Fullscreen HTML5Med Res (??MB)HTML4

Here’s a cheap and simple battery charger which you can leave connected without risk of overcharge. Design by Branko Justic Words by Ross Tester “Intelligent” 12V Charger for SLA and Lead Acid Batteries 78  Silicon Chip siliconchip.com.au F ollowing our look at charging cordless tool batteries last month (Nicad and NiMH), we’re moving on to charging their big brothers: 12V Lead Acid and Sealed Lead Acid types. These are much less forgiving than Nicad and NiMH when it comes to letting them discharge – so if you have anything which uses a 12V battery (and who doesn’t?) this little project could get you out of a lot of trouble. It could even be used to keep your car battery always at maximum charge (which, incidentally, your car’s alternator/regulator normally does not!). Lead acid batteries of any persuasion do not like being left discharged. In fact, a brand new battery can have its life drastically shortened if not charged as soon as possible after discharge. I had to stop myself saying “as quickly as possible” just then – in battery parlance quickly means something completely different. And that can ruin a battery just as easily! As discussed last month, the trouble with most low-cost chargers is that they simply keep pushing charging current into the battery without any method of detecting the amount of charge. So it’s easy to overcharge (and cook) a battery. Perhaps worse, they don’t know how discharged the battery was before you started charging it. If you’ve only slightly flattened it you can, once again, overcharge it even if you do remember to turn it off. Unfortunately, we’ve all gotten used to our mobile phones where we tend to “plug ’em in and charge ’em” regardless of how much they’ve been used – because these days most phones (if not the chargers) have the “smarts” to stop batteries being overcharged. OK, so much for what we shouldn’t do. What should we do? There are two ways to charge a battery. The most unreliable method is to connect a charger for a certain period of time, dependent on the charging current. It’s unreliable for several reasons – one, (the unknown charge state) mentioned above; another is that human “forgettory” takes over and we leave the charger on far too long. Both result in overcharging. The second method is to monitor the voltage. You probably know that as a battery discharges, its voltage drops only slowly for a period, then as its charge diminishes it starts to drop rather rapidly, until it is exhausted (or “flat”), when the curve once again flattens out. Charging a battery is similar – in reverse. The voltage rises quite slowly at first, then much more quickly as it approaches full charge. It then flattens out as it is overcharged. If you were able to sit and graph the voltage for the whole charging period, you would be able to pick the point where you could say the battery was fully charged. But who wants to do that? Fortunately, we can nominate the point at which a battery is said to be fully charged. In a lead-acid battery, that’s generally assumed to be about 13.8V. So all we have to do is monitor the battery voltage and turn the charger off when the battery reaches that level. OK, that’s being a bit simplistic but in effect, that’s exactly what this charger does. It can be left connected permanently to the battery so that when the charge level drops – whether through use or by self-discharge – it will automatically switch itself back on again. siliconchip.com.au It’s a fully self-contained mains charger which will handle anything from small SLAs up to marine and diesel monsters. It might take a while to charge bigger batteries but can be left on indefinitely. It’s intended for mains operation (after all, it is mounted on a plugpack!) but with a little ingenuity, could also be used as the battery charger for a solar, wind or micro hydro installation. How it works The charger is based on an L4949 precision voltage reference and regulator, as was used in the Auxiliary Battery Controller last month. For more information on this chip, refer to last month’s article. It is powered by a 9V AC plugpack, connected to a simple half-wave voltage doubler consisting of 1000mF and 100mF capacitors and diodes D1 and D2. This gives January 2007  79 4.7k ~ ~ +29V DC (NO LOAD) E C K B + Q1 BD682 LEDS A Q2 C8050 B 8.2k 3 IC2b 1 4 2 E D1 1N5819 IC2a 4.7k 82k 8 100 µF 82k CHARGING LED1 D3 1N4148 5 K A IC2c 14 1k 10 6 11 4.7k A λ K SC B IC2: 4093B 100 µF* K 2007 E C +5V 82k 1000 µF* A E 8.2k C 9V AC INPUT 1 2 * HIGH RIPPLE, HIGH TEMPERATURE TYPES C B BD682 8 BD6 K D2 A 1N5819 100 µF C8050 9 12 TO BATTERY 2 7 13 CHARGED A LED2 IC1 L4949 8 λ 7 IC2d 82k 5 47 µF 22nF 18k K INTELLIGENT 12v BATTERY CHARGER 1N5819 1N4148 A A K – K Q1 switches charging current on when the battery voltage sensed by IC1 falls below a preset threshold. Once the battery is charged, it switches off again. This means that the battery will not be overcharged. an unloaded, pulsating DC voltage in the region of 29V. The two capacitors are special types, capable of handling the high ripple current of the voltage doubler and also have a higher-than-normal temperature rating, as they can run rather warm. The current this simple arrangement is capable of supplying is limited largely by the reactance of the 1000mF capacitor and the plugpack supply – it’s in the order of a couple of amperes. But remember that this is a half-wave supply so as it stands it has far too much ripple (hum) to use for anything but a battery charger! Now let’s turn our attention to IC1, the L4949. It’s used in a similar way to last month, detecting a voltage at pin 2 and switching a series of logic gates in IC2, a 4093 quad Schmitt NAND, via its output, pin 7. Once again, the chip’s internal 5V regulator is used to supply a stable voltage to the gates, which in this circuit are connected as inverters (both inputs connected together). The battery voltage is monitored at pin 2 of IC1. As the battery is drained and its voltage falls below IC1’s threshold, an internal transistor connected to the output (pin 7) is turned on, resulting in the output falling to logic level 0. This drives the inputs of paralleled gates IC2c and IC2d low. Their outputs then go high, forward biasing diode D3 and very quickly charging the 47mF capacitor at its cathode. IC2b’s inputs are then taken high, sending its output low. Because there is no drive for LED2, it stays extinguished. But IC2a’s inputs are now also low, sending its output high. LED1 does have drive and now lights, indicating the battery is being charged. At the same time, the NPN transistor Q2 is fully turned on, which in turn pulls the base of Darlington transistor Q1 low, turning it fully on. In this role Q1 is simply an on or off switch. When it is turned on, current can flow into the battery, which starts to charge. The voltage doubler is incapable of maintaining the (unloaded) peak voltage and it drops down to around 15V or so. Eventually, the battery charges and the voltage at IC1’s pin 2 exceeds the threshold voltage. The output (pin 7) is now pulled high by the 82kW resis- The plugpack has two plastic guards (see left) which need to be removed so that the PC board can sit flat. They break out easily with a pair of pliers, then a little judicious paring with a sharp knife removes any remnants. It’s easier to do with the screws out! 80  Silicon Chip siliconchip.com.au (1000mF) is a very tight fit between Q2 and the edge of the transformer, so you might have to juggle it a bit to get it in. Solder in Q2 at the same time as the 1000mF to make sure it fits properly. We decided to drill another hole in the PC board to get the best fit for this capacitor. All that’s left is Q1, the Darlington transistor. It mounts with its metal side up and its heatsink is then screwed down onto it. First, though, you’ll have to bend the three legs down 90° to go through the PC board. This is a bit tricky because you also have to make sure the hole through Q1 aligns with the hole in the PC board. When you think you have the bend right, temporarily secure Q1 to the PC board with the screw and nut before 9V AC IN Building it D1 100 µF + o 1000 µF + 5819 5819 105 82k 1k D3 4148 IC2 4093B 4.7k 4.7k D2 Q2 LED2 + soldering it in – that way, you can be sure it is in the right place and the solder joints won’t be stressed when you tighten the nut on permanently. Solder Q1 in, then fit the heatsink with the single nut and screw from the underside of the board. The heatsink, which is up off the PC board by the height of Q1, hides two resistors and partly obscures two more. Fitting to the transformer The PC board is designed to screw directly to the output terminals of the supplied plugpack transformer. It can also be used with another PC board screw terminal block to connect to a transformer without the screw terminals. Note that it must be a transformer (AC output), not a DC supply. We’ll assume that you are using the TO BATTERY UNDER CHARGE – Q1 V+ + C8050 (Q1 METAL SIDE UP) BD682 GND The same-size photo at left matches the component overlay diagram of the assembled board at right. The 1000mF capacitor (right top) is a rather tight fit! It and the other brown 100mF electro are both 105° types. o 4.7k IC1 L4949 LED1 82k 18k 22nF 100 µF 100 µF 105 8.2k 8.2k CHARGED CHARGING + siliconchip.com.au 47 µF © oatleyelectronics.com The first thing to do is a little surgery on the plugpack. It has a couple of guards moulded into the plastic around its screw terminals – but these are right in the way of where we want to mount the PC board! It’s quite easy to break these out with a pair of pliers. You may need to clean the area a little with a sharp knife because the PC board needs to sit flat. By the way, temporarily securing the PC board upside down onto the transformer makes it a handy little soldering holder! After checking the PC board for any defects, start assembly by soldering in the resistors and non-polarised capacitor. Use the resistor colour code table and/or a DMM to check their values – particularly the 82kW and 8.2kW (they’re easy to mix up!). Next to solder in are the two IC sockets, making sure the notches match the PC board screen overlay, along with the screw terminal connector. The three diodes and two LEDs are next – watch the polarity and note that the two 1N5819 diodes at the top of the PC board mount opposite to each other. Both LEDs can be mounted hard down on the PC board. The cathodes (shorter leads closest to the flat edge of the base of the LED) mount towards the bottom of the PC board. Now solder in the four smaller electrolytic capacitors, taking care with polarity. The largest electrolytic 240V – 9V AC PLUGPACK + tor to +5V. As the inputs to IC2c and IC2d are now high, their outputs are low, IC2b’s output is high and LED 2 lights, indicating that the battery is charged. With IC2a’s input high, its output must be low, therefore Q2 receives no forward bias and both it and Q1 turn off, shutting off the charging current to the battery. This doesn’t remove power from the monitoring circuit because it continues to be powered by the charged battery. We mentioned the 47mF capacitor at the junction of IC2a and IC2b before but not since. It, along with the 82kW resistor in parallel, form a short time delay. The 82kW resistor discharges the capacitor slowly, preventing the circuit from “hunting” back and forth, which it could do as the battery loads down the main supply voltage. 82k 82k K215 HEATSINK MOUNTS ABOVE RESISTORS ON TOP OF TRANSISTOR transformer included in the kit – with the screw terminals. After giving the assembled PC board a thorough check to make sure the components are in the right places, in the right polarity (where appropriate) and are soldered in properly, the board can be attached to the transformer. The photographs show this clearly. The two screw terminals are on the underside of the transformer. The PC board is attached to the transformer with the screws on the same side of the board as the copper tracks. (Mounting it the opposite way around, though possible and will not do any harm, will not allow the charger to fit into a wall-mounted power point because the components will be in the way). Undo the screws enough to slide the board in, copper side up, then January 2007  81 Parts List – 12V Battery Charger 1 PC board, 53 x 54mm, coded OE-K215 1 240V-9VAC/2.22A plugpack 1 mini finned heatsink 1 8-pin IC socket 1 14-pin IC socket 1 2-way screw terminal block, PC mounting 1 M3 x 10mm screw, nut & washer Semiconductors 1 L4949 5V regulator and voltage sensor IC (IC1) 1 4093 or 4011 quad NAND Schmitt trigger (IC2) 1 BD682 PNP Darlington transistor (Q1) 1 C8050 NPN transistor (Q2) 2 1N5819 Schottky diodes (D1, D2) 1 1N4148 signal diode (D3) 1 5mm red LED (LED1) 1 5mm green LED (LED2) Capacitors 1 1000mF 50V 105°C electrolytic (do not substitute) 1 100mF 50V 105°C electrolytic (do not substitute) 2 100mF 35V electrolytic 1 47mF 16V electrolytic 1 22nF polyester (code 22n or 223) Resistors (0.25W 5%) 4 82kW   1 18kW  2 8.2kW 3 4.7kW   1 1kW do them up tightly so that they grip the PC board and make contact with the tinned copper area. It’s as simple as that! If your PC board doesn’t sit flat, it’s probable that you have some remnants of the plastic guard ridges stopping it being screwed right down. This close-up photo of the edge of the PC board shows how the heatsink is bolted to the transistor (Q1) underneath, with some resistors also beneath the heatsink. Q1 lies flat on the PC board with its metal face upwards to make contact with the heatsink. Testing Without any battery connected, plug the charger into a power point and turn it on. The red “charging” LED should light. If it does, you can be reasonably confident everything else is OK. If you measure the voltage at the output connector, it should be somewhere around or above 25V DC. Now connect a length of polarised figure-8 cable to the output connectors and connect the other ends to a 12V battery – watch the polarity! A pair of alligator clips on the cable make this easy. The voltage at the output terminals will drop significantly, depending on the state of charge of the battery. With a known “good” but discharged battery (eg, one that hasn’t been sitting around discharged for months!) this voltage could be somewhere around 10-12V. As the battery charges, this voltage will rise up to a maximum of about 15V, at which stage the green LED will come on indicating that the battery is charged. The green LED flashes? We mentioned earlier that there is a time delay built into the circuit to prevent it hunting back and forth. This also has another effect: periodically, the red LED goes out and the green LED comes on. This is not indicating full charge – the green LED Resistor Colour Codes o o o o o No. 4 1 2 3 1 Value 82kW 18kW 8.2kW 4.7kW 1kW 82  Silicon Chip 4-Band Code (1%) grey red orange brown brown grey orange brown grey red red brown yellow purple red brown brown black red brown 5-Band Code (1%) grey red black red brown brown grey black red brown grey red black brown brown yellow purple black brown brown brown black black brown brown Here’s how the PC board mounts onto the transformer, after the plastic guards have been removed. It will work the other way up but the components will stop the plugpack fitting on a wall-mounted outlet. stays on constantly when the battery is charged. What happens is that the 82kW resistor discharges the 47mF capacitor, switching the charging off. But if the L4949 hasn’t registered a charged battery, the capacitor charges again, turning the charging back on. This happens continuously while the battery is charging. If you wish, the frequency at which this switching occurs can be decreased by increasing the 82kW resistor and/ or the 47mF capacitor. A value of 1MW and 100mF will increase the time to 100 seconds. It’s running hot! Several components in this project run quite warm, even hot, to the touch. The transformer, for example, can get quite warm (but it should never get uncomfortably hot). Q1 (on its heatsink) has to dissipate a fair amount so it can get too hot to touch. Indeed, if you are wanting to charge car batteries, the PC board-mounted heatsink is probably inadequate and should be replaced with a bigger unit. For small SLA batteries, it should be OK. Finally, the 1000mF and 100mF capacitors in the voltage doubler will run fairly warm – but they are 105°, high ripple types and are designed to handle the heat. SC Where from, how much? This project was designed by Oatley Electronics, who hold the copyright. A complete kits of parts, including the special 9V AC plugpack transformer, is available for $18.00 plus $7.00 pack & post within Australia (Cat K215). Contact Oatley Electronics, PO Box 89, Oatley NSW 2223, or via their website, www.oatleyelectronics.com siliconchip.com.au