a safe continuous current is usually around C/50, but trickle charging for NiMH cells is not universally recommended."
As for those autonomous chargers (a market in which Maxim is a fierce competitor), designers may find their task considerably simplified from what it was when the app note was written. To play fair, I'll pick an example from Linear Technology's line card, the LTC4075 Li-ion charger, introduced last spring.
Figures 5a and 5b show a block diagram of the charger and the charging curves it produces. With this charger, programming is achieved via three package pins. The pins labeled IUSB and IDC use resistors to program the currents for USB and wall adapter voltage sources. A resistor on ITERM programs termination current threshold. The other pins provide status indications or accept input power.
This is not to say that all chargers must have this degree of autonomy. For a contrast, take a look at Summit Microelectronics' SMB137 (see "Where Are We?" p. 50).
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sumesh -September 19, 2009
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