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This simple circuit uses your PC’s games card to monitor & display the rate of charge or discharge of a nicad battery pack. It can be built in just a few minutes & requires only the addition of a low-cost software package to make it work. By DARREN YATES A PC-based nicad battery monitor This project is a logical follow-on from our “Experiments With Your Games Card” series in the Computer Bits column. It will allow you to monitor 7.2V nicad battery packs during their charge and discharge cycles, so that you can see how they are progress­ing. What’s more, it can be easily modified to handle 6V, 9.6V or 12V cam­corder batteries, which have become notorious for memory effects. By using some simple software, the project will give an analysis of your battery over time and indicates whether you have a problem with ei- ther memory effect, incorrect charging or exces­sive discharging. In addition, you can save the readings to disc and then pull them out for later analysis and comparison. And if you have a graphics printer, you can also do a print screen to the Clipboard and then paste the image into Paintbrush (and then print out from there), provided you have Windows running in the background. This will also allow you to save the image as a bitmap (.BMP) file and then import it into just about any Windows-based program, just as we have done here to illustrate this article. Games card interface Fig.1: the input control circuitry for a typical games card. There are four such circuits to cover all the controls on a joystick. 62  Silicon Chip By utilising the games card which appears in just about every PC sold these days, we can do away with just about all of the usual circuitry that would otherwise be required. As a re­ sult, our circuit consists of just three resistors and that’s about all. As mentioned in previous issues, the games card has two analog inputs which are used by the joystick’s X- and R1 2.2k 7.2V NICAD BATTERY R2 4.7k 56k PIN 3 GAME CARD CONNECTOR PIN4 NICAD BATTERY MONITOR FOR PCS Fig.2: the circuit uses just three resistors. Note that R1 & R2 must be adjusted to suit the nominal battery voltage – see Table 1. Y-axis controls – see Fig.1. These are just 100-500kΩ pots which vary the charging current to a 0.01µF capacitor on the games card PC board. A software counter keeps tabs on how long it takes for the monostable controlled by the joystick to reset and this number is available to any software. In our case, we can replace the variable pot inside the joystick with a variable voltage (ie, the nicad battery voltage). The reason this works is that in a normal joystick arrangement, a fixed voltage (ie, the 5V rail) and a variable resistor provide the variable current. In our case, the variable current is pro­vided using a variable voltage (the battery under test) and a fixed resistor. So, in the end, we wind up with pretty much the same thing. We simply add the battery to the our three resistor circuit and the PC does the rest via our purpose-designed software pack­age, GAMESBVM.BAS/EXE. Circuit diagram The project is built using the Games Breakout Board de­scribed in the Computer Bits column for April 1994. Fig.3 shows the parts layout for that board, while Fig.4 shows the full size board pattern. However, there is no reason why you could not use your own board pattern if you wish, provided you make all the right connections. TABLE 1 The battery voltage is plotted on screen as a continuous graph which can then be saved & reloaded at a later date. In this case, 500 samples were taken at 0.5-second intervals but this time period can be varied to suit the application. 4.7k Games breakout board This “screen-grab” shows the opening menu that appears when the software is booted up. It offers six choices: (1) set the nominal voltage; (2) set the time between measure­ments; (3) take readings; (4) save readings to file; (5) load and display readings from file; and (6) quit. 56k Vbatt R1 R2 4.8V - - 6V 1k 4.7k 7.2V 2.2k 4.7k 9.6V 4.3k 4.7k 12V 6.8k 4.7k 7.2V NICAD BATTERY 2.2k Let’s now take a quick look at the circuit diagram – see Fig.2. As shown, the battery under test is connected to the circuit and the voltage tapped off by a 2.2kΩ and 4.7kΩ resistive divider. The resulting output is then fed to pin 3 of the games card via a 56kΩ current-limiting resistor, while the common ground connection is made via pin 4. And that’s all there is to it! The voltage divider is set so that the tapoff point gives close to 5V when the battery is at its nominal output voltage (ie, 7.2V). In fact, if you go through the maths, you will find that a 7.2V nicad pack will give 4.9V at the tapoff point. For other battery voltages, we simply adjust the value of the divider to ensure a 5V output (the 56kΩ current limiting resistor remains the same in all cases). Table 1 shows the de­tails. Note that if you have a 4.8V nicad pack, then you only need the 56kΩ current-limiting resistor. Fig.3: the prototype was built onto the Games Breakout Board described in the April 1994 issue. You could also connect the parts directly to the back of the DB15 socket but make sure that all the connections are correct. June 1994  63 PARTS LIST 1 PC board, code 07103941, 95 x 59mm 1 DB15 female PCB-mount socket 1 GAMESBVM software disc Resistors (0.25W, 1%) 1 56kΩ 1 4.7kΩ 1 2.2kΩ GAMESBVM SOFTWARE Fig.5: this is the full-size etching pattern for the PC board. Alternatively, you could even connect the resistor network directly to the back of the DB15 socket. Be very careful if you do this though. If you make a wrong connection or short any of the pins, you could damage the games card. Before you start construction, make sure that the board pattern is correct and that there are no shorts or breaks in any of the tracks. Once you’re satisfied that the board is OK, in­stall the four wire links and the three resistors. After that, it’s simply a matter of installing the DB15 PC-mount socket and adding the test leads for the battery. DB15 cable If you’re lazy, you can buy a DB15DB15 male-male cable for around $30. This will be invaluable if you have other devices to connect to the games card, since all 15 pins are connected, but it’s expensive overkill if used solely for this project. In this circuit, because you only need two connections to the games card, you can easily “roll your own” cable but make sure that the connections are correct. Software As you will have probably suspected with such a simple circuit, most of the work is done by the PC through software. The program is too large for us to publish here but readers can obtain the source code (GAMES­BVM. BAS), the object code (GAME­SBVM. OBJ) and the complied, executable program (GAMES­BVM.EXE) on either a 5.25-inch or 3.5-inch disc directly from SILICON CHIP. The nominal battery voltage is entered in at this screen prompt. This voltage must be 4.8V or greater & the circuit must be adjusted for values other than 7.2V. 64  Silicon Chip The complete GAMESBVM software package is available from SILICON CHIP for $7 + $3 p&p. Please specify either a 5.25-inch or 3.5-inch disc. Send your cheque/ money order to: Silicon Chip, PO Box 139, Collaroy, NSW 2097; or phone in with your credit card details (Visa, MasterCard and Bankcard) on (02) 979 5644; or fax your order to (02) 979 6503 (see the order coupon in this issue). The software was written in Quick­ BASIC 4.5 but should also work quite happily with DOS 5/6’s QBasic should you wish to modify it in any way. The opening menu of the software offers six choices: (1) set the nominal voltage; (2) set the time between measure­ments; (3) take readings; (4) save readings to file; (5) load and display readings from file; and (6) quit. The readings are stored on file in ASCII format so that you can use them in other programs or even modify them if you so wish. The screen grabs in this article give an idea of what to expect as you go through the various stages of the soft­ware package. Note that measurements are displayed on the screen as they are taken but once the voltage falls below 10% of the bottom scale, then the program automatically stops and returns to the main menu. When saving the measurements, you can use the standard drive:\ path\filename.ext format to store them where you wish. By using the Nicad Battery Monitor for PCs, you will be able to keep tabs on just what the battery pack is doing. It should also show up any early signs of “memory effect”, thereby allowing you to take the appropriate counterSC measures before it goes too far.