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Build this advanced small-cell charger and step up to the newest generation of super-capacity rechargeable batteries – Pt.2 Last month, we looked at the features of our new intelligent SuperCharger, described how the circuit worked and showed you how to assemble the main PC board. This month, we complete the construction and give you the driving details. O Pt.2: By PETER SMITH NCE THE MAIN board has been assembled, it's simply a matter of completing the small front-panel board, wiring them together and completing the assembly. But first, there are a couple of minor modifications to the main board. The accompanying panel has the details. Front panel board assembly Referring to the overlay diagram (Fig.9), begin by installing the 11 wire links, followed by the resistors. Next, 68  Silicon Chip turn the board over and install the two remaining resistors on the bottom (copper) side, as shown in Fig.10. Cut the protruding resistor leads off flush with the surface of the PC board (on the top side). Moving back to the top side, install the connector (CON7) followed by the 33µF tantalum capacitor. Mount the capacitor horizontally rather than vertically and fit a short length of heatshrink tubing over its negative lead to ensure that it cannot short out on nearby components. Transistors Q5-Q8 can go in next, followed by the four pushbutton switches. It is particularly important that the base of each switch is seated firmly on the PC board surface during soldering. Be sure to install the red switch in the S4 position and make sure that the flat side of each switch is oriented as shown. The final step involves mounting all the LEDs and fitting the board to the front panel. Start by installing each LED in place but do not solder or cut the leads short just yet. Note in particular the orientation of the anode and cathode leads for each column of LEDs – they differ between the left­hand and righthand columns, as indicated in Fig.9. Follow the details in Fig.11 to mount the PC board to the front panel. That done, place the face of the panel on a flat surface and push the LEDs into their designated panel holes. If www.siliconchip.com.au you would like the LEDs to protrude through the panel slightly, then raise the panel the desired amount and push the LEDs through until they contract the flat surface below. Solder them into position to complete the job. Main PC Board Update Cabling The front panel is hooked up to the main board via a length of 10-way rainbow cable, fitted with header sockets on both ends. Keep this cable as short as possible but allow about 20mm of slack so that it’s not stretched tight when installed. The header sockets must be carefully wired, as it is very easy to mistakenly reverse the wiring order. Fig.12 shows how it’s done. Double-check (with the finished cable connected) that pin 1 of CON4 on the main board connects to pin 1 of CON 7 on the front panel, using the overlay diagrams as a reference. All four discharge globes are wired in parallel with light-duty hook-up wire. Insulate the connections to the rear of the bezels with heatshrink tubing. Route the cabling as shown in the various photos. Use medium-duty (5A or higher) figure-8 cable or similar for the battery connections, keeping the length down to around 400mm or so. Bend the cable sharply as it exits the terminal block (CON5) to avoid Q2’s heatsink, then route it alongside the 1000µF capacitors and out through the rear panel. That done, place a cable tie around the cable at the point where it enters the grommet (on the inside of the case) so that it can’t be pulled through from the outside. Mark the positive battery lead clearly or, better still, use some kind of keyed connector with your chosen battery holder(s). Accidentally connecting your batteries in reverse could easily ruin all your hard work! Initial tests Before installing IC2, IC3 and IC4 on the main PC board and connecting the front panel, it’s a good idea to check that the power supply circuitry is working properly. To do this, you’ll need a digital multimeter and a spare 15kΩ 0.25W resistor. The resistor is needed to provide a minimum load for IC3’s VDD supply. Referring to Fig.13, insert and solder the resistor to the unused pads situated on either side of the 4.7µF tantalum www.siliconchip.com.au Fig.7: the overlay diagram for the top side of the main PC board, updated and reproduced again this month for convenience. Since the first part of this project was published, we’ve had the opportunity to test the SuperCharger with a greater variety of batteries and power sources. Our tests revealed that a few small changes to the original design were required. Two additional parts are needed for the main PC board, as follows: (1) 1 18Ω 1W 1% or 5% metal film resistor (R38) (Farnell 337-640) (2) 1 10nF 250VAC polypropylene capacitor (7.5mm lead pitch) (C19) (Farnell 303-9146) We’ve reproduced a small section of the circuit diagram (Fig.8) to show where these two new components are located. They function as a simple R-C damper (or ‘snubber’), reducing high frequency ringing when Q2 switches on and off. Both components are installed near Q2, with the resistor mounting vertically rather than horizontally. Note also that the capacitor mounts directly above the SMD diode (D3), so it is particularly important to ensure that D3 is positioned so that it does not obscure the capacitor’s mounting capacitor. Once that’s done, plug the 3A fuse into its clips and connect a 16VAC 1.5A plugpack to CON1. Before applying power, however, take a moment to recheck your work against Fig.8: we’ve included an R-C damper on the final version of the main PC board, shown here in red. holes. We’ve also changed the value of inductor L1 from 22µH to 18µH. Finally, we’ve relocated the 470pF (C14) and 1µF (C10) capacitors slightly. The PC board pattern shown in Fig.16 contains all of these changes and an updated parts overlay diagram is reproduced above in Fig.7 for convenience. The PC pattern sent to the board manufacturers also includes these changes. the overlay diagram. Assuming all is OK, hold your breath and hit the power switch. No surprises? Great! All measurements that follow are with respect to December 2002  69 Fig.9: overlay diagram for the front panel PC board. The 33µF capacitor must be mounted horizontally (see above photo), with heatshrink tubing on its leads to prevent short circuits. Note that the lefthand and righthand columns of LEDs are orientated differently. the ground (0V) rail. A handy ground connection point can be found on pin 1 of CON4. First, check the DC (VIN) rail at the cathode of ZD2. It should measure about +21.5V. If you’re using a plugpack other than what we’ve recommended in the parts list, be aware that this voltage must not exceed +24V, Fig.10: just two resistors are mounted on the copper side of the front-panel PC board. Position the resistor bodies so that they are close to the surface of the PC board, as shown in the above photo, before soldering their leads. otherwise the transient suppression diode (TVS1) will conduct and may be destroyed. Next, check the +5V rail, accessible on pin 2 of CON4, pin 20 of IC2 and pin 8 of IC4. Finally, check IC3’s VDD supply by probing the end of the 15kΩ resistor (installed earlier) closest to Q1. You should get a about +15V. WHERE TO GET THE PARTS At time of publication, the Super­Charger was not available as a kit from the usual suppliers. However, all of the parts are available locally (see parts list), with the exception of two items: (1) The LTC1325CN (IC3) can be purchased directly from the manufacturer, Linear Technology. You can buy on-line at www.linear.com (2) The 18µH inductor (L1) used in the prototype is manufactured by Sumida Corp., part number CDRH127-180MC. It can be purchased on the web from Digi-Key at www.digikey.com Inductors from three other manufacturers have also been identified as suitable. These are: (a) Part no. 3631C180ML, manufactured by Meggitt Electronic Components (www.meggittelectronics.com); (b) Part no. TSI1207P-180, manufactured by Selmag Co. (www.selmag.com.tw); and (c) Part no. TPRH1207-180M, manufactured by Top Magnetics Corp. (www. topmagnetics.com). As usual, the PC boards and programmed microcontroller (IC2) will be available from RCS Radio, phone (02) 9738 0330. 70  Silicon Chip If all readings check out, then power down and remove the 15kΩ resistor. Install the three ICs, being sure to align pin 1 of all devices as shown on the overlay. We haven’t specified sockets for IC3 and IC4, as we believe they would reduce the reliability of the project. However, if you’re wary about soldering these (expensive) little devices, then we recommend using high-quality, turned-pin sockets. If you can’t source an 18-pin socket for IC3, then you can cut the two end pins off a 20pin version with a sharp knife and tidy up with a fine jewellers file. Programming the micro If you’ve purchased this project as a kit, then the microcontroller (IC2) will have been programmed for you. Alternatively, if you’ve sourced all the parts yourself, then you’ll need to program the Flash and EEPROM memory in the micro. We’ve provided an ISP (In-System Programming) header (CON3) for connection to an ‘Atmel-compatible’ programmer for the task. Two suitable programmers have appeared in the pages of Silicon Chip, the most recent in October 2002. A www.siliconchip.com.au Fig.11: about 8mm of space is required between the front panel and the PC board. This is easily achieved with 6mm spacers and M3 nuts, as shown here. Fig.12: how to wire the 10-way cable that connects the two PC boards together. Ignore the pin 1 mouldings on the sockets and follow this diagram and the directions in the text closely. simpler design was presented in the October 2001 edition. The necessary program files for the microcontroller can be downloaded from the Silicon Chip web site at: www.siliconchip.com.au Battery holders Almost any style of battery holder can be used with the SuperCharger. www.siliconchip.com.au However, it is unlikely that the lowcost plastic varieties will perform well when rapid-charging high-capacity cells. The current rating of most cheap holders is probably only a few hundred mA at best, which explains why we’ve seen them melt under heavy load! In addition, it’s too easy to accidentally reverse a cell in a multi-cell holder. With this in mind, we’ve designed a PC board that will accept up to six single-cell holders of either the low-cost or high-current variety. The overlay diagram for the battery holder PC board is shown in Fig.15. The board has mounting positions for four types of holders, including three high-current types in sizes AA, AAA & 1/ AA (available from Farnell, see parts 2 list) and a low-cost AA size. The holders are connected in series, so you need only install the number that you require. Populate from the CELL1 end and work up. The high-current holders should be mount­ ed securely with two M3 x 10mm CSK screws, nuts and washers before soldering. These holders include both solder pins and tags for push-on terminals. We cut off the unused tags with sidecutters and cleaned up the sharp edges with a jeweller’s file for a neater appearance. If you’ve opted for the low-cost AA holders, then you’ll need to trim the flying leads to about 10mm in length before stripping and tinning the ends. Secure them with M2 x 6mm screws and nuts. Note that the board will also accept low-cost AAA and 1/2AA sizes but you’ll need to drill additional mounting holes to suit. The charger connects to the holder via a 2-way terminal block plug and PC-mount terminal block sockets. As shown in Fig.14, we’ve made provision for one socket per holder (CON101 – CON106) To determine the number of 2-way terminal block sockets required, first consider the number of cells you will be charging together. For example, it you’ve installed all six holders and will be charging one, two, four and six cells together, then install the first (CON101), second (CON102), fourth (CON104) and sixth (CON106) sockets only. We’ve provided sockets in this ‘series’ configuration to eliminate the need for switches to select the number of cells to be charged. In use, you Fig.13: temporarily solder a 15kΩ resistor in circuit for power supply testing. We’ve provided a couple of spare pads for the purpose, positioned on either side of the 4.7µF tantalum capacitor. simply insert the cells by starting at the bottom (CON101) position and working up. The charger plug is then inserted into the socket adjacent to the last cell. For example, if you have inserted four cells, then plug the charger into the socket next to the fourth cell (CON104). To protect your furniture as well as the underside of the PC board, fit 10mm (diameter) self-adhesive rubber or acrylic feet to the corners of the completed PC board. Note that the feet need to be positioned close to the corners of the board so that it doesn’t tilt over when installing batteries. Operation Driving the SuperCharger is quite straightforward, with all operations selected via the four front-panel pushbutton switches. The ‘Cell Type’ button allows selection of either NiCd or NiMH-type batteries. Essentially, this setting selects either a 1C (NiMH) or 1.5C (NiCd) charge rate for the rapid charge mode. It has no effect in fast charge mode, where both types are charged at their 0.5C rates. Don’t be tempted to charge NiMH batteries on the NiCd setting – you’ll probably damage your batteries! Note also that the maximum charge rate for both battery types is 1800mAh. This means that NiCd batteries larger than 1200mAh will be charged at slightly less than their 1.5C rate. The vertical column of nine LEDs has two functions. Initially, they indicate the chosen cell capacity, which can be increased or decreased December 2002  71 Building the SuperCharger is easy, with virtually all the parts on two PC boards: a main board and a front panel board. Note how the 10-way cable is installed. using the ‘up’ and ‘down’ buttons on the right-hand side. Once charging has commenced, they then indicate elapsed time as a percentage of the maximum expected time for a full charge. Unless you’re charging completely exhausted batteries, you’ll probably find that not all the LEDs in the column light before the charge completes. Once cell type and capacity are set, it’s then just a matter of pressing the ‘Go/Stop’ button once for rapid charge or twice for fast charge. To perform a discharge before charge, hold down the button until you hear two ‘beeps’. We’ve also included a standard (0.1C) 16-hour charge mode for recovering cells that will not accept a full charge at the rapid or fast rates. The operational flow chart in Fig.17 details how to access this mode. It also shows how you can determine the state of any charge as it advances through the various modes to completion. If you need your batteries in the shortest time possible, then you can halt the cycle at the end of the rap72  Silicon Chip id charge period, rather than wait for the 2-hour top-up. At this point, somewhere between 90 and 95% of battery capacity will have been returned (assuming the cells are in good condition!). It is important, however, that you occasionally allow the top-up charge to complete so that all cells in a set can be equalised. Hitting the ‘Go/Stop’ button at any point in a charge cycle will return to the standby state. This is also the recommended way of terminating a trickle charge before removing your fully charged cells! Cycling problem batteries Fig.14: the circuit diagram for the optional battery holder PC board. The new-generation batteries do not suffer ‘memory effect’ but they can exhibit a similar problem called ‘voltage depression’. The most obvious symptom of this problem is low charge acceptance. Even fully discharged cells with this problem will not accept a full charge at the fast (0.5C) or rapid (1C or 1.5C) rates. In our experience, this problem is common amongst newly-purchased cells, probably because they have been stored for long periods before sale. To eliminate, or at least greatly reduce the effects of voltage depression, www.siliconchip.com.au It’s possible to mix different-sized cell holders on the same batteryholder PC board. Here we have used both AA (bottom) and AAA sizes. The shorting link (arrowed) is necessary to allow the top two holders to be used in isolation but must be removed when using any of the bottom (AA) holders. we’ve found that a full charge at the standard (0.1C) rate followed by a number of discharge and charge cycles at the fast/rapid rate is effective. In use, it can take many fast/rapid charge cycles before a set of cells will deliver close to 100% of their rated capacity. Discharge-before-charge The SuperCharger provides a discharge-before-charge function, albeit with several limitations. These are as follows: (1) Do not select discharge-before-charge if your batteries are already Fig.15 (right): the battery holder PC board overlay, shown here with highcurrent AA-size holders installed. Note that if you only ever intend charging a maximum of four cells, then you can cut off the top section of the PC board along the ‘cut here’ line. ‘flat’. The terminal voltage for each cell must be within the nominal range (around 1.2V) in order for the Super­ Charger to correctly determine the number of cells connected. (2) Between two and six cells must be connected for the discharge function to work properly; it does not We made up a selection of battery holder boards to suit our needs. The bottom board has two low-cost holders installed and has been cut-down to accommodate four cell holders only. www.siliconchip.com.au December 2002  73 work with just one cell. In addition, it should not be used with 9V (or 7.2V) PP3 size batteries. The batteries are discharged into a simple resistive load, consisting of four parallel-connected 12V 120mA globes. Therefore, the discharge current will vary according to the number of cells installed. For example, with only two cells installed, the discharge current will be about 120mA, whereas with four cells installed it will be about 240mA. This means that you’ll need to allow a considerable amount of time when cycling high-capacity cells. To speed up the discharge, you can customise the load to suit your requirements. For example, if you only intend discharging a maximum of four cells, then you can replace the 12V globes with 6V versions, thereby roughly halving the discharge time. In-car use Another view of the mixed cell holder board with four AA cells in position. Note that the shorting link has been removed. TABLE 1: BEEP ERROR CODES Beeps Error Description 1 No error Indicates beginning & end of charge cycle. 2 No error Indicates discharge-before-charge sel ected. 3 Reverse cell check Check for reversed cells. If OK, hi t 'Go/Stop' again. 4 EEPROM checksum error EEPROM is corrupted and needs reprogramming. 5 Can't autorange 6 Charge timeout 7 Low vol tage battery 8 High vol tage battery 9 Input vol tage too high 10 Input vol tage too low Unable to detect number of cel ls connected. Battery voltage i s less than 850mV after 3 hours (shorted battery). Battery vol tage decreased below 850mV during charge (possible shorted battery). Battery vol tage too high (high resi stance/open circui t cell or battery di sconnected). Input vol tage exceeds 24V. Di sconnect immediatel y! Input voltage i s less than 12V. 11 No headroom Input vol tage is too low to charge current battery. When an error is detected, all LEDs on the front pane ylash and the piezo buzzer 'beeps' an error code. This table lists all the codes and their interpretations. TABLE 2: WHERE TO GET BATTERY INFORMATION Manufacturer Website GP http://www.gpbatteries.com.hk Eveready http://data.energizer.com Panasonic http://www.panasoni c.com/industri al/battery Sanyo http://www.sanyo.com/industrial/batteri es Powerex http://www.mahaenergy.com/products/prosumer/batteri es.htm Kodak http://www.kodak.com/global/en/consumer/products/batteri es Rayovac http://www.rayovac.com/products/recharge/recharge.shtml 74  Silicon Chip A separate DC input has been provided for connection to any low-impedance 13.8V 1.8A DC source, such as a car cigarette lighter socket. Up to five cells can be charged in series from a 12V car battery. However, a minimum of 13.2V is required to fully charge a typical 6-cell stack (eg, a 7.2V R/C battery pack), so you’ll need to have the engine running. If the voltage dips below the required minimum, the charge will terminate with an error (see Table 1). We strongly recommend that the charger be disconnected from the vehicle’s electrical system during engine start to prevent possible damage to the sensitive electronic circuitry. The Chargemeister’s tips We’ve already talked about some of the more important elements of recharging. Here they are again, grouped together with a couple of new points that you should find useful. (1) Keep all contacts clean. This applies to both the battery terminals and holder contacts. Corrosion on or around contacts should be cleaned up immediately. If a contact’s plating is damaged (eg, if it is pitted or peeling), it should be replaced. (2) Always keep batteries together as a set (as used in the end equipment). This ensures that all cells within a set are roughly equivalent in ‘strength’, thus maximising the life of all. One way of achieving this is to mark each cell with an identifying ‘set www.siliconchip.com.au Fig.16: full-size patterns for the main, front panel and optional battery holder PC boards. number’. In other words, “‘till death do us part!” (3) A maximum of 6 cells can be charged in series. Unless approved by the battery manufacturer, don’t charge cells in parallel. (4) Ambient temperature has a big effect on cell charge/discharge efficiency and reliability. Where possible, charge your batteries at room temperature (about 21°C). Avoid rapid or fast-charging batteries at less than 10°C or greater than 40°C. (5) Avoid totally discharging your batteries. Manufacturers build over-discharge protection into all recharge­ables these days but repeated total discharge will shorten life considerably. Generally, when you notice a sudden drop in output (light, sound, www.siliconchip.com.au We fitted four rubber feet to the bottom of each cell holder to stop them scratching desk tops. Note that these are not close enough to the corners to stop the holder from tilting over when cells are installed. December 2002  75 etc), remove the batteries and recharge as soon as possible. Rechargeable batteries are ideal for use in many high-drain projects. Cells with solder tags, rather than nipples, are often the best choice, so why not make up your own battery packs? Soldering the cells together eliminates potential connection problems and ensures that they’re always part of the same set. Note that the focus of this project has been on recharging small, cylindrical cells in the AA and AAA size ranges. However, the SuperCharger can also handle other NiCd and NiMH batteries with ratings between 200mAh and 1800mAh. Always check the manufacturers specs (often available on the web) for maximum charge rates. Fig.17: the complete operational chart for the SuperCharger. The exact mode of operation depends on whether you select a rapid charge, a fast charge or a standard charge. 76  Silicon Chip www.siliconchip.com.au This is especially important for NiMH batteries! The rear panel of the SuperCharger carries the four discharge globes and the power sockets. Recovering flat/shorted cells Cells that have been over-discharged or reverse-charged can usually be recovered by the SuperCharger’s ‘pre­ charge’ function. This function is automatically invoked before the main charge begins if the total battery voltage is less than 900mV. Using a constant current of about 60mA, the SuperCharger will try (for 3 hours max.) to bring the battery voltage up above 850mV. If successful, the charge progresses to the next stage, otherwise the battery is assumed short-circuit and the charge terminates with an error (see Table 1). Note that if the initial battery voltage is less than 200mV, then the SuperCharger will flash all LEDs and ‘beep’ three times, prompting you to check that you have not accidentally connected any cells in reverse. If all is OK, simply hit the ‘Go/Stop’ button again to continue with the charge. It’s quite common for cells to go short-circuit near the end of their lives. We’ve even seen this happen to comparatively new cells that have been lying idle for a couple of years. So what can you do about it? Some say that if a cell is shorted, it’s at the end of its life anyway, so it may as well be discarded. That’s possibly true but if you’d like to have a shot at resuscitation, take a look at the ‘Nicad Zapper’ project in the August 1994 edition of Silicon Chip. This works by applying a brief, high-current pulse to the cell, ‘blowing out’ the dendrite growth that is usually responsible for short-circuiting the plates. A suggested modification to the Nicad Zapper project appeared in Circuit Notebook, June 1995. It simplifies the original design by eliminating the power supply circuitry. Note that when recovering shorted or reverse-charged cells, charge each cell individually (rather than in series with other cells) at the standard (0.1C) SC rate for the first cycle. Use these photographs to guide you when installing the internal wiring. Keep the rainbow cable clear of the heatsinks. www.siliconchip.com.au December 2002  77