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Charge SLA batteries away from the mains Want to charge a sealed lead acid (SLA) battery away from home? This simple project lets you use your car or boat battery to automatically – and safely – charge 12V SLA batteries. By JOHN CLARKE Sealed Lead Acid (SLA) batteries are used in a host of devices: cam­ corders, spotlights, toys, portable TVs, communications equipment and even go-anywhere vacuum cleaners . . . and these are only a few applications which spring to mind. SLA batteries are great when you’re at home or close to a power outlet and charger. But charging them when you’re miles (or even kilometres) from home? – that’s a different matter! 54  Silicon Chip Despite what many people believe, you cannot simply connect a 12V SLA battery to your car or boat battery via a current limiting resistor and expect it to charge properly. The reason for this is that there is insufficient potential difference between the two batteries to fully charge the SLA battery. What’s more, even when such a system is used to partially charge an SLA battery, it requires constant monitoring to ensure that the battery isn’t cooked by too high a charging current. Main Features • Powered from 12V battery • 2A average current limit • Suitable for 12V 6.5A.h & greater capacity SLA batteries • Efficient switchmode desig n • Fuse protected • Reverse polarity protectio n • Power indication Many readers have asked for a safe, reliable means of charging SLA batteries from vehicle or boat batteries, which is the reason this project was developed. Whether camping, boating, travelling or off-roading, this charger will come in handy. Fig.2: inside the Motorola MC34063 DC-DC converter IC which forms the heart of the circuit. Fig.1: the basic operation of the DC-DC converter. It connects to the car or boat battery and directly charges any SLA battery with a capacity of 6.5Ah or more to an endpoint of 13.8V, without the need for constant monitoring. The charger will initially supply over 2A to a discharged battery and this current will gradually decrease as the battery voltage reaches 13.8V. Another application is as a solar battery charger, using 12V panels. The circuit will step-up the voltage from the panels when it drops below 12V and thereby improve overall effic­ iency. This design effectively super­cedes the design published in Nov­ember 1991. Basic operation In operation, the SLA battery charg­er steps up the voltage from the battery using a DC-to-DC converter. Fig.1 shows the basic principle of the step-up circuit. When switch S1 is closed, current I1 flows through inductor L1. When S1 opens, the field collapses and an induced current, I2, flows through the load via D2. C1 is also charged at this time and discharges through the load when S1 is closed. By using a transistor or Mosfet in place of S1 and by monitoring the output voltage across the load, we can adjust the on and off times for the switching so as to provide a constant voltage output. A Motorola MC34063 DC-DC converter IC has been used to control this operation. This IC contains all the necessary circuitry to produce either step-up, step down or an inverting DC converter. Its principal sections are a 1.25V reference, comparator, oscillator, RS flipflop and a Darlington transistor pair (Q1 and Q2) – see Fig.2. The frequency of operation is set by a capacitor on pin 3. A 0.001µF cap­ acitor, for example, will set it running at about 30kHz. The oscillator drives the flipflop which in turn drives the Darlington transistor. Excess current is sensed at the current peak input (pin 7) and this switches off the flipflop and Darling­ton transistor, to bring the current under control. The on time for the Darlington transistor is set by the comparator. This is used to monitor the output voltage. When the pin 5 comparator input exceeds the 1.25V reference, the comparator goes low to keep the flipflop from setting and thus holds the Darl­ing­ton off. Conversely, if the output voltage is too low, the inverting input of the comparator will be below the 1.25V reference and so the Darlington can be toggled by the RS flipflop at the rate set by the oscillator. The complete circuit Fig.3 shows the full circuit diagram. The Darlington emitter at pin 2 drives the gate of Mosfet Q1 while the 120Ω resistor turns off the gate whenever the Darlington is off. Note that the Darlington collectors at pins 1 and 8 are connected to the positive supply (pin 8 via a 47Ω resistor). A 0.1Ω resistor between pins 6 & 7 sets the peak current delivered to the Fig.3: The similarities between the complete circuit and that of Fig. 1 are obvious, with the IC and Q1 effectively replacing the switch and its functions. July 1996  55 This view inside the assembled SLA charger shows how the PC board clips into place using the integral side pillars. Note that inductor L1 is secured to the board with cable ties ­– don't rely on its leads to hold it in place. inductor to 0.3V/0.1Ω, or 3A peak. The average current supplied to the load via D2 is limited to a little under 2A. A 0.68µF capacitor at the output filters the voltage before it is applied to the SLA battery. Voltage regulation is provided by the 22kΩ and 2.2kΩ voltage divider resistors connected to pin 5. When the output voltage is 13.8V, the voltage at pin 5 is 1.25V. Since this is the reference voltage on the internal comparator (see Fig.2) the IC will maintain 13.8V at the output. The 0.1µF capacitor at pin 5 removes transient voltages which could cause the IC to behave erratically. Diode D1 has two purposes. First, it provides reverse polarity protection for the circuit. This may be of no consequence if a cigarette-lighter plug is used to obtain the battery voltage. However, if clip leads are used, then reverse polarity is a distinct possibility and protection is useful. The second purpose for D1 is to prevent overcharging. This can happen with a step-up circuit when the input voltage rises above about 14V. At this point, the FET will be permanently turned off and so there is a direct path to the batter via D1, L1 and D2. However, the resulting drop across 56  Silicon Chip D1 and D2 (approx. 1.2V) will limit the voltage applied to the SLA battery. Finally, fuse and zener diode protection has been included to limit the short circuit current and prevent transient voltages damaging IC1. LED1 provides power indication. Construction The Silicon Chip 2A SLA Battery Charger is housed in a plastic case measuring 130 x 68 x 42mm. The components are mounted onto a PC board coded 04305961 and measuring 103 x 60mm. A front panel label measuring 62 x 126mm affixes to the top lid. Begin construction by checking the Both power diodes and the Mosfet are mounted on finned heatsinks. There is no need for insulating bushes or washers but make sure that the leads do not contact the heatsink. PC board for shorted tracks or small breaks, then insert all the PC stakes. These are located at the four external wiring points on the PC board. Next, insert and solder in all the resistors, using the accompanying table as a guide to the colour codes. This done, insert the IC and zener diode. The capacitors are next: there are no polarity-conscious capacitors in this circuit so their orientation is unimportant. However, the fuseholder clips must be inserted correctly, otherwise the fuse will not clip in. It is best to fit the fuse into the clips before inserting them into the PC board. D1, D2 and Q1 are each mounted horizontally on the PC board with a heatsink and secured with a screw and nut. Bend the leads for each component at right angles before mounting these devices. LED1 is mounted on the end of its leads so that it will later protrude through the front panel. Inductor L1 is wound with 1mm enamelled copper wire on a ferrite toroid. Draw half the length of wire through the centre of the core and wind on 22 turns neatly side by side. The direction is unimportant. Now, using the other end of the wire, wind on another 22 turns so that the toroid has 44 turns neatly wound around the core. The windings are terminated into the PC board holes as shown on Fig.4. Make sure that the wire ends are stripped of insulation before soldering. The insulation can be scraped off with a knife or melted off with a hot soldering iron. L1 is secured in place with two cable ties which loop through holes in the PC board and around opposite sides of the toroid. Final assembly The assembled PC board can now be installed in the case. First, affix the label to the front panel and drill out the holes for the LED and switch S1. You will also need to drill out holes in the top and bottom of the base of the case to accept the cable grommets. This done, place the PC board in the case and test the lid to check that the LED passes through its front panel hole. Adjust the height of the LED if necessary, so that it just protrudes through the lid. Next, connect the lighter plug to a length of twin automotive wire or heavy-duty figure-8 cable. This done, Fig.4: install the parts on the PC board as shown on this wiring diagram. Fig.5: check your board against this fullsize pattern before installing the parts. pass the other end of the lead through a grommet and terminate the wires to the PC board and S1 as shown. Similarly, connect the battery clips to one end of the second length of twin automotive wire, pass this wire through the second grommet and connect the leads to the output terminals on the PC board. You are now ready to test the unit. Apply power from a 12V supply (or battery) and check that the LED lights. If it doesn’t, check that the LED is oriented correctly and that the supply is connected with the correct polarity. Now measure the voltages on IC1 with a multimeter. There should be about 12V across pins 4 and 6. Now connect a 470Ω resistor (or there­ abouts) in parallel with a 100µF 16VW (or larger) electrolytic capacitor across the output terminals (positive of capacitor to positive terminal, negative to negative) and check that the output voltage is about 13.8V. Note that some power supplies will not cope well with the battery charger since it draws high RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ No. 1 2 1 1 Value 22kΩ 2.2kΩ 120Ω 47Ω 4-Band Code (1%) red red orange brown red red red brown brown red brown brown yellow violet black brown 5-Band Code (1%) red red black red brown red red black brown brown brown red black black brown yellow violet black gold brown July 1996  57 PARTS LIST SILICON CHIP SOFTWARE Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. ORDER FORM Semiconductors 1 MC34063 DC-DC controller (IC1) 1 MTP3055E, BUZ71 60V Mosfet (Q1) 2 BY229 fast recovery diodes (D1,D2) 1 16V 1W zener diode (ZD1) 1 5mm red LED (LED1) PRICE ❏ Floppy Index (incl. file viewer): $A7 ❏ Notes & Errata (incl. file viewer): $A7 ❏ Alphanumeric LCD Demo Board Software (May 1993): $A7 ❏ Stepper Motor Controller Software (January 1994): $A7 ❏ Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7 ❏ Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7 ❏ Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7 ❏ Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7 ❏ I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7 Capacitors 2 100µF 16VW electrolytic (for testing) 2 0.68µF 250VDC MKT polyester 1 0.1µF 63VW MKT polyester 1 .001µF 63VW MKT polyester Resistors (0.25W, 1%) 1 22kΩ1  120Ω 1W 2 2.2kΩ 1 47Ω 1 470Ω (for testing) 1 0.1Ω 5W POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5 Disc size required:    ❏ 3.5-inch disc   ❏ 5.25-inch disc 1 PC board, code 14305961, 103 x 60mm 1 plastic case, 130 x 68 x 42mm 1 front-panel label, 62 x 126mm 1 SPDT toggle switch (S1) 3 TO-220 mini heatsinks (19 x 19 x 9.5mm) 3 3mm screws and nuts 2 M205 PC board fuse clips 1 3A M205 fuse 1 battery clip, red 1 battery clip, black 1 cigarette lighter plug (or two more battery clips) 2 cable ties 2 cord grip grommets 1 Neosid 17-742-22 iron powder ring core 4 PC stakes 1 1.5-metre length of 1mm enamelled copper wire 1 3-metre length of 5A twin red/ black automotive cable TOTAL $A Enclosed is my cheque/money order for $­A__________ or please debit my Bankcard   ❏ Visa Card   ❏ MasterCard ❏ Card No. Signature­­­­­­­­­­­­_______________________________ Card expiry date______/______ Name ___________________________________________________________ PLEASE PRINT Suburb/town ________________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number (Bankcard, Visa Card or MasterCard). 58  Silicon Chip ✂ ✂ Street ___________________________________________________________ current pulses. However, a 100µF capacitor connected to the power supply terminals will usually prevent the supply from going into overload. If this fails, use a 12V battery instead. Now you can test the charger on an SLA battery. Connect the charger to the lighter socket in your car and to the SLA battery and check that the battery charges to 13.8V. Check the temperature of D1, D2 and Q1. They SC should run warm.