Fullscreen HTML5Med Res (??MB)HTML4

Off grid? On grid with battery backup? How do you monitor the state of your batteries? y r e r t e t Ba ti Logg l u M By TIM BLYTHMAN Knowing the condition of your batteries is essential for keeping them healthy long-term. A system that can monitor and log vital battery statistics is a great aid, and can help you to avoid having to shell out for expensive replacements. It can also be used for troubleshooting, such as when you don’t know which device is responsible for periodically discharging a battery. S olar and wind power is growing in use and getting cheaper, so there is a need to maintain batteries associated with such systems. You might also have a large battery in a shed, caravan, boat or another vehicle that you need to monitor. Backup batteries for mains power failures are another case where you might need a battery monitor or logger. Our new Battery Monitor Logger is versatile and capable, being able to handle a charger and two separate loads out-of-the-box. It is based on a Micromite LCD BackPack, so can be reprogrammed in MMBasic, Micromite’s variant of the BASIC language. But as we have written software with many useful features, you don’t need to do any programming. We last published a Battery Capacity Meter in June & July 2009 (www.siliconchip.com .au/Series/44). It featured a PIC microcontroller capable of monitoring a battery’s voltage and current via an external current meas28 Silicon Chip uring shunt. It could log data as well as calculate such things as battery capacity and estimated battery run time. New features The 2009 Battery Capacity Meter used a single shunt so it could only monitor the overall current moving into or out of the attached battery. Our new design supports up to three shunts, so it can monitor three separate current paths, helping you to split out the charging or discharging figures across multiple loads and/or generators. It even includes a fourth internal shunt for monitoring its own power usage. For example, you might have a solar panel array and a wind generator (or several) and want to keep track of the energy they generate separately. Or you might have several loads like a fridge, lights and a kettle and want to see which one is consuming the most energy. Australia’s electronics magazine siliconchip.com.au The old design was also limited to around 60V at its input (compared to 100V for this one) and could also store a minimal amount of data in the PIC. The PIC32 we have used in this design has much more storage space, so it can record more data for longer. The battery voltage and currents are sampled at 10-second intervals. That data is averaged every hour to give up to two days of hourly samples. The hourly samples are also averaged over each day to give about a fortnight of daily values. The flow of both charge and energy is logged, to provide capacity values in Ah (amp-hours) and Wh (watt-hours). You specify the full and empty voltages of your battery, plus the battery capacity, so that the unit can self-calibrate when the battery is either fully charged or discharged. A simple, linear voltage state-ofcharge value is also calculated, giving a rough indication of battery state when the more accurate information is not available. 10A, you can use the same arrangement except with external shunts. These will typically have a lower resistance and also can handle higher dissipation, both factors allowing greater currents to flow safely. For example, you can get 100A shunts quite easily, or even 500A shunts. Circuit design The circuit of the Battery Monitor Logger is shown in Fig.2. It has been designed as a complete Micromitecompatible board, rather than an add-on board for a Micromite LCD BackPack. This allows us to control its power usage better, reducing the current drawn from the battery. Operating concept Fig.1(a) shows the simplest way to use the Battery Monitor Logger. The battery connects to a two-way screw terminal (CON3) while the positive ends of up to three loads or charging sources connect to the contacts of three-way screw terminal CON3a. The negative ends of those loads/ charging sources connect directly to the battery negative (ground). This allows the Battery Monitor Logger to independently measure and display the current flowing to or from each load or charging source. It also produces a total current in/out figure and uses this to keep track of the battery’s state-of-charge in amp-hours (Ah). Multiplying this by the battery’s current voltage gives a nominal watthours (Wh) figure for the current state of charge. If you have more than three external devices to connect, they can share terminals on CON3a, as shown in Fig.1(b). For example, one terminal is shared by two loads (LOAD1 & LOAD2). The measurement on that channel will be the total load current for these two devices. Another terminal is shared by two charging sources (SOLAR & WIND), and likewise, their currents will be summed. The third terminal is shared by LOAD3 and a mains charger. In this case, the unit will measure the net current flow in/out – ie, it will see a flow into the battery if the charger current exceeds the current drawn by LOAD3, a flow out if the situation is reversed, and will measure zero if the two currents are equal (ie, the LOAD3 current is supplied by the charger). If you need to monitor currents over siliconchip.com.au Fig.1: three examples of how you could use the Battery Logger/Monitor. The simplest configuration, at top, uses its internal shunts to monitor the currents (up to 10A) into or out of three loads/charging sources. Or as shown in (B), you can connect more than three loads/charging sources, with some of them sharing shunts. For higher-current applications (up to hundreds of amps), external shunts can be used, as in (C). Australia’s electronics magazine February 2021  29 As with any battery-operated device, it’s important to consider power consumption during the design phase. The battery and load/charger terminals are at lower right, with the bottom half of the right-hand page showing the sensing circuitry. Other external connections (USB, serial, programming etc) are arranged along the left-hand side, with the BackPack circuitry occupying most of the left-hand page, plus the display at centre-right. The unit’s power supply is across the top of both pages. The Micromite V2 BackPack (May 2017; siliconchip .com. au/Article/10652) is the closest BackPack variant to our design. This comparison is only for the sake of explaining some of our design choices; it is not important if you are coming to this circuit without knowing about the earlier designs. We’ve opted to use the 2.8in (7cm diagonal) LCD touch- l SC Ó BATTERY multi-logger Fig.2: the circuit includes the equivalent of an entire Micromite V2 BackPack, a precision multi-channel ADC and a switchmode regulator capable of running the device from a DC supply between 6V and 100V. It monitors the battery voltage, the current to/from three external points and its own current consumption and logs all this (plus the current battery state-of-charge) to the internal flash memory of microcontroller IC1. 30 Silicon Chip Australia’s electronics magazine siliconchip.com.au screen in this design, rather than the 3.5in (9cm) version we’ve been using more recently (eg, in the V3 BackPack), as the smaller display uses slightly less power. The V3 BackPack also has many features which simply aren’t needed in this case, hence our choice of the V2 BackPack as the basis for this design. The main advantage it has compared to the original Micromite BackPack is the inbuilt USB-Serial interface. siliconchip.com.au Battery sensing The main battery sensing circuitry centres on IC5 (an AD7192) and REF1 (a MAX6071). IC5 is a four-channel 24-bit ADC (analog-to-digital converter) with an SPI serial interface. It is supplied from REG2’s 3.3V output, with its analog rail filtered by a 10µH inductor. Each of its 3.3V supply pins is bypassed by a 100nF capacitor. IC5 shares the SPI bus with the LCD touchscreen, with Australia’s electronics magazine February 2021  31 IC1’s pin 24 used for the If larger external CS function, to indicate shunts are used instead, when IC5 is being adyou just need to run low• Battery voltage: 6-100V dressed. current sensing wires • Current monitoring: up to three chargers or loads, IC5 needs a stable reffrom both their ends, monitored separately erence voltage to convert back to CON3/CON3A. • Current handling: limited only by the shunts used voltages into digital valThe shunt values can be ues, and this comes from set in the software to ac(10A with onboard shunts) REF1, a MAX6071 2.5V count for practically any • Current resolution: 0.1% (10mA with onboard shunts) reference. It is a very lowresistance value. • Operating current: <1mA while logging (with display off) noise and precise voltage A local analog ground • User interface: 2.8-inch colour touchscreen reference chip, and it is net separates the analog • Firmware: Programmed in BASIC supplied with 3.3V from voltages from digital SPI • Data logging: can be viewed on device graphically, REG2, with 100nF casignals. pacitors on its input and or downloaded as CSV files Supply current output. Its output sup• Measurements: current charge (Ah) and energy (Wh) plies IC5’s REFIN1+ (pin The current drawn by • State of charge: displayed based on voltage and charge 15), while IC5’s REFIN1the circuit itself is mod(pin 16) is tied to analog est but not insignificant, ground. and needs to be accountEach of the four analog inputs to IC5 is fed by a ed for to get accurate measurements. Since it is a fairly low 390kΩ/10kΩ divider, bypassed at the bottom by a 100µF current, we use a different technique to monitor it. Any capacitor. This means that the nominal full-scale reading current flowing into our circuit from the battery at CON3 is 100V with a resolution of around 6µV, and settling times flows out through a 100mΩ shunt resistor, generating a of around ten seconds. We use the ADC to perform a convoltage below ground proportional to the current. version cycle (of all channels) about once every ten secIC6 is a single-channel op amp in a five-pin SOT23-5 onds, a slow rate needed to obtain maximum resolution. SMD package. It is wired as an inverting amplifier with a One of the dividers is connected directly across the gain of 100 (100kΩ/1kΩ), presenting a voltage to IC1’s pin battery at CON3. The other three monitor the voltage at 4 where the micro’s internal ADC can read it. the load/charger end of the three shunts which connect The 100nF capacitor and 100kΩ resistor provide simibetween the BAT terminal of CON3 and the terminals of lar smoothing on this signal (a time constant of around ten CON3A. By measuring the difference between the voltages seconds) so that it too can be sampled at similar intervals fed to the ADC, we can determine the current flow into or to the other channels. out of each terminal. When the Battery Monitor Logger is operating, the LED The PCB provides pads for 15mΩ shunt resistors which backlight of the LCD panel consumes the most power, so allow a theoretical resolution under 10mA. These are 3W a high PWM frequency is used to ensure that this measparts, notionally allowing up to 14A to be sensed. In pracurement is accurate. tice, the terminals limit this to around 10A. Features & specifications Power supply There are two possible power sources in this circuit; USB socket CON5 can supply 5V, while the battery connection at CON3 handles up to 100V from the battery being monitored. There are several components on the board that have a 100V maximum rating, so this is a hard limit and should not be exceeded. A switchmode buck regulator chip, IC4 (LM5163) efficiently steps the battery voltage down to 5V. Its supply from the battery via CON3 is bypassed with These photos show an earlier prototype, which was missing the MISO series resistor and CON6 (which is not used by the current version of the software). Some of the resistor and capacitor values are slightly different too, but overall it looks quite similar to the final version. Take note of the values shown on the silkscreen PCB overlay diagram during construction. 32 Silicon Chip Australia’s electronics magazine siliconchip.com.au a 2.2µF capacitor and fed into pins 2 (VIN) and 1 (GND). A voltage above 1.5V on pin 3 (EN) enables the regulator, which is equivalent to a voltage of around 5.5V at CON3 due to the 1MΩ/390kΩ resistive divider. Apart from accepting up to 100V at its input, IC4 also has an extremely low idle current of just 10.5µA with no load, and not much more at light loads. Its efficiency varies with the input voltage and load current, but is typically in the 75-90% range. See the panel below for more details on this handy little chip. It switches its pin 8 output (SW) alternately between VIN and GND using a pair of internal N-channel Mosfets. The upper Mosfet has its gate voltage supplied from the 2.2nF capacitor on pin 7 (BOOST). The pulses are smoothed by the 120µH inductor and a 22µF capacitor to provide the output voltage. The voltage on feedback pin 5 (FB) is internally compared to a 1.2V reference, so the 30kΩ/10kΩ divider sets the output voltage to 4.8V. This is set to be slightly less than 5V so that if an alternative 5V supply is available, it takes over from the battery. Schottky diode D2 feeds the 4.8V into a pi filter formed of two further 10µF capacitors and a 10µH inductor. The 1nF capacitor across the 30kΩ resistor at the top of the FB divider helps with the stability of the circuit that drives the output pulses, by ensuring sufficient ripple at the FB pin for the circuit to operate correctly. See our panel for more detail on this. Microcontroller details This approximately 5V rail then feeds the Micromite sec- tion of the circuit. MCP1700-3.3 REG2 and its associated bypass capacitors provide the 3.3V supply for microcontroller IC1. This is a 32-bit, 50MHz micro (PIC32MX170F256B) and is surrounded by its own complement of bypass capacitors. IC1 is programmed with the MMBasic firmware and runs a BASIC program to implement the Battery Monitor Logger functions. While some Micromite BackPacks used the 28-pin DIP version of this IC, the Battery Monitor Logger uses the 28pin SMD (SOIC) part. It works identically but is smaller, so we can cram more onto the PCB, and most of the other ICs are only available as SMDs anyway. In this case, its pins are relatively far apart (on a 1.27mm/0.05in pitch) so it is not difficult to solder. To save power, the micro can switch 5V power on and off to the touchscreen via the 14-way LCD header. A high level on IC1’s pin 10 turns on N-channel Mosfet Q4, which is otherwise held off by a 10kΩ pull-down resistor. When Q4 is on, it pulls P-channel Mosfet Q3’s gate low, which allows 5V to flow from Q3’s source to drain and into the LCD panel’s supply pin. A similar arrangement, controlled by IC1’s pin 26 via Mosfets Q2 and Q1, switches power to the LCD panel’s LED backlight. Typically, a PWM signal is applied to pin 26, modulating the backlight brightness. Unlike the Micromite BackPack V2, which had PWM brightness control, we have omitted the option of manual backlight control as the backlight is easily the biggest user of power in the circuit. So it needs to be fully shut off during logging and monitoring. DS3231 MEMS variant The DS3231 real-time clock IC has been the go-to choice for keeping track of time for the last five years or so. Its appeal is no doubt enhanced by the fact that it is available in an easy-to-use module typically sold as an Arduino accessory. Such a module was the subject of our first El Cheapo Modules feature from October 2016 (siliconchip.com.au/Article/10296), which we used in several projects, typically in combination with a Micromite. The module includes I2C pullup resistors, an I2C EEPROM and a cell holder. The module simplifies connection as it includes all that is needed for the DS3231 chip to work, but sometimes it’s too big. We used the bare DS3231 IC (which comes in a wide 16-pin SOIC SMD package) in our Micromite BackPack V3 (August 2019; siliconchip.com.au/ Article/11764) and the Ol’ Timer II clock (July 2020; siliconchip. com.au/Article/14493). To support those projects, we kept a stock of those ICs. One day, we were surprised to receive a package of small 8-pin SOIC parts instead of the wide 16-pin SOICs that we were expecting. Had we been conned? No; we had received the DS3231M variant instead. Those familiar with the DS3231 will know that it only uses eight of its pins; the lower pins are marked NC (“not connected”). The reason for siliconchip.com.au the large package is not that it needs 16 pins, but because it includes a temperature-compensated crystal oscillator inside the plastic IC case, which would not fit inside an 8-pin package chip. But with the advance of MEMS technology (see our article in the November 2020 issue: siliconchip.com. au/Article/14635), the crystal oscillator inside the DS3231 has been superseded by a smaller MEMS device. So given their small size and decent performance, we decided to try them out in this project. We found the DS3231M to work the same as the DS3231. The nominal accuracy is slightly worse at ±5ppm compared to ±3.5ppm, but for situations where size is of concern, the smaller package is the overriding concern. The MEMS part doesn’t appear to suffer from crystal ageing either, which means that in the longer term, it could be more accurate unless this is compensated for in the earlier version of the chip. The backup battery current draw appears to be higher for the MEMS part in typical cases, but in most cases, the battery life will still be close to its shelf life. In this particular project, we’ve made allowances for either part in the PCB design, with a dual footprint that suits both the wide 16pin SOIC part and the narrower 8-pin SOIC part. We don’t know if the DS3231M will end up more popular than the original DS3231, but we’re ready for either eventuality. Australia’s electronics magazine February 2021  33 Screen1: The main screen provides all the critical statistics for your battery, as well as three simple menu options for accessing other features. The greyed values seen are capacity calculations which are not yet valid, as the Logger has not detected a complete charge and discharge cycle; they will light up brighter when that happens. Screen2: The Data screen provides a graphical view of the logged data. Different timespans can be shown, and the display will automatically scroll once a minute to show current data. The Weeks option provides around a fortnight of data. Data can also be dumped as CSV rows over the console serial port with the Export button. Serial communications Both IC1 and IC2 have their in-circuit serial programming (ICSP) pins broken out to the edge of the PCB at CON2 and CON1 respectively. This is a feature not seen on the other BackPacks, but we have included it here because the SMD ICs used here are more difficult to program out-of-circuit than through-hole (DIP) chips. A DS3231 real-time clock, IC3, provides accurate timekeeping over long periods. Its I2C serial bus pins 15 and 16 (SDA and SCL) connect to IC1 at pins 18 and 17, the I2C pins used by the Micromite firmware. Two 4.7kΩ resistors provide the pullups needed by the I2C protocol. The PCB is also fitted with a SOIC-8 footprint to allow the similar DS3231M (which uses a MEMS oscillator rather than a crystal) to be used instead. See the separate panel explaining the differences. IC1 sends display data and gets touch events back from the touchscreen using an SPI serial bus on its pins 3, 14 and 25 (MOSI, MISO and SCK). These connect to the LCD panel’s pin 6 and 12 (MOSI), pin 13 (MISO) and pins 7 and 10 (SCK). MISO stands for “master in, slave out” while MOSI stands for “master out, slave in”. The MISO line has a series 1kΩ resistor so that it can still operate when the LCD panel is switched off. These signals, plus a chip select signal from IC1’s pin 9, also connect to the SD card header at the other end of the LCD panel PCB via a four-pin header. We had planned to use the SD card to store data, but flash memory limitations in the micro mean that there isn’t enough space to include the (rather large) libraries needed to do this. IC2 is an 8-bit PIC16F1455 microcontroller programmed with the Microbridge firmware. This allows it to act as a USB-Serial bridge, and it can also program the PIC32 microcontroller. Pushbutton S1 is used to switch IC2 between USB-Serial and programming modes, with LED1 flashing to indicate that it is passing serial data, or lighting up solidly when in programming mode. Mini USB Type-B socket CON5 is used both for USB communications (D+/D-) as well as optionally supplying 5V power. Schottky diode D1 feeds USB 5V to the Micromite 5V rail. Jumper JP1 provides the means to bypass D1 if needed. REG1 is identical to REG2 and supplies 3.3V to IC2 independently. Serial TX and RX signals are bridged to and from the virtual USB-Serial port by IC2. These connect between its pins 5 and 6, via 1kΩ resistors, to Micromite console pins 11 and 12 on IC1. IC2’s pins 2, 3 and 7 can be used to program IC1 via its ICSP interface; they are connected to IC1’s pins 4, 5 and 1 respectively. The PGD signal travels via JP2, which allows IC1’s pin 4 to be used as an analog input when it is not being used for programming. 34 Silicon Chip Software operation Some of the following may seem obscure to those not familiar with MMBasic, but this information could come in handy if you want to change the code. MMBasic certainly makes driving the LCD (TFT) panel easy, as it performs startup initialisation and has built-in BASIC commands for drawing on and writing to the display. But it needs some help to work with our circuit arrangement, which starts with the LCD panel powered off, and therefore not ready to accept the initialisation commands that are automatically sent. So we need to add a routine (in the MM.STARTUP subroutine) to set pin 10 as an output and set it high, then rerun the LCD initialisation code. Every time we power up the display after shutting it down, we need to trigger that code. We also need to control the other lines that run to the LCD panel, as some of these idle high by default and would therefore waste power. MMBasic does not allow direct control of these, as the firmware reserves them to control the LCD panel, so we need to ‘POKE’ directly to IC1’s registers and then run a command to reinitialise the LCD controller. Similarly, shutting down the controller requires direct POKEs to shut down those pins. No software deinitialisation Australia’s electronics magazine siliconchip.com.au The LM5163 switchmode regulator IC Our initial design plans for the Battery Logger set the ambitious target of designing it to work at up to 80V, improving on the 60V limit of the old Battery Capacity Meter. That one used an LM2574HV integrated switchmode IC operating at a fixed frequency of 50kHz, requiring a sizeable toroidal inductor and electrolytic capacitor. Hoping that that state of the art had progressed in the last decade, we decided to look for newer parts. We found plenty of parts capable of working with a 100V supply, which is impressive. 1MHz switching frequencies are no longer uncommon. This much higher switching frequency means that a smaller inductor and capacitors are needed, helping us to keep our board compact. Many parts we found could only deliver 100mA. While this might have been sufficient with careful control of the LCD backlighting, we wanted more headroom. The LM5163 came in as the cheapest part capable of more than 100mA (500mA) in an easily-soldered SOIC-8 package, which is a good compromise between size and ease of handling. As is typical of modern buck regulator designs, it is a synchronous type, meaning it has two internal switches. The incoming voltage is switched to the inductor by a high-side internal Mosfet. When the Mosfet is off, a second low-side Mosfet is switched on to provide a path for the inductor current to circulate. This removes the need for an external diode to serve this role and increases its efficiency. The LM5163 is a COT (constant on-time) design; the time that the high-side Mosfet is switched on is set by an external resistor, after which it is switched off. The feedback pin monitors the output voltage, and when the output voltage has decayed, another on-cycle begins. So the duty cycle is modulated to maintain the desired output voltage, but the constant on-time means that the switching frequency varies, although it can be predicted. When we built our first prototype, everything worked as expected; we were truly impressed with how flexible and easy-to-use this tiny part was. But then, it started squealing! The tone would change with load (which we could easily modulate by adjusting the LCD backlight intensity) and input voltage. It was bad enough, especially around 12V, that we needed to do something about it. The cause was electrical noise, which was affecting when it would switch on. It might switch on early, which causes the output voltage to rise. This will cause the next switch-on to be delayed, as the controller will be waiting for the output voltage to drop below its threshold. The output pulses start to cluster into bursts, and it is these clusters that occur at audible frequencies, causing the high-pitched squealing we were hearing (‘subharmonic oscillation’) – see below. As we found with our Switchmode 78xx replacement (siliconchip. com.au/Article/14533), trying to get these sort of parts to operate optimally over a wide range of input voltages can be tricky. In that case, extra output capacitance helped. Fortunately, a section of the LM5163 data sheet (reproduced in Fig.4) describes methods to avoid this. The aim is to increase the ripple seen by the FB pin, so that the regulator has a clearly defined time to switch on, despite the presence of noise. We tried the Type 1 method, which involves adding series resistance to the output capacitor. The extra resistance means that the voltage seen at the FB pin is influenced less by the capacitor and more by the pulses from the inductor. But it also means that the output capacitor is less effective at filtering the output voltage, and we found it did little to reduce the squealing. So we tried part of the Type 2 method (omitting the series resistor from Type 1) and simply added the ‘feedforward’ capacitor in parallel with the top feedback divider resistor. This means that the FB pin sees the full amplitude of the output ripple voltage, as it is coupled directly by the capacitor rather than being simply divided by the resistor chain. This effectively quadruples the ripple seen by the FB pin with our 30kΩ/10kΩ divider, without degrading filtering. That eliminated the squealing, so we have kept it in our final design. Any switching device which depends on a feedback voltage from a divider to switch its output elements can benefit from having a feedforward capacitor. It depends on the frequency of operation, capacitor value and divider ratio, though. A word of caution: while this capacitor may appear to be a cure-all, it does have the side-effect of slowing down response to transients as it reduces the closed-loop gain for higher frequency components. Fig.4: Texas Instruments’ recommended solutions for subharmonic oscillation or ‘squegging’ in the LM5163. We tried Type 1, and it didn’t work, but Type 2 did. It only requires the addition of a low-value feedforward capacitor, Cff, across the upper half of the feedback divider. Type 3 is similar but adds another pole for improved transient response; that’s overkill in our application. Fig.3: usually, low ESR is considered desirable in a capacitor as it gives superior filtering, but when it filters out the ripple too effectively, it affects the regulator’s ability to produce pulses regularly. siliconchip.com.au Australia’s electronics magazine February 2021  35 Parts list – Battery Multi-Logger 1 double-sided PCB coded 11106201, measuring 86mm x 50mm 1 2.8in LCD touch panel with ILI9341 controller 1 UB3 Jiffy box (optional, depending on desired mounting) 1 laser-cut acrylic panel to suit LCD and UB3 box [SC3456, SC3337, SC5063 or sim.] 2 5-pin right-angle headers (CON1, CON2; both optional, for programming IC2 & IC1) 1 2-way 5/5.08mm-pitch screw terminal (CON3) 1 3-way 5/5.08mm-pitch screw terminal (CON3A) 2 2-pin headers (CON4 & JP1; both optional) 1 SMD mini-USB socket (CON5) 1 3-way pin header (CON6, serial port; optional) 1 3-pin header (JP2) 2 jumpers/shorting blocks (JP1,JP2) 1 SMD coin cell holder (BAT1) [BAT-HLD-001 – Digi-key, Mouser etc] 1 CR2032/CR2025 cell or similar (BAT1) 1 120µH 6mm x 6mm SMD inductor (L1) [eg, SRN6045TA-121M – Digi-Key, Mouser etc] 2 10µH 1206/3216-size SMD chip inductors (L2,L3) 1 SMD or through-hole 4-pin tactile pushbutton switch (S1) 1 14-pin header socket strip (for LCD) 1 4-way female socket strip (for LCD) 8 M3 x 6mm panhead machine screws 4 M3 x 12mm tapped spacers 4 M3 x 1mm untapped spacers (eg, stacks of 3mm ID washers) 3 heavy-duty current shunts [eg, Jaycar QP5415, Altronics Q0480 – optional, see text] hookup and heavy-duty wiring to suit shunts, batteries and load (see text) Semiconductors 1 PIC32MX170F256B-I/SO 32-bit microcontroller programmed with MMBasic or 11110620A.hex, SOIC-28 (IC1) 1 PIC16F1455-I/SL 8-bit microcontroller programmed with Microbridge firmware, SOIC-14 (IC2) 1 DS3231/DS3231M real-time clock IC, wide SOIC-16 or SOIC-8 (IC3) 1 LM5163DDAR synchronous buck regulator, SOIC-8 (IC4) 1 AD7192BRUZ 24-bit ADC, TSSOP-24 (IC5) 1 NCS325 CMOS op amp, SOT-23-5 (IC6) 1 MAX6071AAUT25+TT high-precision 2.5V reference, SOT23-6 (REF1) 2 MCP1700-3.3 low-dropout 3.3V regulators, SOT-23 (REG1,REG2) 2 IRLML2244TRPBF P-channel MOSFETs, SOT-23 (Q1,Q3) 2 2N7002 N-channel MOSFETs, SOT-23 (Q2,Q4) 1 3mm or SMD M3216/1206 LED (LED1) 2 SS14 (or equivalent) 40V 1A SMD schottky diodes, DO-214AC (D1,D2) Capacitors (all SMD M3216/1206 size) 4 100µF 6.3V X5R 1 22µF 16V X5R 7 10µF 50V X7R 1 2.2µF 100V X7R 10 100nF 50V X7R 1 2.2nF 50V C0G/NP0 1 1nF 50V C0G/NP0 Resistors (all 1% SMD M3216/1206 size 1/8W metal film except where noted) 1 1MΩ (code 105 or 1004) 5 390kΩ (code 394 or 3903) 2 100kΩ (code 104 or 1003) 2 30kΩ (code 303of 3002) 8 10kΩ (code 103 or 1002) 2 4.7kΩ (code 472 or 4701) 8 1kΩ (code 102 or 1001) 1 0.1Ω (code R100 or 0R10) 3 15mΩ 1% 3W (M6331/2512 size; not needed if external current shunts are used) 36 Silicon Chip Australia’s electronics magazine is needed as the LCD can simply be powered down from any state. Despite this complication, it’s relatively easy to sense touches on the LCD panel even if it is shut down. This is necessary, as the user needs some way to wake the unit up if it is in a low power state. Even when the LCD is powered off, the TIRQ pin (which is connected to IC1’s pin 15) is pulled to GND whenever the panel is touched. As the Micromite firmware provides a weak pullup on this pin, simply monitoring the state of this pin is sufficient to know if a touch has occurred. The main job of the MMBasic program is to read the battery voltage and the voltage across the three shunts to infer battery voltages and currents. It logs these to variables which are kept in RAM and they are regularly saved to internal flash memory. With the circuit running from the battery it is monitoring, it would take a major fault to shut it down and lose the contents in RAM, so only longerterm samples are saved to flash memory hourly. If the unit needs to be disconnected to work on the battery, at most one hour of data will be lost. When saving to flash, the data is averaged over a period before being archived. This means that less data needs to be stored, but a good amount of data can be kept for historical purposes. For example, you might like to compare how much power your solar panels are putting into your battery over a period of a few weeks. Data about current and power usage is also used to calculate parameters such as battery capacity and state of charge. The MMBasic program also provides a user interface to allow settings to be changed and values to be graphed and viewed. Plus there is the option to dump the data over a serial port so that it can be exported to a PC program for graphing and analysis. We’ll delve more into the software operation during the setup procedure next month. Next month In the second and final part of this feature, will have the complete PCB assembly details, microcontroller programming procedures, setup and operation instructions, calibration information along with the final construction procedure. SC siliconchip.com.au