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Project by Tim Blythman Battery BackPack for GPS Clocks USB-rechargeable lithium-ion battery LED charge indicator Mid-rail tap for GPS Analog Clock driver Mounts directly to the 2022 GPS Clock Driver PCB Pin headers for general-purpose use Regulated 3.0V/3.3V output at up to 500mA (other voltages possible) While this design sounds like it has a niche application, it can be used in many places that need an uninterrupted power source at a low voltage and with a modest current requirement. It neatly replaces the battery pack in our 2022 GPS Clock Driver, making it a simple upgrade. This add-on PCB (shown attach to the GPS-Synchronised Analog Clock Driver) is much the same size as two AA cell holders and can replace the battery on the Analog Clock Driver. It can also be used as a general-purpose, uninterruptible, low-voltage power supply. I have a GPS-Synchronised Analog Clock (September 2022, siliconchip. au/Article/15466) on my office wall. I originally built it to test my WiFi Time Source for GPS Clocks (June 2023, siliconchip.au/Article/15823) but, perhaps unsurprisingly, I found it quite useful. Unfortunately, I’m finding that non-rechargeable AA batteries don’t last as long as I had hoped. I think I may have a pack of dud cells, or perhaps the current draw of the clock is a bit higher than Geoff’s prototype. Since there are a few steps to reset the clock when the batteries need to be replaced, it is a hassle when the battery goes flat. I figured that a rechargeable battery with a USB socket could replace the two 1.5V cells and would have uses in other places where a simple, rechargeable power supply is needed. The circuit is quite simple, but there is one small catch to make it work with the Clock, which we’ll discuss shortly. Effectively, the Battery BackPack is powered from a mains USB power supply, keeping the onboard lithium-ion cell fully charged. The USB power supply provides power to the connected clock or other device while it is present. If the power source is removed, the lithium-ion battery takes over. Circuit details Fig.1 shows the circuit. You might recognise that this is quite similar to the rechargeable battery circuit we have used in some other projects, such as the August/September 2025 USB-C Power Monitor (siliconchip. com.au/Series/445) and the September 2024 Compact OLED Clock and Timer (siliconchip.au/Article/16570). The main component is IC1, an MCP73831 charge regulator IC. USB power is provided at CON1, a USB-C socket. For USB-C, the two 5.1kW resistors are needed to communicate that this device is a power sink and should be supplied 5V from a power source. CON2 provides a simple pin header that can be used as an alternate connection for an incoming 5V DC supply. The two 10μF capacitors and the 10kW resistor are the minimum data Fig.1: this design provides circuitry to charge a lithium-ion cell and provide a regulated 3.3V output. CON4 matches a header on the GPS-Synchronised Analog Clock Driver, allowing it to replace the existing battery arrangement directly. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au The header on the right connects to the Clock Driver PCB. If you are not using it with a Clock Driver, this tab can be removed to save space. The 470μF capacitor and two 100kW resistors are also not needed in this case. Scope 1: the blue shows the voltage on the 470μF capacitor as the clock advances, while the red trace is the 3.3V supply rail. Even with the deviation seen here, there is more than enough voltage to drive the clock mechanism. sheet requirements needed for the IC1 charge regulator chip to function. The 10μF capacitors provide input bypassing and output filtering, while the 10kW resistor sets the battery charge current to 100mA. IC1 provides a STAT output that is low during charging and high when the battery is fully charged. We have connected bicolour LED1 as shown in Fig.1; the arrangement of the 1kW resistors means that it will show red while charging and green when charging is completed. The resistor chain is powered from the USB supply, so it does not waste battery power, and the LED will be off if USB power is absent. Thus, you can quickly know if the unit is running from USB power or running down the battery. The three schottky diodes allow a seamless transition when USB power is applied or removed. When USB power is present, current flows through D2 and D3. D1 is reverse-­ biased, so no current is drawn from the battery, and the charge controller can accurately sense the battery voltage and current for correct and complete charging. D1 also prevents USB power from being directly fed into the battery. We use D2 and D3 in series from the USB supply to help share some of the dissipation in the downstream regulator. With typical load currents, the extra diode will drop no more than 0.3V, so even at 4.5V, a badly sagging USB supply will still provide power in preference to a fully charged lithium-­ ion cell at 4.2V. REG1 and its two 10μF bypass/filter capacitors provide a regulated 3.3V that is used to power the attached clock circuitry (or other load). The expected voltage from a pair of AA cells is 3V, but it is possible to see over 3.2V from a pair of fresh alkaline cells. So virtually any circuit designed to run from two alkaline cells should work with a 3.3V supply. In any case, the GPS-Synchronised Analog Clock Driver specifically can operate with a supply up to 3.6V, limited mainly by its microcontroller, so 3.3V is suitable and uses a commonly available regulator value. If you plan to power a particularly sensitive circuit, you could swap it for a 3.0V regulator, which is also available. However, the dissipation in the regulator would be a little higher, so the maximum load current will probably be reduced somewhat. The MIC37100 has been specifically chosen for its low dropout voltage, ensuring that it can provide 3.3V (or near enough), even with a fairly flat Li-ion cell. The SOT-223 package allows more dissipation than a smaller SOT-23 part would. Its quiescent current is slightly higher than other similar parts, and there is no low-voltage cutout, so it is important that a protected lithium-­ion cell is used. siliconchip.com.au Australia's electronics magazine The remaining circuitry provides the centre tap that is derived from the midpoint of the two AA cells in the original circuit. This allows the Clock Driver to produce a bipolar drive signal to the clock movement. While there are many possible ways of doing this, the drive signal current is quite modest, and importantly, it should be symmetrical about the supply mid-rail. So we have simply provided a fairly large half-rail bypass capacitor (470μF). It is biased towards the midrail voltage by a 100kW/100kW resistor divider, which draws only a few extra microamps from the rechargeable cell. The time constant of this arrangement is around 23s, so it does take a while to settle. Fortunately, the Clock Driver does not start driving the clock mechanism immediately, giving time for the settling to occur. The Clock Driver output will also tend to pull the capacitor towards mid-rail. Scope 1 shows the voltage on the capacitor as the pulses alternate. You can see that the alternating pulses cancel out their effects on the capacitor, and its average voltage remains near the mid-rail as desired. Performance Figs.2 & 3 show some charts of the output voltage and component dissipation at various output currents. Since REG1 is a linear device, the September 2026  79 input current roughly matches the output current, although there is a small amount of current through its ground pin. The data sheet notes a typical ground current of 11mA for a 1A output; Fig.3 takes this into account by assuming a constant 11mA ground current. We expect it will be lower than this in most cases (around 0.5mA at the current drawn by a typical clock). Fig.2 indicates that the regulated 3.3V output will be maintained at all times with the USB supply or a fully charged cell. We expect that will cover most scenarios, since the device is not intended to be used for long periods without USB power. Even with a discharged Li-ion cell, which could drop as low as 3.3V, the unit still provides 2.5V at a 1A load, well above the 2.25V that causes the Clock Driver to switch to low-power mode. Typically, a device designed to run from two alkaline cells will run down to 2V (1V per cell), although occasionally you will find a device that stops working at a higher voltage. 2.25V should be pretty safe for most such devices. With good output voltage performance, Fig.3 is more critical to checking the range of safe and proper operation. The 1N5819WS data sheet notes a maximum dissipation of 250mW at 25°C; you can see that is reached at around 600mA of current. This part does require derating as the temperature rises, with 40°C ambient reducing that to 200mW, reached at 500mA output current. The SOT-223 package of the regulator is typically rated for over 1W, so it should have no issues with continuous operation up to 500mA. The PCB has large copper areas and thus good thermal mass, so it should be able to handle brief excursions above this. In practice, our clock drew just over 100mA on power-up, and the normal operating draw is around 3.3mA, which appears to be on the higher side of what’s expected (perhaps the cause of our frequently flat battery!). So even a power-hungry clock should be able to operate for a few weeks without USB power. PCB design The Battery BackPack has been designed to replace the pair of AA cells on the GPS-Synchronised Analog Clock Driver and thus the BackPack PCB is the size of two AA cells. So it is a suitable size for other applications that expect a pair of AA cells. If you aren’t using the Battery BackPack with the Clock Driver, power can be supplied via either the CON1 USB-C socket or the CON2 header. To avoid problems with two power supplies feeding each other, we recommend choosing and fitting just one power input socket. If using CON2, the standard 5V USB range of 4.75V to 5.25V is safe. Lower voltages should not cause damage, but IC1 may not be able to charge the cell unless there is at least 0.3V of headroom. IC1 can operate up to 6V, but you will need to derate REG1 if the supply voltage is higher than 5.25V. Half of the PCB is taken up by the single AA (14500-sized) cell holder, while the remainder is covered with the circuitry seen in Fig.1. The CON4 output connection has the same pinout as CON3 on the GPS-Synchronised Analog Clock Driver PCB, allowing direct mounting of the new board on the Clock PCB. CON4 is on a small, protruding tab which can be easily snapped off if CON4 is not needed. The so-called mouse-bites encourage the board to break at the desired location. Breaking the PCB can release fibreglass dust, so we recommend doing so outside wearing a mask. If you wish to remove the tab, do this before assembling the PCB. Carefully run a sharp knife over the traces near the edge of the tab. Doing so will reduce the chance of the traces being torn, which could ruin the PCB. A light scoring on both sides will encourage a clean break. Use pliers to gently flex the tab. It will take some force, but flexing should cause it to break along the line of holes. Clean up the rough edge with a file and ensure any stray fibreglass is removed. Make sure that there are no loose traces or other copper that might cause a short circuit. Once you have broken off the tab, there is no external connection for the mid-rail tap, so the 470μF capacitor and two 100kW resistors are useless and should be omitted, which will also save a small amount of drain on the lithium-ion cell. CON3 is a simple header that breaks out the regulated and unregulated outputs and ground. We expect constructors might use this connection if they are using the BackPack to power some other device that expects around 3V but does not need the mid-rail tap. The PCB also has a pair of pads to suit a AAA or 10440-sized cell holder. Lithium-ion cells of this size are not as common as AA or 14500-sized cells, but might be necessary in tight spaces too, due to the thickness of the resulting assembly. Construction Fig.2: the output of the Battery BackPack is fully regulated while a USB power supply is available or the lithium-ion cell is near capacity. In any case, there is always enough voltage to power the Clock Driver, even with a nearly flat Li-ion cell. 80 Silicon Chip Fig.3: the power dissipation in the individual components under various output loads. The offset from zero at low current draws is due to the quiescent current of the regulator, which we have estimated conservatively. Since we will be fitting SMD parts, you’ll need the usual gear, such as flux paste, tweezers and a magnifier. Refer to the Fig.4 overlay diagram for the component locations. Start by applying flux to the SMD pads on the righthand side of the PCB. Rest IC1 in place; Australia's electronics magazine siliconchip.com.au Fig.4: check the component locations during assembly with this overlay diagram. We recommend fitting IC1 and the USB-C socket (CON1) before the other components, since they have the most closely spaced pins. We used some foam-backed double-sided tape to secure the Battery PCB to the Clock Driver since the header only attaches at one point. Make sure that no other parts of the two PCBs can come into contact. it is asymmetrical, so should only line up one way. Tack one lead and confirm it is flat against the PCB and aligned before soldering the remaining leads. Check for bridged pins and use solder-wicking braid and extra flux to draw out the excess solder from any bridges. Next, solder the USB-C socket (if you are using it). After placing it flat on the board and soldering the signal pins, apply a generous amount of solder to the side pads that secure the shell to the PCB. That will give it good mechanical strength. Work through the smaller passives. Fortunately, all the SMD capacitors are the same value. Note that all three diodes have their cathodes facing the same way. Make sure to get this right, as there is a risk of directly supplying power to the lithium-ion battery if one of the diodes is reversed! Solder REG1 next. It too should only fit one way. After that, use a flux cleaner or solvent such as isopropyl alcohol to clean off the excess flux from the board. Let it dry and inspect it closely for bridges and dry solder joints. If you find any problems, rectify them before continuing. At this stage, you should be able to connect a USB-C power supply for testing. You can probe some voltages with respect to ground; use the middle connector of CON3 or the lower pad of the unfitted 470μF capacitor as the ground connection. You should see between 4V and 5V on the V+ pad of CON3 and 3.3V (3.2-3.4V) on the 3.3V pad of CON3. This should be the same as the top + pad of CON4. The two middle pads of CON4 should show around 1.6V. If your voltages are significantly different, remove power and check the component placements and soldering. Disconnect the power supply and fit the remaining through-hole parts. Make sure that all leads are trimmed as flat as possible to reduce the chance of them contacting the PCB this will mount on. The battery holder pads appear to align with those on the clock driver, but should be kept well clear as they are at different voltages and contact might allow the lithium-ion battery to inadvertently discharge. Bend the leads on the electrolytic capacitor, making sure that it will be fitted with the polarity shown on the PCB, then solder it in place. Follow with the LED. The K cathode marking corresponds to the green element of the LED; you can use a light-emitting diode tester or multimeter in diode test mode to confirm this. When it lights green, the red probe is on the anode and black on the cathode. Finally, fit the cell holder, BAT1, taking care with the polarity. As for the other through-hole components, cut the leads flush so there is no chance of them shorting from the underside of the PCB. If you like, you can fit the cell now and connect a USB-C power supply to charge it. Check that the LED lights up red and then turns green, indicating it is fully charged. If the LED lights up green initially, it may be fitted with siliconchip.com.au Australia's electronics magazine the wrong polarity. Remove the cell before proceeding. Fitting it to the Clock Driver If you are upgrading an existing GPS Clock Driver PCB, you will need to remove the existing cell holders first. The four-way CON4 header should be soldered to the underside of the Battery PCB. The four-way header makes the electrical connections to the Clock Driver and will provide some mechanical strength. It’s a good idea to place a few lengths of thick, foam-backed, double-sided tape between the two PCBs in the area where the AA holders used to sit on the Clock Driver PCB. These will insulate the exposed pads and provide extra support for the added Battery BackPack PCB. The larger pad (located in the centre of the cell holder) is isolated on both the Clock Driver and Battery PCBs, so it could also be used for soldering a sturdy wire between the two. Watch that no wire fouls the cell. Whatever option you use, also be sure to solder the four-way header between the two PCBs and trim the leads short on the underside. You can now start the Clock Driver with our modified procedure to test for correct operation. Manually adjust the clock hands to about 10 seconds before the next half hour and then insert the cell. The LED on the Clock Driver should flash twice to indicate a normal startup. The attached time source (GPS module or WiFi Time Source) should start September 2026  81 Silicon Chip PDFs on USB ¯ A treasure trove of Silicon Chip magazines on a 32GB custom-made USB. ¯ Each USB is filled with a set of issues as PDFs – fully searchable and with a separate index – you just need a PDF viewer. ¯ Ordering the USB also provides you with download access for the relevant PDFs, once your order has been processed ¯ 10% off your order (not including postage cost) if you are currently subscribed to the magazine. ¯ Receive an extra discount If you already own digital copies of the magazine (in the block you are ordering). up and run for maybe 10 seconds or so – long enough to acquire the time. Then the LED on the Clock Driver will switch to a long flash sequence until the next half-hour is reached. Refer to the article in the September 2022 issue if there are problems with the Clock Driver. Powering the time source is probably the greatest load the Battery BackPack will see, so you should be able to identify any problems early on. If the Driver appears to lock up, it may not be able to power the time source, so you should check that the battery is charged. You can then use the S1 button on the Clock Driver to manually advance the clock hands and test that the Battery BackPack can power the clock mechanism. If the 470μF capacitor is still charging, it might take a few ticks before the mechanism starts moving, but after this, it will be charged to the correct bias point. Advance the time (using S1) until it is exactly at the next half-hour. Check that the clock starts as expected; it should take no more than 30 minutes. About 12 hours after being powered on, the Clock Driver will check its time source again. Check that the clock continues to run after this; if so, then all is well. Other uses The Battery BackPack can also be used as a general-purpose replacement for a pair of AA or AAA cells in lightduty applications. It will not have the high current capability of AA cells, with the limit being about 500mA, as noted earlier. This is partly limited by the dissipation and dropout voltage of the regulator, so it will vary with the tolerance of the load to voltage sagging and the state of charge of the lithium-ion cell. Brief bursts of higher current draw may be fine, as long as they do not overheat the regulator or diodes. Refer to Figs.2 & 3 to check the behaviour at your desired operating current. CON3 also provides a connection that is upstream of the regulator, so could be used if your circuit can tolerate voltages above 3.3V. In this case, you might also wish to bridge out one of D2 or D3 to reduce the voltage drop. Modules like the Raspberry Pi Pico include a buck-boost regulator that can operate between 1.8V and 5.5V, so that is an example of a case where feeding the raw battery voltage to the device is ideal and will give the best efficiency. If you don’t need the 3.3V output, you could also omit REG1 and the capacitor for the 3.3V rail (the 10μF part closest to the 470μF capacitor). Removing the regulator will delete the main quiescent current draw, so it might be preferred for extreme lowSC power applications. Parts List – Power Supply for GPS Clocks EACH BLOCK OF ISSUES COSTS $100 NOVEMBER 1987 – DECEMBER 1994 JANUARY 1995 – DECEMBER 1999 JANUARY 2000 – DECEMBER 2004 JANUARY 2005 – DECEMBER 2009 JANUARY 2010 – DECEMBER 2014 JANUARY 2015 – DECEMBER 2019 OUR NEWEST BLOCK COSTS $150 JANUARY 2020 – DECEMBER 2024 OR PAY $650 FOR THEM ALL (+ POST) 1 double-sided 56 × 39mm PCB coded 11105261 1 AA-size (14500) through-hole cell holder (BAT1) 1 14500-sized lithium-ion rechargeable cell with protection circuitry 1 USB-C power-only SMD socket (CON1) 1 2-way 0.1in/2.54mm pitch pin header (CON2; optional, only for non-USB supplies) 1 3-way 0.1in/2.54mm pitch pin header (CON3; optional, for general-purpose use) 1 4-way 0.1in/2.54mm pitch pin header (CON4) electrical tape or foam-backed double-sided tape for insulation Semiconductors 1 MCP73831T-2ACI/OT lithium-ion cell charge controller, SOT-23-5 (IC1) 1 MIC37100-3.3 3.3V LDO linear regulator, SOT-223 (REG1) 3 1N5819WS SMD schottky diodes, SOD-323 (D1-D3) 1 3mm red/green bicolour through-hole LED (LED1) Capacitors 1 470μF 10V radial electrolytic, up to 12mm tall 4 10μF 25V X5R M3216/1206-size SMD MLCC Resistors (all SMD M3216/1206-size ±1%, ⅛W) 2 100kW 1 10kW 2 5.1kW 2 1kW WWW.SILICONCHIP.COM. AU/SHOP/DIGITAL_PDFS Kit (SC7707, $25 + P&P): includes all the parts listed above except the Li-ion cell. 82 Australia's electronics magazine Silicon Chip siliconchip.com.au