Actual Dimensions: 47 x 35 mm

Since mid-1999, deepTec™ has been supplying professional clients with charging solutions for Palm® handheld computers, and we are continually honing our techniques and products to provide the best quality recharging solutions for these devices.

The latest result is our 3rd-generation charger board, VIC™, which can recharge an empty battery pack in less than 90 minutes and enables continuous operation of the Palm without involving a battery-damaging high trickle-charge current.

Based on an ATMEL RISC controller, we created custom firmware to provide tailored charge functionality. This architecture also provides the ability to produce custom versions for clients with unique requirements.

Theory of Operation: essentially, the hardware consists of two functional blocks:

1. the controller, incorporating A/D converters, PWM control (for the switched current source), EEPROM memory (for parameter storage) and communication ports.
2. the power section, which controls the charge current.

To achieve high charge currents while maintaining a small form factor, VIC utilizes a switched current source, and is able to supply up to a 1000 mA charge current without heating up. As a result, the board can be installed in a completely closed case as small as 50 x 36 x 8 mm without requiring any auxillary cooling.

In the standard version, VIC operates from an input voltage of between 7.5V and 16V without losing efficiency (in fact, the input current decreases with higher input voltages). Supplied with 12 VDC and charging with 500 mA, the board typically consumes only 240 mA. Custom versions for input voltages between 5.1 V and 35 V can be produced for higher volumes orders.

The controller also permanently monitors the supply voltage, and a charge process is only initiated if the supply voltage is within acceptable limits (if not, an error condition is indicated and the user can check the power supply).

A watchdog circuit ensures a proper CPU reset if the system fails due to power spikes or other external influences. The charger output is fully protected against shorting.

Charge Process: the actual charging process is divided into 4 sequential phases, plus periodic maintenance cycles:

1. conditioning phase,
2. fast-charge phase
,
3. top-off phase,
4. survey (idle) phase, and
5. maintenance cycle.

Conditioning: When a battery is first connected to the charger or the charger is powered up, a conditioning phase is initiated. This uses a reduced current of about 150 mA for approximately 1 minute, preparing the battery for the following fast-charge phase.

This process also has the ability to help recover deep-discharged batteries. If the battery pack doesn't register at least 0.5 V/cell after the one minute period, the conditioning phase is extended for up to 4 hours (in accordance with the specifications of battery manufacturers). This procedure protects both the battery and the charger: a highly-discharged battery will be damaged by a fast-charge current, and this type of load can damage the charger's output drivers at high current rates.

Fast charge: Following the conditioning phase, a fast charge cycle is started, applying 500 mA to the 2 AAA battery pack. During this cycle, a negative slope detection algorhythm monitors the battery voltage. Once a negative slope is detected, the fast charge cycle terminates.

To accurately measure the battery voltage, the charge current is switched off for a few milliseconds once every second during the A/D conversion. This eliminates the influence of the battery impedance (basically the resistance of the battery) which increases with battery age. This unloaded voltage (no charge current applied) remains essentially unchanged over the lifetime of a cell. In addition, noise generated during the A/D conversion is greatly reduced, so VIC achives a measurement accuracy below 0.005V. As a result, VIC is also suitable for Lithium Ion charging if custom firmware is used.

To protect against an exceptional case where the slope detection mechanism fails, a safety timer automatically terminates the fast charge cycle after 90 minutes.

Top-off charge: Because NiMH batteries are quite sensitive to overcharging, the fast charge process is terminated with a certain 'safety margin'. The subsequent top-off cycle fills the battery to 100% with a reduced current of approximately 80 mA. The top-off cycle lasts for a specified 30 minutes and cannot harm the battery pack, even one which is fully charged from the fast charge cycle.

Survey (idle) phase: Unlike commonly-available "premium" chargers, VIC does not use a "trickle charge" current to maintain a full battery. As noted above, NiMH cells are sensitive to overcharging, and most NiMH battery manufacturers recommend trickle charge rates below 1/60 C (1C = battery capacity), if at all. For the Palm battery pack, 1/60 C equates to a trickle charge current of approximately 10-12 mA. If the Palm is off, this 10 mA would be continuously overcharging the cells. If the Palm is on, the trickle charge current isn't sufficient to supply the 17-20 mA the device consumes in idle mode. And when active applications, backlighting or serial activities are drawing additional current on the Palm, it's not nearly enough. Eventually, the result would be an empty battery.

Instead of using a trickle charge, VIC constantly surveys the battery voltage. Once the battery voltage drops below a pre-defined value (the default value is set at 2.4 V, but this is configurable, as noted below), a new cycle is immediately started. This ensures continuous operation, regardless of the amount of current the Palm draws.

Maintenance cycle: To compensate for battery self-discharge and the handheld's small data retention current, every 10 minutes VIC applies a low current for a brief period, equivalent to about 4 mAh (or 1/150 C). This maintenance charge is used as a safe alternative to a constant-current "trickle charge" (a technique battery manufacturers recommend against), and ensures that the battery is completely full when removed from the cradle.

Charge Process Flow Diagram Click the icon below to view or download a PDF file of the Charge Process Flow Diagram.

Please note: this flow diagram is an overview of the charge processes. Background tasks such as A/D conversion, interrupt handling, RS-232 buffer handling etc. are not shown for reasons of simplicity.

VIC Charge Process Flow Diagram (60 kB, 6/26/01)

Serial Port VIC includes an RS-232 port, allowing direct communication with the Palm handheld. We have developed VIC-mon™, a companion Palm OS software application which directly displays detailed charging information on the Palm, and permits viewing (and modification where desired) of essential charge parameters.

Charge Parameters VIC is pre-configured for optimal charging of the deepTec Palm battery pack; however, some non-standard applications may require a different configuration of charging parameters. All relevant charging parameters are stored in VIC's non-volatile EEPROM memory, and these can be changed using VIC-mon via VIC's built-in asynchronous serial port.

Auxilliary Port VIC offers one TTL-level line which can be used as an input or output for special control functions. The target system could control additional functions of VIC, or VIC could trigger additional tasks on the target system. The auxilliary port can also be used to supply a basic "battery full" signal.

Please contact deepTec if your application requires any of the available custom features.

For additional technical information on VIC, including LED codes, serial port pinouts and programming port pinouts, please refer to our online VIC Operating Manual (1.4 MB, 6/26/01)

VIC - Technical Specifications

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June 26th, 2001