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Pt.2 By JOHN CLARKE A charger for deep-cycle 12V batteries In this second article on the 3-Step Battery Charger, we present the full construction details, the parts list and set-up procedure. It is designed to fully charge and maintain deep-cycle lead acid batteries, so that they can deliver their full capacity. T HE BATTERY CHARGER is built on three PC boards. These are the Power PC board coded 14111041 and measuring 224 x 77mm, the Control PC board coded 14111042 and measuring 92 x 69mm and the Display PC board coded 14111043 and measuring 141 x 66mm. These are housed inside a metal case measuring 88mm high x 279mm deep x 304mm wide. The Power Controller components are mounted on a fan-assisted heatsink which is cooled using an 80mm 12V 28  Silicon Chip fan. The transformer is a 300VA toroid. On the front panel are the power switch, control switches and the LCD module. At the rear of the case are the charger leads, the temperature sensor input socket, the fuses and the heatsink fan. Also there is a finger guard to cover the fan blades. Building the PC boards Before installing any parts, check all the PC boards for any shorts between the copper tracks or for breaks in the connections. Also check the hole sizes. You will need 3mm holes for the mounting screws and the regulator screw. The holes for the power connections at each side of sense resistor R1 and adjacent to transistor Q2 need to be 4mm in diameter to suit M4 screws. The component layouts for all three boards are shown in Fig.6. Start by installing the PC stakes at the wiring and test points, then install the links and low-wattage resistors on the Power PC board. Use Table 1 as a guide to selecting each resistor and check each value using a multimeter. Next, insert the diodes (taking care with their orientation), then install IC1 and IC2. Be sure to install the LM358 in the IC1 position, while IC2 must be the LM393. Both ICs must be installed with the correct orientation, as shown on Fig.6. The trimpots, capacitors and 5W resistors can be inserted next. Note that siliconchip.com.au the 5W resistors should be mounted about 1mm above the PC board to allow cooling. That done, insert and solder in the R1 sense resistor. When installing the capacitors, make sure that the polarised types are inserted the right way around and that they have the correct voltage ratings for each position. Leave transistors Q1-Q5 off until the final assembly, to ensure they are set to the correct height for the heatsink mounting holes. Three M4 x 10mm screws should now be soldered to the underside of the PC board – one on either side of R1 for the power connections and one adjacent to transistor Q2. Each screw is installed from the underside of the PC board and soldered to the large copper areas provided. This will make it easier to complete the connections on the top of the PC board. Now for the Display PC board assiliconchip.com.au sembly – see Fig.6. Begin by installing the diodes, the resistor, the capacitors and trimpot VR5. Note that the 10µF capacitor needs to lie on its side, as shown in the diagram. The pushbutton switches must be orientated with their flat sides facing up, as shown. Positioning of the LCD will depend on the particular module. The Jaycar module is connected via a dual 7-way header strip, while the Dick Smith and Altronics modules are each connected via a single in-line 14-pin header. Note that the DSE module reads upside down compared to the other two modules. This is solved simply by mounting the PC board upside down in the case. The ends of the display adjacent to the header connection are supported using M2.5 x 15mm screws and M2.5 nuts. The 4-way and 6-way header pins are right-angle types. Make up the plugs by terminating the 6-way Main Features • Suitable for 12V lead acid batteries • LCD shows charging phase and settings • Temperature, voltage and current • • • • • • • • • metering 3-step charging Optional equalisation phase Battery temperature compensation 16.6A charge capacity Initial trickle charge when battery voltage is low 4 preset battery chemistry settings 2 adjustable specific battery settings (can be set for 6V batteries) Correction for voltage drop across battery leads Wide battery capacity range (4250Ah) in 18 steps December 2004  29 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o No.   1   1   1   1   4   5   1   2   1   5   1   1   1   1   1 Value 1MΩ 910kΩ 100kΩ 43kΩ 22kΩ 10kΩ 5.6kΩ 3.3kΩ 1.8kΩ 1kΩ 330Ω 270Ω 150Ω 120Ω 10Ω and 4-way rainbow cable into the header clips and insert the clips into the header shells. The plugs are placed at each end of the wire cabling. Note that the PC board overlays show a pin 1 orientation for these connectors. Ensure that their polarities are correct. You can now start work on the Control PC board by installing the resistors, diodes and PC stakes. IC3 and IC4 are soldered directly to the PC board while IC5 (the PIC microcontroller) is mounted in a socket (don’t plug IC5 in yet). The regulator mounts horizontally and is secured to the board using an M3 x 10mm screw and M3 nut. Next, install trimpots VR3 and VR4, making sure the 200Ω trimpot goes into the VR4 position, near the regulator. The capacitors can then be installed, again making sure that the electrolytics are corectly orientated. Testing the PC boards The Control and Display PC boards can now be tested for correct operation. First, connect them together using the 4-way and 6-way cabling as shown in Fig.9. Make sure IC5 has not been installed and set trimpot VR4 fully anticlockwise. Next, apply power (ie, any DC voltage between 12V and 25V) between the +25V supply pin and the GND pin. That done, connect a multimeter set to read 10V between the GND pin and the tab on the regulator (REG1). Adjust VR4 for a 5.0V output. Assuming all is OK, switch off the 30  Silicon Chip 4-Band Code (1%) brown black green brown white brown yellow brown brown black yellow brown yellow orange orange brown red red orange brown brown black orange brown green blue red brown orange orange red brown brown grey red brown brown black red brown orange orange brown brown red violet brown brown brown green brown brown brown red brown brown brown black black brown power, install IC5 into its socket and solder SENSOR 1 to the PC stakes. Wait a minute or so until the sensor sensor cool after soldering, then install the shorting jumpers JP1 and JP2. Re-apply power and adjust trimpot VR5 on the display PC board for best display contrast. Check that the display shows BATTERY AMP HOUR and <60Ah>. Also, press the Set switch and check that the display now shows BATTERY TYPE and <LEAD ACID>. Press Start and check that the display shows BULK and 26 Deg C 0.0V 0.0A (for example). Next, adjust trimpot VR3 so that the display shows the same temperature as the ambient (this can be measured using another thermometer). Note that the display reads in 2°C increments, so set the display to the nearest value available. This may need to be rechecked to be sure the reading tracks the ambient value correctly. To test the Power PC board, apply between 12V and 25V DC to the +25V and GND pins. That done, check the voltage between pins 8 & 4 of IC1 and IC2 – this voltage should be similar to the input supply. The voltage across ZD3 should be close to 5.1V if the supply is around 25V but may be lower than this if the power supply is only 12V. Drilling the case You will need to drill quite a few holes and make cutouts in the case. We should note at the outset that the case used in our prototype has a drawback 5-Band Code (1%) brown black black yellow brown white brown black orange brown brown black black orange brown yellow orange black red brown red red black red brown brown black black red brown green blue black brown brown orange orange black brown brown brown grey black brown brown brown black black brown brown orange orange black black brown red violet black black brown brown green black black brown brown red black black brown brown black black gold brown Table 2: Capacitor Codes Value 470nF 220nF 100nF 1nF μF Code 0.47µF 0.22µF 0.1µF .001µF EIA Code 474 224 104 102 IEC Code 470n 220n 100n 1n in that its bottom panel has a section of mesh right where we wanted to mount the transformer. This means that it needs an additional large metal plate underneath to provide secure anchoring for the transformer bolt. On the rear panel, holes and cutouts are required for the fan and finger guards, mini XLR socket, bridge rectifier BR2, blade fuseholder, cable glands for the charger leads, mains cord-grip grommet and the M205 safety fuseholder. The fan is mounted towards the far edge of the panel. It requires a 75mm diameter cutout and this is made by drilling a series of holes around the perimeter and then knocking out the centre piece. File the cutout to shape. On the front panel, cutouts and holes are required for the fan outlet and finger guard mounting, the LCD window, the control and power switches and the mounting pillars for Display PC board. The latter holes for the PC board pillars need to be countersunk, to suit countersink screws. The LCD window cutout required siliconchip.com.au Fig.6: install the parts on the three PC boards as shown in these parts layout diagrams. Take care to ensure all polarised components are correctly orientated, including switches S1-S4 on the display board. siliconchip.com.au December 2004  31 Fig.7: this diagram shows the hardware codes (above) for the main wiring diagram (Fig.8). At right are the wiring connections for an alternative transformer which is available from Jaycar and Altronics. is 67 x 19mm, suitable for fitting the Perspex window. The fan cutout is half a circle and this is on the side where the heatsink is positioned. The cutout position for the LCD window depends on the particular module that is used. Fit the front panel label and the Perspex window to the front panel. The front panel can be assembled now. Insert the M3 x 5mm Nylon countersunk screws for the Display PC board mounting and secure these Fig.8: the mains earth solder lug must be securely attached to the metal chassis as shown here. Tighten the first nut very firmly before winding on the second “lock nut”. Note: scrape away any paint or powder coating from around the mounting point, to ensure a good connection. 32  Silicon Chip with the 10mm tapped standoffs. The Display PC board is attached to these screws and held in place with M3 nuts. The fan guard is secured with M3 x 10mm screws and M3 nuts, while the the mains switch is simply clipped into its rectangular cutout (make sure that it is a tight fit, so that it doesn’t come adrift. Holes in the box baseplate are required for the mains earth solder lug mounting, the transformer mounting bolt and for mounting the PC boards and the heatsink. In addition, two holes are required for the 3-way mains terminal block. Our prototype box included side panels which are used to increase the case height and these have side flanges. The panel for the transformer side of the box needs to have these flanges removed. We used a nibbling tool to cut these flanges away and filed the edges down to a smooth finish. The flanges on the fan side also need to be removed near the fan to allow it to be positioned without fouling. In addition, the front panel requires a 50 x 4mm notch adjacent to where the heatsink mounts on the top and bottom mounting flanges. These are required to allow the heatsink to fit without touching the panel. Insulated heatsink mounting An important aspect of the charger design is the heatsink which is completely insulated from the case. This was done so that no mica or silicone washers are required when mounting the bridge rectifier and the five power transistors (Q1-Q5) and this maximises heat transfer from those components. As well, it greatly simplifies the high-current power connections to the collectors of the five power transistors. The +18V output from the main bridge rectifier simply connects to the heatsink and since all the transistor collector tabs are also bolted to the heatsink, that makes the connection. Simple – but you do have to make sure the heatsink is properly isolated, so there is no chance of shorts between it and the case. The heatsink requires tapped holes to mount the bridge rectifier (BR1), the two thermostats, Q1-Q5 and the positive supply connection. In addition, the top and bottom surfaces require six holes each. siliconchip.com.au Fig.9: this is the main wiring diagram for the battery charger. Be sure to use heavy-duty cables where indicated (see text) and take care with the mains wiring. The terminals of switch S5 should all be insulated (see text), as should the terminals for Fuse F1 (using heatshrink). Once the wiring is in place, it should be secured using cable ties, as shown in the photos. siliconchip.com.au December 2004  33 This view shows how the parts are arranged on the rear panel and also shows how the display board is mounted. Be sure to insulate the mains switch terminals and use cable ties to secure the wiring. A sheet of 0.5mm plastic insulating sheet (225 x 45mm) is fixed to the top and bottom surfaces of the heatsink to ensure that it cannot short to either the base or the lid of the case. The bottom holes are tapped M4, so that the heatsink can be attached to the baseplate using Nylon M4 screws (the plastic sheet is sandwiched between the heatsink and the base of the case). The holes in the top of the heatsink are tapped M3 so that the second 225 x 45 x 0.5mm plastic sheet can be secured using M3 countersunk Nylon screws. Do not use glue to secure the plastic sheeting. Fig.10 shows the drilling and tapping details for the heatsink. Again, if you are building this project from a kit, you will not have to worry about and drilling and tapping the heatsink. Attach the thermal cutouts to the heatsink using a smear of heatsink compound to the back of the sensor before securing them with M3 x 10mm screws and star washers. That done, install the heatsink in the case. Check 34  Silicon Chip that it is isolated from case by measuring the resistance between them with a multimeter – the meter should indicate an open circuit. Installing the power board Next, install the PC board standoffs onto the baseplate using the M3 tapped 6mm Nylon standoffs and the M3 x 15mm screws and place the Power PC board in place. That done, insert the five power transistors (Q1-Q5) and adjust their height above the PC board so that the holes in the transistor tabs line up with their respective tapped holes in the heatsink. You can then carefully lift the board out and solder the centre leg of each power transistor. When you’ve done this, replace the board and re-check alignment before soldering the other transistor leads. The power transistors can now be bolted to the heatsink. To do this, first, apply a smear of heatsink compound to their mounting faces, then secure the Power PC board in place with M3 nuts. The power transistors can then be fastened to the heatsink using M4 x 10mm screws and star washers. Q1 (BD649) is secured using an M3 x 10mm screw and star washer. Similarly, apply heatsink compound to the back of bridge rectifier BR1 before securing it in place with an M4 x 15mm screw and star washer. Make sure it is oriented with its positive (“+”) terminal positioned as shown. The Control PC board can now be attached to the baseplate using 6mm standoffs, M3 x 15mm screws and M3 nuts. Note that the lower right mounting point connects the 470nF capacitor to chassis via a solder lug. Check that this is earthed using a multimeter between chassis and this earth point (you should measure this as a short circuit). Note that one of the extra securing points for connecting the rear panel to the baseplate is located beneath where the fan mounts. The M3 securing nut for this should be glued in place so as to make assembly easier. Fig.9 shows how the hardware is installed on the rear panel. Secure the bridge rectifier (BR2), the blade fuseholder, the mini XLR panel socket, siliconchip.com.au Fig.10: here are the drilling and tapping details for the heatsink. It is completely isolated from the chassis, to avoid using insulation washers for the power devices. siliconchip.com.au December 2004  35 Parts List – Battery Charger 1 Power PC board coded 14111041, 224 x 77mm 1 Control PC board coded 14111042, 92 x 69mm 1 Display PC board coded 14111043, 141 x 66mm 1 front panel label 1 88.1mm high x 279mm deep x 304mm wide metal case (Jaycar HB-5556 or equivalent) 1 240V to 18V 300VA mains toroidal transformer (2 x 9V or 2 x 18V secondaries) (T1) 1 fan-assisted heatsink, 225 long (Jaycar HH-8530 or equivalent) 1 80mm 12VDC 2.4W fan 2 80mm fan finger guards 1 NO 50°C thermostat switch (TH1) 1 NO 70°C Thermostat switch (TH2) 1 M205 panel-mount safety fuseholder (F1) (Altronics S 5992 or equivalent) 1 3A M205 slow blow fuse 1 30A chassis blade fuseholder (F2) (Altronics S 5970 or equivalent) 1 20A blade fuse 10 100mm long cable ties 6 20 x 20 x 8mm large adhesive rubber feet 2 4-8mm waterproof cable glands 1 cordgrip clamp for mains cord 1 7.5A mains cord and plug 1 3-way 10A terminal strip 2 50A insulated battery clips (1 red, 1 black) 2 2.54mm jumper shunts (JP1,JP2) 2 2-way header terminal strips 1 LCD Module (DSE Z 4170, Altronics Z 7000A or Jaycar QP 5515) 1 14-way SIL header strip for Altronics and DSE LCD module the fan and the fan guard, as shown in this diagram. That done, secure the cable glands for the battery leads and the M205 fuseholder. The fan should be orientated so that it blows air inside the case. If you look closely, you will see arrows on the fan that indicate the blade direction and airflow (ours was installed with the labelled side facing inside the box). Note that we have specified extra 36  Silicon Chip 1 14-way DIL header strip for Jaycar LCD module 1 6-way polarised header plug 1 6-way polarised right angle header plug 2 6-way polarised header sockets 1 4-way polarised header plug 1 4-way polarised right angle header plug 2 4-way polarised header sockets 6 5.3mm ID eyelet crimp connectors suiting 6mm wire 9 female insulated 6.4mm spade connectors suiting 4.8mm wire 2 female insulated 6.4mm spade connectors suiting 6mm wire 1 solder lug 1 mini XLR 3-pin line plug 1 mini XLR 3-pin chassis mount socket 1 SPST Neon illuminated 240V 6A rocker switch (S1) 4 White SPST PC board mount tactile snap action switches (S2-S5) 1 18-pin DIL IC socket 10 M3 tapped x 6mm Nylon standoffs 5 M3 tapped x 10mm standoffs 3 M4 x 15mm screws 12 M4 x 10mm screws 6 M4 x 12 Nylon screws 5 M4 nuts 19 M4 star washers 12 M3 x 15 screws 5 M3 x 15mm Nylon countersunk screw 12 M3 x 10mm screws 6 M3 x 6mm Nylon countersunk screws 10 M3 nuts 12 M3 star washers 2 M2.5 x 12mm screws 2 M2.5 nuts 23 PC stakes 1 50 x 50mm piece of Prespahn star washers in the parts list. This is so that you can place them under the screws attaching the panels to assemble the case. The star washers will bite into the metal to ensure the panels are earthed correctly to the baseplate. Note also that you should scrape away any paint or powder coating around the screw holes, to ensure good metal-tometal contact. Once the case has been assem- insulating material 1 67 x 19mm sheet of 2.5-3mm clear Acrylic or Perspex 2 225 x 45mm pieces of 0.5mm flexible sheet plastic to insulate heatsink top and bottom (eg thin plastic chopping mat) 1 225 x 80mm piece of 0.5mm flexible sheet plastic to insulate below the power PC board 2 30 x 70mm pieces of 0.5mm flexible sheet plastic for covering ventilation holes in fan airway tunnel 1 1.5m length of single core shielded cable 1 200mm length of 6-way rainbow cable 1 200mm length of 4-way rainbow cable 1 200mm length of 0.7mm tinned copper wire 1 1.5m 3.3mm squared 30A red automotive wire (12G) 1 1.5m 3.3mm squared 30A black automotive wire (12G) 1 500mm length of 24 x 0.2mm red hookup wire 1 500mm length of 24 x 0.2mm black hookup wire 1 200mm length of 32 x 0.2mm brown hookup wire 1 200mm length of 32 x 0.2mm blue hookup wire 1 100mm length of 16mm heatshrink tubing 1 100mm length of 10mm heatshrink tubing 1 50mm length of 6mm heatshrink tubing Semiconductors 1 LM358 dual op amp (IC1) 1 LM393 dual comparator (IC2) 1 4051 analog 1 of 8 selector (IC3) 1 TLC548 8-bit serial A/D converter (IC4) 1 PIC16F628A-20P microcontroller bled, each separate panel should be checked for a good connection to the baseplate using a multimeter set to read low ohms. Don’t skip this step – it’s vital to ensure that all panels are correctly earthed, to ensure safety. Wiring Fig.9 shows all the wiring details. First, strip back 250mm of the sheathing at the end of the 3-core mains cable. That siliconchip.com.au programmed with battchrg.hex (IC5) 1 BD649 NPN Darlington transistor (Q1) 4 TIP3055 NPN power transistors (Q2-Q5) 1 LM335 temperature sensor (SENSOR1) 1 50A 600V bridge rectifiers (BR1) 1 35A 400V bridge rectifier (BR2) 3 1N4004 1A rectifiers (D1-D3) 6 1N4148 diodes (D4-D9) 2 15V 5W zener diodes (ZD1,ZD2) 1 5.1V 1W zener diode (ZD3) Capacitors 1 2200µF 50V PC electrolytic 1 220µF 50V PC electrolytic 4 100µF 25V PC electrolytic 1 22µF 63V PC electrolytic 11 10µF 35V PC electrolytic 2 1µF 35V PC electrolytic 1 470nF 50V MKT polyester 1 220nF 50V MKT polyester 1 100nF 50V MKT polyester 2 1nF 50V MKT polyester Resistors (0.25W 1%) 1 1MΩ 4 1kΩ 1 910kΩ 1 1kΩ 0.5W 1 100kΩ 1 330Ω 1W 1 43kΩ 1 270Ω 4 22kΩ 1 150Ω 5 10kΩ 1 120Ω 1 5.6kΩ 1 56Ω 5W 2 3.3kΩ 1 10Ω 1 1.8kΩ 4 0.1Ω 5W 1 0.005Ω 3W 1% resistor, Welwyn OAR-3 0R005 (Farnell 327-4706) Trimpots 3 10kΩ horizontal trimpots (VR1,VR2,VR5) 1 20kΩ horizontal trimpot (VR3) 1 200Ω horizontal trimpot (VR4) Miscellaneous Heatsink compound, solder. done, secure the cord into the back panel using a cord-grip grommet. This grommet needs to be really tight, so that the cord cannot be pulled out. Cut the Active (brown) and the Earth (green/yellow) wires to 70mm length. The earth wire connects to the solder lug – it should be soldered in place rather than crimped to ensure a good earth connection is made. Also, scrape away the paint from the earth position on the baseplate to siliconchip.com.au Fig.12: here’s how to wire the temperature sensor. It’s protected by covering it with heatshrink tubing. ensure a reliable connection to the case and use an M4 x 15mm screw, a star washer and an M4 nut to attach the lug in place. A second M4 lock nut is then fitted, so that the assembly cannot possibly come undone. Now measure the resistance between chassis and the earth pin on the mains plug. This should be zero ohms. If not, re-check the connections to chassis. Check also that you get a zero ohm reading between the earth pin of the mains plug and all case panels. The Active lead is connected to the fuseholder by first passing the wire through a 50mm length of 16mm heatshrink tubing and then soldering it in place. The other terminal of the fuseholder also passes through the heatshrink tubing before it is soldered in place. Finally, slip the heatshrink tubing over the fuseholder before shrinking it down with a hot-air gun. Note: be sure to use a safety fuseholder for fuse F1 (see parts list), so that there is no danger of receiving a shock if the fuse is removed while the unit is plugged into the mains. Transformer mounting Typically, the mains transformer will be supplied with two circular rubber washers, a dished metal mounting plate and a mounting bolt. As noted previously, the prototype’s case required an additional plate underneath to provide secure anchoring for the transformer bolt. The 3-way mains terminal block is placed over a 50 x 50mm piece of prespahn insulating material and is held in place using two M3 x 15mm screws and M3 nuts. The wiring to the mains side of the power transformer depends on its windings (the power transformer will be supplied with one of two different winding arrangements). Our transformer came from Oatley Electronics and has 2 x 120V windings and 2 x 9V windings. This requires the two 120V windings to be connected in series, suitable for a 240V mains input. The two 9V windings need to be connected in series to obtain 18V. Other transformers (eg, from Altronics and Jaycar) will have a single 240V winding and two 18V windings. The 18V windings will need to be connected in parallel. The different wiring arrangements are shown in Figs.7 & 9. Use 250VAC-rated wire to connect between the power switch terminals and the terminal block. Insulated 6.3mm crimp spade lugs make the connections to the switch, while the remaining wiring is as shown in Fig.9. Note that heavy-duty 12-gauge wire is used for the connections to BR1 and BR2 and for the screw terminals on the PC board via crimp eyelets. The battery leads are also run using 12-gauge wires. These leads must be exactly 880mm long and they pass through the cable glands in the rear panel and are terminated to the battery clips. The battery clips we used require the jaw to first be removed and the wire then passed through the handle of its plastic clip before the lead is soldered. Note: if you want leads longer than 880mm, you can use heavier gauge wire so that you get 0.01Ω total resistance in both the positive and negative leads (this exact resistance is required for the control circuit to accurately calculate the voltage loss in the battery leads). For example, you can use 2m each of 8-gauge wire (8mm2). Alternatively, the voltage sensing lead that connects to fuse F2 inside the unit can be extended to the full length of the charger lead and connected to the positive battery clip. This provides remote sensing in the positive lead. In this case, the negative lead could either be 1.76m long using 12-gauge wire or 4m long using (thicker) 8-gauge wire. Heavy-duty hookup wire is used to make the remaining connections to the PC boards, except for the shielded cable used for the temperature sensor lead. This runs from the back of the mini XLR socket to the control PC December 2004  37 board. Cable ties are used to hold the wires together and stop them from coming adrift. This is important for the mains wires – use cable ties to secure the wires at the rear of fuse F1, at the mains switch and at the terminal block. Temperature sensor The temperature sensor (Sensor 1) is mounted at the end of a 900mm length of shielded cable. It can be inserted into a 5mm inside diameter tube and covered in heatshrink tubing. Alternatively, wire the sensor and cover it in heatshrink without the tube. Make sure the probe does not have exposed metal that can short to the battery terminals or to the case. The other end of the wire connects to pins 1 and 3 of a mini XLR plug. Use the shield connection for pin 1. Final tests Do a thorough check of all your wiring, then fit the lid on the case before applying power. Check that the power switch lights up when on. The LCD should also be on. Next, disconnect power and remove the lid. That done, reconnect power and measure the supply voltage between the GND pin and pin 8 of both IC1 and IC2. These should be around 30V and 25V respectively. Check also that ZD3 has 5.1V across it. Adjust VR1 so that the voltage between TP1 and TP GND is 1.7V, then adjust VR2 for 1.8V between TP2 and TP GND. This sets the circuit over-voltage and over-current levels to 17V and 18A respectively. The over-voltage adjustment sets the maximum allowable voltage when charging during equalisation. You may wish to raise this to allow the equalisation to operate for the full three hours. Alternatively, you can reduce the value to prevent damage to any equipment connected to the battery during equalisation. Note that the over-voltage value is restricted to 10-times the TP1 voltage. Final points Fig.11: this artwork can be used as a drilling and cutout template for the front panel. Note that the display cutout position shown here is for the Jaycar LCD and is different for the Altronics and DSE LCDs. You can also download a copy of this artwork from www.siliconchip.com.au and print it on polyester “Scotchmark” sheet using a laser printer or photocopier (available from www.wiltronics.com.au). 38  Silicon Chip The cooling fan will cycle on and off during charging, particularly at the higher currents. To ensure adequate cooling, the air inlet at the rear of the unit should not be blocked. When using the charger, make sure that the battery clips are correctly connected to the battery terminals. Check the Ah setting for the battery. Remember that a battery with a reserve capacity (RC) rating will need this value to be multiplied by 0.42 to convert it to the Ah capacity. Also, be sure to set the correct battery type. At the start of bulk charging, it will take a few seconds to bring the current up to the 25% of Ah current. Note that an already charged battery will cycle through to the float charge in a short space of time. This fast cycling through to float can also indicate a faulty battery, if it has not been charged recently. The temperature sensor does not necessarily need to be placed on the battery case during charging. In most cases, the sensor can be located adjacent to the battery, to monitor the ambient temperature. However, the sensor does need to be placed on the battery if it has been brought to the charger from a different temperature environment, such as a cold room. You can secure the sensor to the battery using masking tape. Alternatively, you can use adhesive-backed Velcro material if the battery SC is to be charged regularly. siliconchip.com.au