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AMATEUR RADIO By GARRY CRATT, VK2YBX Build this intelligent charger for 12V gel batteries Do you use 12V gel cells to power your transceiver or other equipment? If so, you need to know the best way to charge them so that they last as long as possible. The "intelligent charger" described here does all the right things to look after the welfare of your 12V gel cells. When it comes to charging sealed lead acid batteries used in an amateur station, most operators use their standard 13.BVDC power supply. There is a big drawback to this method because, by definition, a 13.BV DC power supply puts out a constant 13.8 volts. Such a power supply may deliver a very much higher current to a discharged or partially discharged battery than is advisable. In fact, gel cell or sealed lead acid batteries are very fussy about being over-charged, even at low rates. They should not be charged SOURCE COMPENSATION 15 14 Charging current +VIN ,------6-----6-----< r + l - - - 0 1 3 VOLTAGE SENSE ....,,...--1----012 CHARGE ENABLE POWER 7 INDICATE 9 OVER-CHARGE INDICATE OVER-CHARGE B TERMINAL Fig.1: block diagram of the UC3906 IC. It controls both the output voltage and charging current to ensure optimum charging conditions. 66 SILICON CHIP at a constant current or 'trickle charged' unless the terminal voltage of the battery is monitored and the charging current is terminated immediately full charge is reached. Voltage limited charging is most suitable for this type of battery. The correct charging procedure for sealed lead acid batteries is to charge them at the optimum current, until a maximum voltage of 2.42 volts per cell is reached. This equates to a voltage of 14.52V for a "12V" battery comprising 6 cells. At this level, the charging current should drop to a level sufficient to maintain the full charge voltage of 13.68V (2.28V per cell). The "optimum" current depends on the battery temperature, its state of discharge and lastly, the battery capacity which is expressed in ampere/hours. Typically though, the recommended charging current varies from a maximum of C/5 to C/10 where C is the capacity expressed in ampere hours at the 20-hour rate. For example, a 4.5 amp/hour battery can deliver 225mA at the 20-hour rate. At C/5, the charging rate would be 900mA. At C/10, the charging rate would be 450mA. Provided sealed lead acid batteries are carefully charged and are not over-charged to the point where significant gassing occurs, they can have very long life in standby or "floating" use where they sit across a fixed power supply. They can last between 5 to 10 The parts are installed on a small PC board fitted with quick-connect lugs for the input and output terminals. It delivers a maximum charging current of 500mA. A flag heatsink must he fitted to Qt for input voltages greater than 20V. R1 0.5P. 5W + SUPPLY INPUT -i 01 1N5404 BATTERY R3 1k 5 3 16 i- R2 68k 1% 15 11 12 R4 22k IC1 UC3906 1% 13 R5 10 270k 1% R6 18k 1% .,. 14 BCE 12V GEL BATTERY CHARGER Fig.2: this circuit shows the UC3906 connected as a dual level float charger. It's pin 16 output controls PNP power transistor Qt which in turn handles the charging current. years. However, designing a charger for such a standby application is not an easy task, at least until recently. Fortunately, there is now an "intelligent" battery charging chip available, which has been designed specifically for this purpose by Unitrode Corporation of the USA. The UC2906 and UC3906 battery charger controllers contain all the necessary circuitry to optimally control the charge and hold cycle for sealed lead acid batteries. These integrated circuits monitor and control both the output voltage and current of the charger through three separate charge states: a high current bulk charge state, a controlled over charge, and a precision float charge or standby state. Optimum charging conditions are maintained over an extended temperature range with an internal reference that tracks the nominal temperature characteristics of the lead acid cell. Separate limit amplifiers regulate the output voltage and current levels in the charger by controlling the onboard driver. The driver will supply up to 25mA of base drive to an external power transistor. Voltage and current comparators are used to sense the battery condition and provide inputs to the charge state logic. In addition, a charge enable comparator with a trickle bias output can be used to obtain a low current turn-on mode for the charger. This feature prevents high current charging during abnormal conditions such as a shorted battery cell. Other features include a supply under-voltage sense circuit with a logic output to indicate when input JULY 1989 67 r------------._ AMATEUR RADIO; Hobbyists communicating world wide using state-of-the-art electronics. I I I I Are you into computers? ; Like to access BBS around 1 the world by radio? I Interested in different forms• of digital communication - AMTOR - PACKET? WHY NOT BECOME A RADIO AMATEUR? Want to know more? I Join the WIA - the oldest and most experienced radio society in the world - always at the forefront of radio communications for hobbyists. Receive AMATEUR RADIO, the monthly magazine for members of the WIA, full of news of DX, clubs, satellites, technical articles and lots more. • • • • • • Other WIA services include: A world wide QSL card service Weekly news broadcasts Classes for all grades of amateur licences Correspondence lessons available Meetings, contests, field days Representation for radio amateurs at Government level STATE 2 l STATE 3 I IT !OCT ,..L, }-vT IMAX STATE 1 : BULK CHARGE STATE 2 : OVER•CHARGE STATE 3 : FLOAT CHARGE CHARGER OUTPUT CURRENT Fig.3: the circuit starts off in state 1 (bullc charge) and then switches to state 2 (over-charge) and finally to state 3 (float charge). power is present. In addition, the over-charge state of the charger can be externally monitored and terminated using the over-charge indicate output and the over-charge terminate input. Fig.1 shows the block diagram of the UC3906. It comes in a standard 16-pin dual inline plastic package. Maximum input voltage is 40 volts DC. A practical circuit Fig.2 shows the UC3906 connected as a dual level float charger. CHARGE VOLTAGE ----::---.---:svoc O ~-----:F,-;;;;~V31 -r--------:---,--·:~---------, G IT Registered address: 3/105 Hawthom Road Caulfield North, 3161 Please send aWIA information package to: NAME: ........................................................... . ADDRESS: ..................................................... . STATE LEVEL OUTPUT oc INDICATE OUTPUT oc TERMINATE INPUT (C/S OUT) ~~] __ _ I OFF ................................... POSTCODE ............... .. L-------------.1 E001qii I ""~---- i - SILICON CHIP Ail high currents are handled by the external PNP pass transistor Ql which is a readily available plastic pack MJE2955. This circuit uses the trickle bias output at pin 11 and the charge enable comparator at pin 13 to give the charger a low current turn-on mode. The output current of the charger is limited to a low level until the battery reaches a specified voltage, preventing high current charging if a battery cell happens to be shorted. Of course, if you did have a battery with a shorted cell, you would have to discard it. Fig.3 shows the various charge states of the circuit. At switch-on, the charger goes into state 1, the high rate bulk charge state. In this state, once the enable threshold has been exceeded, the charger will supply a peak current that is determined by the 250mV offset of the current limit amplifier (monitoring between pins 5 and 4) and the sensing resistor Rl. Our circuit shows Rl with a value of 0.50 so the peak current value will be 500 milliamps. To guarantee full recharge of the battery, the charger's voltage loop has an elevated reguhiting level _J _______________ _ INPUT SUPPLY VOLTAGE CHARGE CURRENT P.O. BOX300 CAULFIELD SOUTH VICTORIA 3162 68 y STATE 1 Learn more about the WIA and Amateur Radio Forward this coupon, or write to: WIA EXECUTIVE OFFICE - - v oc - -v 12 - -v L r- - v 31 ~ STATE 1 .. 1. I i ~--.....,_J-_-STATE 2 I• STATE 3 • I. STATE 1 Fig.4: this diagram shows a charge and discharge cycle for a dual level float charger. Once the battery is charged, it is maintained at a precise float voltage (Vf). RESISTORS D D D D D D No. 1 1 1 1 1 1 Value 270k0 68k0 22k0 18k0 1k0 0.50 SW 4-Band Code not applicable pot applicable not applicable not applicable brown black red gold not applicable 5-Band Code red violet black orange black blue grey black red black red red black red black brown grey black red black brown black black brown brown not applicable Fig.5 (above): parts layout for the PC board. Fig.6 at right shows the actual size PC pattern. (Voc) during state 1 and state 2. When the battery voltage reaches 95% of Voc, the charger enters the over-charge state, state 2. The charger stays in this state until the "over-charge terminate" pin goes high. The charger uses the current sense amplifier to generate this signal by sensing when the charge current has tapered to a specified level, Ioct. So pin 1 is connected to pin 8. As an alternative, the over·charge terminate point could have been controlled by an external source, such as a timer, by using the "over-charge indicate" signal at pin 9. If a load is applied to the batte'ry and begins to discharge it, the charger will contribute its full output to the load. If the battery drops 10% below the float level, the charger will reset itself to state 1, the bulk charge condition. When the load is removed, a full charge cycle will follow. A graphical representation of a charge and discharge cycle of the dual level float charger is shown in Fig.4. When the charger is in the float state, the battery is maintained at a precise float voltage, Vf. The accuracy of this float state will maximise the standby life of the battery, whilst the bulk charge and over charge states guarantee rapid and full recharge. All of the voltage thresholds on the UC3906 are derived from the internal reference. This reference has a temperature coefficient that tracks the temperature characteristic of the optimum charge and hold levels for sealed lead acid batteries. This further guarantees that proper charging occurs, even at temperature extremes. External supply Because the series pass transistor, Ql, is a PNP type, the supply input does not need to be very much PARTS LIST 1 PCB, code SC 11107891, 77 x 41mm 4 quick connector lugs Semiconductors 1 UC3906 battery charger controller (IC1) 1 MJE2955 PNP transistor (01) 1 1N5404 power diode (D1) Capacitors 1 1 OOpF ceramic Resistors (¼W unless stated) 1 270k0 1 % 1 18k0 1 % 1 68k0 1 % 1 1kO 5 % 1 22k0 1% 1 0 .50 5W higher than the maximum output voltage of the charger. This means that the input voltage can range upwards from 15V DC, with 18-20V being ideal. The higher the input voltage to the circuit, the higher the dissipation in Ql. For the circuit as shown, which delivers a maximum of 500mA, no heatsink is required for Ql for input voltages up to a bout 20 volts DC. For higher voltages, a flag heatsink will be required. Construction To provide a basis for experimentation, we have designed a small printed circuit board measuring 76 x 40mm (coded SC 11107891). The component layout diagram is Fig.5. Our prototype board has been fitted with quick-connect lugs for the input and output terminals but these are optional. Note that all the resistor values, with the exception of Rl and R3, should be 1 % tolerance to make sure that the design targets for over-charge voltage and float voltage are obtained. For this design, the float voltage is 13.8V and the over-charge voltage is 14.56V. The UC3906 is available from VSI Electronics Pty Ltd, PO Box 578, Crows Nest, NSW 2065. Phone (02) 439 8622. ~ JULY 1989 69