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Higher Intelligence: Solar Power Battery Charger By Ross Tester Elsewhere in this issue there is a feature which doesn’t portray grid-connected solar power in a particularly good light. To show that we’re not against solar power per se, here’s an intelligent battery charger specifically intended for storage-type solar power systems. 82  Silicon Chip www.siliconchip.com.au T here are many people across Australia, nay, around the world, who rely on “free” power from the sun, courtesy of the solar cells mounted on their roofs. For many of those, solar power is their only source of power: typically, these are people who live too far away from the electricity grid to make connection economic. For them, the somewhat questionable economic returns of solar power don’t come into the equation: if you want power, you have to make it yourself. As we mentioned in that feature, the basic choices are hydro, wind, bio-mass or solar. And while there are plenty of micro-hydro systems, wind generators and even some small-scale bio-mass systems, by far the largest percentage of people opt for solar power. However, there are many others, city and country who, for many reasons –environmental, experimental, (or perhaps just plain mental!) have decided that they too would like some of this “free” power. The main difference between solar power in the suburbs or towns and remote solar power is the way the power is used when it is generated. Where increasing numbers of city dwellers with solar power these days probably have “grid-linked” systems, invariably, remote solar power generators must use some pretty muscly batteries to store the power when the sun is out, ready for use when it is (a) needed, or (b) dark/cloudy/rainy/etc. Typically, banks of storage batteries are used. In the past, a lot of people have used (expensive!) traction-type batteries (eg, fork-lift, etc) because these are designed to be deep cycled. Such treatment destines your typical car or truck battery to a very short life. In recent years, batteries have come onto the market which are specifically intended for power storage (eg, solar power) applications. Most systems use series and parallel connected batteries to give both high current and high voltage (well, higher than six or twelve volts!) systems. The reason for this is mainly in the higher efficiency of DC/AC inversion from a higher voltage and lower I2R losses in the system. 24V is common, as is 48V. Above this, though, you Q2 MTP2955 (SEE TEXT) could start to get into difficulties with running from 12V solar cells. That’s not to say a 12V battery system is not perfectly practical; in fact, you can use a commercially-available 12V/240V (or more usually 230V) inverter and save a lot of hassles. Some of these are very efficient, these days. And we aren’t saying that anyone in the middle of suburbia shouldn’t put in a solar power system, if that is your want. Whether you want to save the planet or not (or perhaps you’ve come across some cheap solar panels!) you have every right to put in your own system. Where the situation does become a bit muddied is when you want to connect your solar system to your home wiring, using existing power outlets and so on. The power authorities have some pretty strict rules about how this is done, especially in the way your system is isolated from theirs. We suggest if you do want to put in a solar power system, keep it completely separate from the domestic mains supply. Besides, unless you’re a licenced electrician, you’re not allowed to do D1 A K 0.33 E Q1 BC557 2.2M B C 22k MBR1645 D2 D A 1k 100F 35VW 100 1W S G ZD1 15V D3 1N4004 S D MBR1645 G Q3 MTP2955 100F 35VW 6.8k LINK FOR 12V ONLY 120k 22k 1 7 120k CHARGING 0.033F LED2 22k IC1b 5 13 6 IC1d 4 12 1 7 1k 11 14 FAN CHARGED 9 IC1c 3 S0 +5V IN IC2 L4949 S 2 VR1 2k GND 5 100F 16VW 12k C D5 1N4148 Q4 2N5551 B 120k OUTPUT TO BATTERIES (CON2) 2.2M 1k 10 8 IC1a 8  LED1  22k D4 1N4004 100F 35VW INPUT FROM SOLAR PANELS (CON1) 120k K 0.033F + 1k E 2 – MBR1645 SC  2002 INTELLIGENT SOLAR CHARGER K A MTP2955 D S G D The intelligent charger is built around a specialised IC, an L4949 made by On Semiconductor. It can suit 12V and 24V systems. www.siliconchip.com.au March 2002  83 anything with your home wiring. But that’s another story in itself! Charging the batteries Having invested sometimes thousands of dollars in batteries, it is important to “treat them right” to maximise not only their life but also the power you can store in them and get from them. “Treating them right” means not only the way they are stored (eg, batteries don’t normally like being placed directly on concrete floors), maintained (eg, distilled water level where appropriate) but also in the way they are charged and discharged. It has been fairly common practice to simply connect the solar cells in series with the battery, usually via a series diode to prevent the battery discharging through the cells when they’re not producing power. As the solar cells are essentially a constant current device, this is not a real problem when the batteries are either fully or partially discharged. However, it is not good for the batteries when they are charged. The solar cells don’t know this and they keep on pumping out power while ever the sun shines. Result: overcharged batteries. This will certainly lower the battery life – and that’s why you need a regulator. It senses the state of charge: while the batteries are less than fully charged, it allows the solar cells to pump in as much power as good ol’ Sol will allow. But when they are nearly charged, it starts throttling back the electrons so the battery won’t overcharge. Circuit operation This circuit is designed for either 12V and 24V systems with the chang- Larger-than-life view shows the input and output connectors at the front of the PC board along with the (optional) fan. This fan should not be needed for solar panel systems (a small heatsink will suffice). ing of just one link. At the heart of the circuit is IC2, an L4949 monolithic integrated 5.0V voltage regulator with a very low dropout voltage and additional functions such as power-on reset and input voltage sense. In this circuit we use the 5V regulator because of its extremely low quiescent current. When there is no power source (ie, solar cells) connected, the total current drawn from the battery is around 300uA. We also employ the voltage sensing comparator section of this IC as the main switching device with hysteresis. The power-on reset circuit is not used. Incidentally, a specification sheet for this IC can be found at the manufacturer’s (ON SEMICONDUCTOR) web site: www.onsemi.com/pub/prod 0,1824, p ro d u c t s m _ P ro d u c t S u m m a r y _ BasePart Number%253DL4949,00.html Instead of typing all that, it is probably easier to search for L4949 at google. com as it will be the first item to come up, in less than a second! For the following explanation, assume that there is a supply voltage present at the source (Solar Panel etc), therefore the voltage at pins 9 and 13 of IC1 would be at logic 1. Pin 2 is the input pin for the battery sensor section of the IC. When the voltage at this pin falls to 1.24V the open collector output pin 8 is pulled internally to ground. This pin would normally be connected in series with a resistor and a Battery Low indicator LED to a positive supply. In this circuit pin 8 pulls the input of IC1b to logic 0 level via a 120kΩ resistor so the output from this inverting gate would be at logic 1. Since both the inputs of IC1d are now logic 1 the output would be at logic 0 and LED2 K&W HEATSINK EXTRUSION. SEE OUR WEBSITE FOR THE COMPLETE OFF THE SHELF RANGE. 84  Silicon Chip www.siliconchip.com.au D1 MBR1645 B 15V 22k 22k .033F 100 1W + FAN � C Q1 BC557 K A CON1 INPUT D2 MBR1645 K CON2 OUTPUT 1k 6.8k 22k 120k 120k 100F + � 1 LINK 2 A 100F LINK 1: IN FOR SOLAR PANELS OUT FOR POWER SUPPLIES 1k 120k IC1 4093 D5 120k 4148 G + 1k ZD1 D + E S K 100F + G K 1 LED1 GREEN IC2 L4949 D D3 D4 S A 4148 4148 2.2M LINK 1 0.33 5W Q3 MTP2955 2.2M Q2 MTP2955 2N5551 Q2 & Q3 MOUNTED METAL SIDE UP B E C D1 & D2 MOUNTED METAL SIDE UP Q4 22k A 1k LED2 RED GREY OUTLINE IS AREA OF HEATSINK/FAN (IF USED) .033F 12k VR1 2k 100F + (Red) would light to indicate that the battery was charging. Because of the inverting action of IC1a, the level at the output of IC1c would remain at logic 1 and LED1 would not light. Q4 is turned on via the 120kΩ resistor and the gates of P-channel Mosfets Q2 and Q3 are pulled low via the 22kΩ resistor. Q2 and Q3 conduct, allowing the battery to charge. A small amount of current is fed by the forward biased diode (D5) and the 2.2MΩ resistor to the voltage divider network, thus effectively slightly increasing the voltage at the sensing pin, (pin 2). The addition of this resistor effectively reduces the hysteresis voltage of this part of the circuit. When the voltage at pin 2 rises to 1.34V, the internal transistor at the output is turned off and the voltage at the input of IC1b is pulled high (to +5V), again via the 120kΩ resistor. LED2 is turned off and LED1 (Green) is turned on, indicating that the battery is fully charged. Transistor Q4 and the Mosfets are turned off so the charging ceases. For a 12V battery (LINK2 in) and with the values selected in the resistor divider network and a centred potentiometer, the voltage of the battery being charged will need to reach approximately 14.2V before the charging is stopped. Charging will will resume when the battery voltage drops to 13.7V. For a 24V battery (LINK2 out), the voltage of the battery being charged will need to reach approximately 28.4V before the charging is stopped. Charging will resume when the battery voltage drops to 27.4V. + � LINK 2: IN FOR 12V BATTERIES OUT FOR 24V BATTERIES Same-size views of the component overlay and matching straight-on photograph. The 3.3W resistor in the pic below is actually in the “Link 1” position – but it doesn’t matter ’cos they’re in parallel. Charging from a supply While the circuit is designed for use with solar panels, it can (with a minor modification) be used with other sources of power. Solar panels have a limited current output so it does not matter if they are connected directly across the battery: the current will be similar in value when the battery is “full” or “flat”. When this charger is used as a regulator for solar panels, the 0.33Ω, 5W resistor should be shorted with a link for most efficient operation. In this case the only loss is due to the “on” resistance of the Mosfets and the low forward drop of the Shottky diode/s. www.siliconchip.com.au However if the charger is used in conjunction with power sources that do not have current limiting (for example a bench power supply or an automotive battery charger) the circuit can be made to current limit by removing the link across the 0.33Ω resistor. When the voltage across the current limiting resistor exceeds 0.6V transistor Q1 is turned on, thus reducing the gate voltage applied to the Mosfets. This serves as a simple constant current source, the value of which equals 0.6/0.33A. To increase the current, reduce the value of the resistor. To minimise battery drain when the solar panel is not supplying power, the voltage at pins 9 and 13 of IC1 are logic low and both the LED’s are at turned off no matter what the state of the battery is. Two series diodes, D3 and D4, were added to reduce the supply voltage to IC2 by approximately 1.2V. This is necessary for a 24V battery as although the IC has a transient supply voltage of 40V, its maximum continuous supply voltage is 28V. In each kit are one 10A Shottky diode and two power Mosfets. The total dissipation in the two Mosfets would be approx. 0.15W at 1A, rising to 2.4W at 4A. Doubling the number of March 2002  85 Parts List – Intelligent Solar Charger 1 PC board, 98 x 70mm, code K009B (Oatley Electronics) 1 U-shaped heatsink (or fan/ heatsink – see text) Semiconductors 1 4093 quad NAND Schmitt gate package (IC1) 1 L4949 voltage regulator (IC2) 1 BC557 PNP transistor (Q1) 1 MTP2955 P-channel mosfets (Q2) (Can use two – see text) 1 2N5551 NPN transistor (Q4) 1 MBR1645 Schottky diodes (D1) (Can use two – see text) 3 1N4148 small signal diodes (D3-D5) 1 15V 0.5V zener diode 1 Green LED (LED1) 1 Red LED (LED2) Capacitors 3 100µF 35VW electrolytic 1 100µF 16VW electrolytic 2 .033µF MKT polyester (code 33n or 333) Resistors (0.25W, 1%) 2 2.2MΩ 4 120kΩ 4 22kΩ 1 6.8kΩ 1 12kΩ 4 1kΩ 1 100Ω 1W (for optional fan) 1 0.33Ω 5W (only required if power supply is used instead of solar panel) Optional: 1 12V fan/heatsink mosfets would reduce this total power dissipation by 1/2. Increasing the number of Mosfets results in better efficiency but is hardly needed. Other types of Mosfet with a lower “on” resistance could be used (an MTP2955 has an on resistance of 0.3Ω). As an example a 60W solar panel is rated to deliver approxiamtely 4.3A into a floating lead acid battery (14V). With this panel the mosfets would dissipate a total of about 2.8W. A small heatsink would be necessary but a fan is not. The fan shown in our photographs is an option, for use when the link is removed and the circuit is used as a constant current source. Here the total dissipation in the Mosfets becomes the supply voltage minus the battery voltage times the current. A 1Ω/1W 86  Silicon Chip resistor is supplied in the kit. With this the current is limited to 0.6A, so the dissipation in the two mosfets would be a total of 1.5W for a 2.5V voltage difference (this figure applies when the optional Kenwood plugpack is used). Construction With the exception of the (optional) fan, all components mount on a single PC board, coded K009B. As usual, inspect the board before assembly for any defects – shorts between tracks or broken tracks – and if necessary, repair them. Most of the construction is pretty much standard: start with the lowest profile components first (resistors, small capacitors) and move from their to the larger capacitors (watch the polarity on the electrolytics) and then the semiconductors. Naturally, all semiconductors are polarised so ensure they go in the right way. Leave the two Mosfets and one or two Schottky diodes for a moment. Whether you use sockets or not for the ICs is up to you but if you do, be careful to align the sockets the same way as shown on the PC board overlay, and be even more careful to get all the pins in straight when inserting the ICs. Now’s the time to decide what format you’re going to build the regulator in – ie, for a 12V or 24V system, and whether it is for solar panels or for use with a power supply. For 12V, a small link shorts out the 120kΩ and 22kΩ resistors near the lower right corner of the board (leave the link out for a 24V system). Of course these two resistors are redundant and could be left out but for the sake of ten cents, they might as well be included. The second choice (solar cells or power supply) determines whether the 0.33Ω resistor is in circuit or not. For solar cells, it can be shorted out via a link (left side of PC board) but if you are going to use it on any device without current limiting (or want to make it dual purpose), keep the resistor in circuit (ie, don’t solder the link in). The Mosfet(s) and diode(s) are the last components to solder in. They may look quite similar so don’t mix them up! The one or two Mosfets (depending on your requirements) mount at the top of the board with their metal side(s) up – that is, opposite to the way you would normally solder them into a circuit. This is to allow contact with the heatsink. The one (or two) Schottky diode(s) mount at the bottom of the board (closest to the connectors) and solder in the “normal” way – metal side down. Finally, solder in the two PC board mounting screw connectors, CON1 and CON2 and the board is finished. Setting up To set the charge, you will need to have the 12V or 24V battery connected and the solar panel(s) or power supply connected. You can set it with a power supply and use the same setting for a solar panel but make sure the 0.33Ω resistor is in circuit if you do! Turn VR1 fully clockwise. Monitor the battery voltage (with a multimeter) and when the battery reaches its correct charge voltage (14.2V or 28.4V for 12V and 24V systems respectively), slowly turn VR1 anti-clockwise until the green LED lights. Optional fan If you decide you want to fit the fan (as shown in the prototype) this simply clips over the PC board along with its integral heatsink. However, as we mentioned, for use with solar panels this fan should not be necessary – a small heatsink will suffice. The 100Ω resistor on the PC board allows the nominally 12V fan to run from the higher voltage produced from the solar panels (up to 18-20V). Wheredyageddit? This design is copyright Oatley Electronics (PO Box 89, Oatley, NSW 2223). Phone 02 9584 3563; Fax 02 9584 3561. website: www.oatleyelectronics.com; email sales<at>oatlelectronics.com SC Kit/Component Prices BASIC KIT: PCB and all components but with one Shottky diode: $21.00 Optional clip-on fan/heatsink: $4.50 Extra Mosfets: $3.00 Extra Shottky diodes: $3.00 16.5V/650mA Kenwood plugpack with non-standard mains connector: $4.00 Postage for any qty/mixture: $7.00 www.siliconchip.com.au