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USB Solar Charging System Simple Electronic Projects with Julian Edgar Charge and power all your USB devices from solar with this low-cost system. It’s inexpensive to put together, and once you’ve built it, charging your devices won’t cost a cent! I have lots of solar-powered devices. A solar-powered smart watch, a solar-powered iPhone, solar-powered noise-reducing headphones and a solar-powered mini floodlight. In fact, every one of my devices charged from a USB adaptor is now solar powered. This is achieved very simply and, if you’re careful with your purchasing, very inexpensively too. The parts required Only a few parts are required: a solar panel (Photo 1), a charge controller (Photo 5), a 12V battery and a one or more 12V to 5V USB converters (Photos 2, 3 & 4). You could spend hundreds of dollars on assembling these parts – or you could do as I did, and buy mostly second-hand, via online market places and/or make use of parts that others have thrown away. The solar panel needs to have an output voltage suitable for charging a 12V battery through a charge controller. This means getting a panel that has a maximum open circuit voltage of about 18-30V, depending on the selected controller. The maximum solar panel output power will also be governed largely by the controller you select. Small charge controllers may have a 60W limit, while larger controllers are good for 250W. So selecting the correct panel is done in conjunction with the charge controller you’ve picked. Second-hand solar panels are now ridiculously cheap – expect to pay from about $25 for a suitable used panel. Also remember that, in this application, the panel’s original maximum output power is probably not needed. To put that another way, this is a good project for reusing degraded panels that otherwise would go to scrap. The appropriate charge controller Photo 1: I mounted this 100W solar panel on a disused satellite dish on the roof. The panel can also be wall-mounted, or even just anchored to the ground. Second-hand solar panels are now very cheap. 26 Silicon Chip Australia's electronics magazine will also depend on the battery type you’re going to use. Sealed lead acid (SLA) batteries are ideal for this application as they can be mounted inside without concern for acid spills or venting of gases. However, SLA batteries tend to be expensive – even second-­ hand – so you may wish to use a conventional car battery. A major benefit of using a car battery is that you can get one free of charge. Simply visit a local mechanic or car battery supply shop. There you will find literally dozens of batteries that have been discarded – they’ve been replaced as no longer being suitable for cranking engines. However, starting an engine is very demanding on a battery; the current draw is in hundreds of amps. So these batteries often still have sufficient capacity to work as a storage battery in a solar system of the type being covered here. Photo 2: this 12V-to-USB converter/ charger provides two outputs and a voltmeter to allowing monitoring of system voltage. It cost about $11 from Banggood (similar units can be found on AliExpress & eBay). siliconchip.com.au Solar panel and battery ratings The greater the power of the solar panel, the better it will work through poor weather and the more battery charge you will have to work with. Also, the larger the battery, the longer the system will cope with cloudy days when little solar output is available. To give you a guide, where I live about 100km north of Canberra, in four years I have never gone close to running out of power using a 100W panel and a 26Ah SLA battery – the latter bought as defective and so probably having only half this nominal capacity. That includes charging power tool batteries as well as my phone, watch, camera, etc. Note also that smaller panels and batteries tend to be more expensive second-­hand, so there’s a further advantage in going big. When selecting a battery from the discard pile, use a multimeter to find a battery that still has an open-circuit voltage above 12V. If you are going to use the battery inside, select one that is fully sealed. A battery with flat terminals to which lugs can be bolted will be easier to wire than a battery with round terminal posts. If you have one with round posts, you’ll need to get matching terminals to attach wires to it (eg, Jaycar Cat HC4038), which will be an extra cost. Once you have selected a solar panel and battery, you can pick a charge controller to suit. Here it’s worthwhile buying new. At the time of writing, a 30A (about 400W) 12V charge controller costs around $12, including freight. They can be bought from AliExpress and similar suppliers. Ensure the controller can be configured to suit the battery type you’re using – most can. Many controllers also have built-in USB 5V outputs. If you choose to use these, you don’t need to buy an additional USB converter. However, more for convenience and appearance, I added three USB output converters to my system. Each of these has two USB outputs (giving six in total), on/off switches and an onboard LED display showing the battery voltage. These units are about $11 each. Note that when selecting these, ensure you don’t get ones designed to plug into a cigarette lighter socket – you want wired-in ones. If you want an even cheaper approach, just buy a 12V-to-USB wired converter that comprises a sealed box, USB output socket on a lead, and input power connections. In addition to these parts, you will need assorted cabling, terminals, an inline fuse holder and fuse, and siliconchip.com.au possibly a panel on which to mount the USB outlets. Building the system Before starting to build the system, consider the cabling requirements. I have the charging system working in my home office, so I had to run a cable from the roof-mounted solar panel through the wall of my house. In my case, that was easy but, in many houses, that will be quite hard! If that’s Photo 3: this non-switched panel with two USB outputs costs about $6 from AliExpress. Ensure you get a device that runs from 12V. the situation, consider having the charging system in an outside workshop or shed. It will still be very useful there – for example, most battery-powered tools have 12V car chargers available for their batteries, so the system can be used to charge power tool batteries. Also, consider how the solar panel will be mounted. In my situation, a disused satellite TV dish antenna was on the roof, pointing north. Mounting Photo 4: in my system, three 12V-to-USB adaptors provide six outlets at the desk in my home office. Note the inconsistency in the voltage readouts – at these prices, you can’t expect perfection! This is the charging voltage on a sunny day. Photo 5: this solar panel charge controller from AliExpress incorporates two USB outputs so, if you wish, you can charge your items directly from this module. Australia's electronics magazine July 2025  27 the panel was just a case of attaching the panel to the dish – quick and easy! However, again, that may not be the situation in your case. If roof-­ mounting is difficult, consider mounting the panel on a wall or even on the ground. The panel should face north and be tilted at an angle that approximately corresponds to your latitude, although horizontal panels will generally work OK. We don’t need to squeeze every drop of power out if it! Fig.1 shows the wiring – it is very simple. Ensure you place the fuse close to the battery; it should be rated at the maximum charging current, as dictated by the controller. If you are not used to working with 12V storage batteries, keep in mind that although the voltage is low (so you won’t get a shock), the ability to deliver current is very high and so you must be careful to ensure that the battery is never short-circuited. I have seen this done when someone inadvertently dropped a spanner across the battery terminals... not good! Always take great care when attaching battery connectors; the battery terminals must be insulated when the battery is in use. An easy way to achieve this is to place the battery in a dedicated box. Boxes designed to house car batteries are available from about $15 from local suppliers. First, connect the battery to the solar Parts List – USB Solar Charging System 1 12V solar panel 1 12V charge controller (type to suit panel and battery) 1 12V rechargeable battery 1 pair of battery terminals (may not be required depending on battery type) 1 inline fuse, rated to suit charge controller 1 or more 12V-powered USB chargers various lengths of wire, rated to handle the maximum charging current Fig.1: the wiring is straightforward, but ensure you maintain the correct polarity of all the connections. Don’t insert the fuse until you have wired the battery to the controller. controller, ensuring the polarity of the connections is correct. Once these connections are made and insulated, insert the inline fuse. The controller should then come alive. Set the controller to the correct battery type, and if the controller has these facilities, ensure the settings for float charge and auto-disconnect (that occurs if the battery is discharged too far) are correct – many simpler charge controllers won’t have these functions. Next, connect the solar panel to the controller, again ensuring correct polarity of the wiring. If you’re unsure of which wire is which (an easy confusion to occur if you’ve extended the solar panel wiring), use a multimeter to identify the positive and negative wires – the solar panel will need to be exposed to light when you perform this check. After the solar panel is connected, most controllers will confirm the panel is generating power, either on the LCD screen or by the simple illumination of an onboard LED. Finally, if you are not using a built-in USB outlet, connect the external USB converter(s). Ensure that no timers are activated on the controller output – you want the output on all the time. Conclusion Photo 6: a used car battery that is no longer strong enough to crank a car engine will often be suitable for this application. Such batteries are available free of charge from car workshops and battery replacement shops. However, it’s best to get a battery with bolt-on terminals rather than round battery posts like this one. If mounting the battery inside, ensure it is of a sealed design. 28 Silicon Chip Australia's electronics magazine What has surprised me over the four years that I have been running the system is its ease of use and convenience. I just plug in the devices, and they charge – obvious, huh? But they charge purely on solar power, and they charge irrespective of the weather. Also, they charge through blackouts, so there’s that safety advantage to conSC sider as well. siliconchip.com.au