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A look at the UC3906 SLA battery charger IC The Unitrode UC3906 is designed to correctly control the charging of sealed lead acid batteries so that over-charging is avoided and life is maximised. It compensates for the change in battery voltage with temperature so that overcharging is avoided, regardless of the ambient temperature. By DARREN YA TES In the July 1989 issue of SILICON CHIP, Garry Cratt featured the UC3906 battery charger in his Amateur Radio column. The circuit presented was suitable for 12V sealed lead acid (SLA) batteries and the article created quite a lot of interest. Since then, many readers have wanted to know how to use the circuit to charge 6V batteries and how to add other features . With this in mind, we are presenting this follow-up article on the maSINK 16 jor characteristics of the UC3906. We have also designed a more comprehensive charger circuit which is featured elsewhere in this issue. With careful use, sealed lead acid batteries can be expected to give a service life of 5 to 10 years. But if they are over-charged, which happens all too often with most charger circuits, their service life will be only a fraction of this figure. A typical car battery charger is not suitable for SLA batteries and will SOURCE COMPENSATION f5 14 +VIN 1---+--013 VOLTAGE SENSE , - - - - - - - U 1 1 TRICKLE I VREF BIAS 1----012 CHARGE ENABLE VREF POWER 7 INDICATE 9 OVER-CHARGE INDICATE OVER-CHARGE 8 TERMINAL Fig.1: internal circuitry of the UC3906. It monitors the battery voltage and switches to one of three charging modes: trickle, charge or float. 10 SILICON CHIP almost always result in overcharging. By the way, there is some confusion about "sealed lead acid" and "gel" batteries. Most manufacturers now call them sealed lead acid but still refer to the term "gel". Since the electrolyte in a SLA battery is in the form of a gel, we don't think there is much wrong with referring to them as gel batteries but since that term now seems to have fallen out of vogue, we will call them SLA batteries from now on. OK? Now back to the UC3906. Charge states Fig.1 shows the schematic diagram for the internal circuitry of the UC3906 while Fig.2 shows the chip connected into a circuit which is suitable for charging a 12V SLA battery. You need to look at both of these circuits together to understand how the chip works. Essentially, the UC3906 has three main modes or states of operation and these depend on the voltage of the battery under charge. These are shown in Fig.3. State 1 is called "bulk charge" whereby the battery is charged at close to or the maximum charge rate. This can be seen on the plot for charge voltage in Fig.3. As the battery voltage builds up to V12, the circuit changes to State 2, the "over-charge" state (point C on the charge voltage curve). When the battery reaches point D on the voltage curve, the charging current begins to taper off. Upon reaching point E, the battery voltage is equal to Voc (overcharge voltage) and the charger circuit switches abruptly to State 3, the "float" state. From here on, the battery may be charged at any level up to 1/lOth the maximum charge rate but it will not be allowed to exceed the "float voltage", Vp. D2 1N4DD7 + + SUPPLY INPUT -i D3 1N5404 .,.. 5 3 4 2 RA 180k 1% 15 6 BATTERY .,. i- 12 1) VT = VREF (1 + 2)-VOC 3) VF = VREF (1 + = = D.95VOC = 0.9VF = B) IT WHERE RX = R: :c RB + RA R~ RB ) :c .t~c 0 RB) o,::v = .o::v = VIN - VB - 2.5V RT Fig.4: all the key voltages and currents can be designed into the circuit using these equations. RC 39k 1% 14 .,. R: RX ) RA + RA 5) V31 6) IMAX IC1 UC3906 VREF (1 4) V12 7) IOCT RB .D39+ Fig.2: basic circuit for charging a 12V SLA battery at currents up to 500mA. For practical applications, both the positive supply input and the output positive lead should be fused to protect D1 & D3. Also shown at the start of the charge voltage curve of Fig.3 is the "trickle charge" condition (points A and B). In this condition, the battery is completely flat and below the trickle voltage, VT, which is typically 10.5V for a 12V battery. When a sealed lead acid battery is in this flat condition, it cannot accept a high charge rate. Therefore, the circuit trickle charges it at IT, until the voltage rises above VT whereupon the circuit switches into State 1, bulk charge. All the key voltages and currents mentioned above can be designed into the circuit using the formulas shown in Fig.4. Four resistors in the circuit determine the three voltage levels, VT, Voc and VF, These resistors are Ra, Rb, Re and Rd and to ensure accuracy, they should be ~~-~---:,,;-VOC C D .__IFF"i~VgF=..V:!131 G CHARGE VOLTAGE - A ------ -- -------- CHARGE CURRENT mc_:_ __ IT --------- STATE LEVEL OUTPUT 1 ::=-i 0 oc TERMINATE INPUT (C/S OUT) I - ~- STATE 1 I I I '1 ON-i---- • OFF -ri I I___ .I.__ I oc INDICATE OUTPUT l __ _ I I I STATE 2 ... I~ ---t--___ . _ _ , _ I- STATE 3 • I. STATE 1 Fig.3: voltage and current waveforms for the various charge states. If the battery is flat, it is triclcle charged until voltage VT is reached. The circuit then switches to State 1 (bulk charge), followed by State 2 (over-charge) & State 3 (float). 1 % tolerance. Using the formulas in Fig.4, you can check that the circuit has a VT of 10.6V (formula 1), Voc of 14.6V (formula 2), and a VF of 13.8V (formula 3). Formulas 4 and 5 are of academic interest only, since they are controlled by the internal functioning of the IC. As it stands, the circuit of Fig.2 will give a ma ximum charging current of 500mA , as may be calculated with formula 6. If you want a lower charge current, the resistor Rs should be varied according to the formula . As a practical example, if you want a maximum charge current of 250mA, resistor Rs should be increased to rn. However, if you want a higher charging current, say 1 amp, the circuit must be upgraded by changing Ql to a higher gain transistor such as a Darlington BD650. This is because the UC3906 can only deliver a maximum base current of 25mA from its pin 16. So for higher charge currents, Ql must be changed as well as Rs. And since diode D2 has a rating of 1 amp continuous, it should be upgraded to, say, a 3-amp 1N5404, if currents of more than 500mA are required. For charging currents of 1A or more, Ql should be mounted on a heatsink. If a load is applied to the battery while it is connected to the charger (which is how we envisage the unit would be used), the charger will contribute its full output to the load. If the battery drops 10% below the float level, VF, the charger will MARCH 1990 11 Charge & Discharge Currents the battery could be expected to deliver around 450 milliamps for 10 hours. Now what do we mean by terms such such C/4 and C/5? When we start bandying about figures of C/4 or C/5, we are talking about charge and discharge rates based purely on the battery capacity and with none of the derating mentioned above. C is the battery capacity in ampere-hours. C/5 refers to a 5-hour rate of discharge or charge. So C/5 for a 5 amp-hour battery is 1 amp. C/4 for a 5 amphour battery is 1.25 amps. Let's make it clear again that rates such as C/4 and C/5 do not Many people are confused about battery capacity and what it means as far as charge and discharge currents are concerned. Battery capacity is rated in ampere/hours. For example, you might have a 5 ampere-hour battery. Now this does not mean that the battery is supposed to deliver 5 amps for one hour, although it might not be far short of it. The capacity is normally rated for 10 hours or 20 hours. At the 20-hour rate, a 5 amp-hour battery could deliver 0.25 amps (250 milliamps) for 20 hours. At the 10-hour rate, the capacity would be reduced by about 10%, so that reset itself to State 1 and full charge current will be delivered. An interesting aspect of the circuit is that it does not drain any power from the battery via the sensing resistors, Ra to Rd, when the input power is off. This is because the chip senses whether the input voltage is low (see "UV sense" section on Fig .1) and if that is the case, the internal transistor at pin 7 is off. Consequently, no current flows through Re and battery loading is negligible. circuit published in July 1989. They have been included because the UC3906 has been found to be vulnerable to reverse voltages. D2 protects the circuit against reverse current flow which can occur if a battery is connected to the output when no power is connected to the input. D3 protects the circuit against reverse connection of a battery and should itself be protected by a fuse (see our complete charger circuit elsewhere in this issue) otherwise it can be destroyed. Dl protects the circuit against reversed input supply connections and could be eliminated if the circuit is permanently wired. Diode protection By the way, diodes D2 and D3 were not included in our original D3 1N5404 RT 470fl ., 5 3 4 1% 16 2 .,. 11 12 RB 18k 1% IC1 13 UC3906 RD l0 390k 1% 7 14 .039-+ RC 43k 1% 15 470fl.,. Fig.5: basic charger for 6V SLA batteries (500mA max). As before, the positive input & output lines should be fused to protect D1 & D3. 12 SILICON CHIP refer to the recommended discharge current of a battery. If you discharge a battery at a C/5 rate you will only get about 85% of its quoted capacity (assuming that is quoted at a 20 amp-hour rate). Finally, regardless of the capacity, SLA batteries can deliver very high discharge currents for short periods (20 seconds or less). For example, a 5 amp-hour battery may well be able to deliver a short term current of 50 amps or more. However, such a rate of current should not be maintained for very long as not only will it discharge the battery very quickly but it may also cause internal damage. Fig.5 is a version of the charger which is suitable for 6V SLA batteries. Its over-charge level, Voc, is 7.4V. VF is 6.9 volts and VT is 5.1 volts. Again, you can check that these voltages are obtained by substituting the values for Ra, Rb, Re and Rd into formulas 1, 2 and 3 in Fig.4. Maximum charging rate As noted above, the UC3906 controls the maximum charging current delivered to the battery, according to formula 6 in Fig.4. If you have access to the specifications for a battery, you will see a figure quoted for maximum charging current. If you don't have access to the specs, you are safe to assume that you can charge any SLA battery at C/5. To give an example, if you have a 10 amp-hour battery, you can safely charge it at a maximum current of 2 amps. For an explanation on charging rates and terms such as C/5, see the panel accompanying this article. Above VT, the battery is charged at the maximum current recommended by the manufacturers. This is another area of confusion as different manufacturers set different maximum charging rates fOi their batteries. Typically though, the maximum charge current is C/5 or C/4 or higher. ~