Three Years 'EMCOMM Ready' 
EMCOMM
My Emergency Power Supply |
Each station must decide on how much power and energy are needed in an emergency. I took the QRP route to some extremes: My shortwave digital mode station consumes less than 20 W or less than 0.5 kWh/day. For a week of 24/7 service I would need around 4 kWh or 300 Ah of batteries. I decided to install 200 Ah/12.8 V LFP (LiFePO4) batteries, a 445 Wp solar panel and a 25 A (230 V) charger. Contents
Basic ConstructionI decided to mount the system on a scrap board that I mounted to a rack on hinges. If you want to follow me here, bolt the rack to the wall! The rack stands next to the station table. This minimizes the length of 12 V lines. The rack stands next to a window as you can see in the picture. The electronics disappear into the rack, protected from direct sunshine. Only a minimum can be seen. On top is a box with analog meters for solar current and voltage. 50 V and 10 A do nicely and show the activities of the MPPT Solar Charge Controller much better than any app. This box was especially valuable to examine the reaction of the solar panel to any form of shades – from clouds, from the houses and from trees. Next you see the 12 V outputs in the form of four Power Pole connectors with separate fuses. Lower rated fuses allow to use thinner cables. On the station table I have another Power Pole distributor. At the bottom are the battery connectors. The two batteries stand below, connected with 1 m long 16 mm2 (AWG 5) wires. 10 cm from the plus pole of each battery is a 20 A fuse. The maximum output is therefore 40 A or 500 W. The relatively long connections should even out high load currents. This is only a precaution: I hardly ever draw more than 100 W. The cable channel on the left hides and organizes most wire runs. It is right on the hinges. The cables are not moved much when turning the board open. A High Current InstallationThe thickest DC cables most radio amateurs know come with transceivers in the 100 W class. They are designed for moderate voltage drop at some 20 A. These currents only flow intermittently during transmitting and even then only for a part of the time. For digital modes with a high duty cycle like RTTY or FT8 most transmitters cannot run full power. They simply would overheat and distort the output signal. The highest currents appear during charging. Also consider that these currents often flow for hours! This is a real fire hazard. Build accordingly. 25 A is the maximum charging current I have designed for, despite the very moderate power draw of my station. On a sunny day I must be able to harvest and store the energy for several days of operation. Another technical topic: The voltage drop between the charge controllers and the batteries must be limited so the charging algorithms can work. For a 12 V system with LFP batteries this voltage drop should be limited to perhaps 200 mV, making it possible to fully charge the batteries with relatively high currents. Consider that solar energy comes with a relatively short maximum around noon. I set the saturation voltage of both charge controllers to 13.9 V, mostly because of the station computer (link within web site, German). It is rated for 12 V and gets up to 13.6 V in my installation. This has worked for years because those miniature computers I use contain no components that use 12 V. All the computer circuitry is behind switching regulators providing 5 V, 3.3 V or even less. The limiting factor might be the loading capacitor at the input. Most often that should be a 16 V capacitor. For a long life of the capacitor you should stay well below the rated voltage. I am using the second of these computers because of a nearby lightning strike – not because the first could not cope with the battery voltage. For LFP batteries you must rise the charging voltage well above 13.6 V, at least every few months. Generally the voltage curves of LFP batteries are very flat, influenced more by he current than the state of charge. From 13.6 V up the charging voltage rises sharply. This is why the battery management systems need this voltage range to start operating, trying to equalize the state of charge of all cells in the battery. 
The CircuitryOn top is the Bluetooth dongle of the 250 W inverter that has no internal Bluetooth. On the right you see one of the hinges that connect the board to the rack. Within minutes I can unplug all connections and dismount the board. 25 A ChargerNext is the charger. I nearly exclusively use it during the winter when hardly any solar energy is available. My special problem are high trees south of my garden. From the end of October to the end of February my house mostly stands in the shadows of the trees. I chose the 25 A model to balance the 200 Ah of battery capacity. In an emergency, power grid interruptions are likely. My station consumes around 40 Ah/day. Two hours of grid power would give me enough energy for another day of operation. This includes that I would need to charge my tablet computer as a minimalistic user interface. Most of the time, especially in winter, I configure the charger as a constant voltage source set to 13.2 V. This keeps the batteries charged in the 70-85% range. If the sun shines, I still have enough storage space to use the available solar energy. The charger is specified to consume 0.5 W without load. During the summer the charger is either unplugged or set to 13.1 V. This voltage leads to a slow discharge as the batteries must cover any current spikes. At this level 12.8 V LFP batteries have a charge/discharge offset of about 200 mV. Charging therefore happens at discharge voltages from 12.9 V down. This maximizes the solar harvest as more charge space is available. On the other side it keeps enough charge available at all times. I see this as the best setting for relays over the summer: Maximum solar harvest and enough charge to cover a rainy day or two. 250 W InverterThis part is not needed for the shack to work. But I can use a soldering iron while the grid is down or power my computer in the home office when the batteries are full. Remember: This is a stand alone system without a possibility to feed energy to the grid. Additionally I could move this inverter to the solar system in the cellar if the 800 W inverter there should die. My food storage scheme relies heavily on a working fridge (link within web site, German). 250 W is not much. I decided against a bigger unit to reduce the basic power consumption. The 250 W model is specified with 4,2 W without load, the 800 W model with 6,5 W. This appears to be not much, But multiply it with 8600 hours per year: 36 kWh/year and 56 kWh/year. Fuse HolderFor most, fuses are a secondary topic, especially at low voltages. But consider the currents that flow, especially if a fault occurs! A 100 Ah LFP battery can provde 100 A or more until its internal battery management systems turns off. The basic rule: Between any power source and any wire must be a fuse that blows before the wire gets too hot. This means:
The fuse holder I chose provides a minus bar on one side and fused plus poles. I use the minus bar only for smaller consumers, that is the power pole connector and the 250 W inverter. The central minus reference is the user side of the 500 A shunt – see below. The fuse holder I use solves several problems quite elegantly, but has a grave problem: The plus bar is a thin metal strip. The fuse contacts were stamped out of this strip. This leaves only 2 mm wide links to the right and left of the stamped out windows. I found this after the plus bolt got quite warm during solar charge with around 20 A. It is quite easy to solder two 2.5 mm2 wires to the underside of the plus bar. The plus bar is short and the heat can be easily distributed. The maximum current in the fuse holder is around 50 A. This should be no problem. A detail that troubles some but is standard practice: The solar charge controller is rated for 35 A. I had to fuse it to at least 40 A. I therefore wired the two leftmost 25 A fuses in parallel. Battery Monitoring SystemTo the left of the fuse holder is the battery monitoring system (BMS), consisting of a 500 A shunt resistor to measure the battery current and a bit of electronics. These days Victron also offers 100 A shunts, but I bought earlier. The battery monitoring system needs a bit of power and measures the battery voltage. The fuse for the BMS is the third from the left. Even this 10 cm long wire is fused! Otherwise a short in the BMS could start a fire. The BMS measures the battery current all the time and integrates the current over the time. This way it can compute the Ah going in and out of the batteries. This is important because I do not use any lead chemistry in the batteries: For lead batteries the battery voltage is a usable measure for the state of charge. Most lithium batteries have a very flat curve over most of the discharge cycle. The only reasonable point to decide the state of charge is when the battery is full. In an emergency power supply it is important to know how much energy is still available, for example to start the gas generator in time. Central Ground PointThe lower contact of the BMS shunt is the central ground point of the emergency power system. Solar charge controller, 230 V charger and fuse holder are connected here. In principle, the inverter should have been connected here. But you also should not add more than three lugs to one bolted contact. I therefore connected the inverter through the fuse holder. My typical loads are below 100 W. The battery is on the upper pole of the shunt – naturally. Building a ground system is more complex than many think:
Solar Charge ControllerThe solar charge controller reduces the solar voltage from around 40 V to the battery voltage. Both voltages are variable. The charge controller sets the solar voltage to the value that provides most output power – therefore Maximum Power PoinT (MPPT). On the output side it works like any good battery charger, but with variable charging current. This variable charging current influences the design: You cannot simply rise the maximum charging voltage to compensate the voltage drop to the battery as the voltage drop can also be considerably lower with lower charge currents. Therefore my recommendation to limit the power drop to about 200 mV at most currents. If that reduces the maximum state of charge around noon, it should be no big topic: In the afternoon should come enough energy to top up the battery. This consideration made me install the solar charge controller top down. All wire lengths around the solar charge controller have been minimized. The solar charge controller is rated to 35 A or 500 W. It only gets only moderately warm. The heat sink is below. Victron rates it for a much higher temperature than it will ever see in my shack. A 25 A model would have been possible. But power electronics always die first and this is the most important power source of my shack. 
MonitoringEspecially in an emergency the state of the power supply is important information. A major reason to go all Victron was their Android app. It provides an overview over all the blue boxes and details of each unit. The picture on the right shows the situation of a mostly overcast winter day.
The solar charge controller provides 2.00 A and 0.89 A arrive at the battery. The difference, 1.11 A, fed to the station. This is a bit below normal, but computers are known for their very variable power draw. This is why I specify the energy consumption of my station to < 0.5 kWh/day. Consider that all values can only be correct to +/- 1 digit. Therefore the nominally 20 mV difference means something between 10 and 30 mV. ...and What About All That Noise?Switching regulators have a very bad reputation in ham radio. Too many of us have struggled like OH8STN did [1]. From my experience the biggest hole, especially on a portable station, is the receiver input. I know what I am talking about from my mobile shortwave station: I started with my car at 59++ on all bands and ended with watching JA stations on 40m while driving through the winter night. Obviously you should use equipment where no Chinese backyard manufacturer has optimized production costs by forgetting certain blocking capacitors (link within web site, German). But the most important measure in all my tests are current chokes on both ends of the antenna cable. Its function is important enough to explain it here once again:
Short end of a long speech: I have absolutely no noise problems with three switching regulators in the multi 100 W range one meter away from my station. The only further measure I presently have are two thin ferrite rings over the 12 V cables of my, extremely cheap from a sale, 2 kW inverter in the cellar. There is a direct 12 V line between both power supplies. If some of this sounds familiar: Scroll up to Central Ground Point. What Is Missing – Or Not?Battery MaintenanceIf this project contained any lead battery, something would be missing: A low voltage cutoff. The same would be valid for any lithium chemistry cell. But nearly all LFP batteries contain a battery management system (BMS). A BMS has two tasks:
Some producers like Eremit use that to proclaim their batteries virtually bullet proof, easily chargeable with any experimental DC power supply that can limit its output current to safe levels. One thing that is clearly missing here is protection against deep discharge, despite the undervoltage cutoff: Nearly all batteries suffer from self discharge. You need to put some charge into any rechargeable battery before you put it a way for a longer time. A small solar module that remains connected and gets some light should be enough. Prevention of Unsupervised, Automatic OperationDigital modes often give stations useful functions without an operator like sending beacons or files. Many countries have limitations in this field. An emergency power supply as discussed here naturally leads to such situations. One problem is how to turn off the station or at least the transmitter. A traditional audio interface might give access to the PTT line to the transceiver. An USB interface on the transceiver calls for more creative software solutions. Simply turning off the computer power supply is definitely no good solution. Windows Plug and Pray is especially stupid with multiple sound card interfaces we nearly always have. This might be the most important reason for Windows sounds appearing on the bands. The other question is how to detect the presence of the operator. A movement detector in the shack with some delay timer might work in some situations. Another is to detect the smartphones of any hams in the family through the WLAN. House automation systems can help here. I experimented with the hm_pdetect add-on for Homematic that searches for the MAC address of the smartphone. It proved to be unreliable as the smartphone might lose the WLAN connection. Since I have retired, I returned to the manual method: I deactivate the PTT checkbox of VarAC. ConclusionThis system is the essence of 7 years tinkering with solar technology. It is not presented as a standard, but as a possible way. A ham that simply wants to be prepared, should first think about additional batteries for his handheld FM transceiver. But if you operate an EMCOMM station of any kind, you might find some ideas here. If you are responsible for a relay station you should think about something like my system. Do not be afraid if your relay consumes 100 W or more, you have lots of possibilities to reduce power consumption. I propose that you shut down your normal relay electronics completely and connect a 10 W handheld transceiver to the relay antenna. A miniature computer like an Arduino evaluates the RX audio, for example decodes CTCSS tones. Through a small audio module it then sends a startup message and turns on the relay electronics. This miniature system could also transmit short messages like "here is DB0ZZZ" every 15 min. In an emergency, status messages like "Next EMCOMM net on this station at 15:00 hours" could be uploaded and sent out repeatedly. If you relay system contains an X86 computer, see my remarks about my shack computer. An emergency power system as I describe here could also greatly reduce your energy costs. At many exposed sites electricity is very expensive. References
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Alexander von Obert * http://www.dl4no.de/thema/myemerge.htm Letzte Änderung: 04.11.26 (Erstfassung) |