1.5 KW Amplifier Off Grid

Running a full kilowatt from battery power has it’s challenges. This note steps through the design and installation of the amplifier, inverter, battery bank, and solar panels. First, the amplifier. It is a Flexradio 4O3A PGXL. To reach 1500 Watts output it requires 220 Volts AC. It does not run on DC. Here it is waiting for some AC while sitting next to the Flexradio 6600 exciter .

The next item is the inverter which provides the AC. A Victron Energy model Phoenix Inverter Smart 24/3000 has been chosen. It provides 3000 watts at 230 volts when connected to a 24 volt battery. This inverter was chosen because it has RFI certification, a reputation for meeting specifications, and it was less expensive than some others.

Next component is the 24 volt battery bank. LiFePO4 lithium ion batteries were chosen, of course. The capacity of each cell is 180 AH. Sixteen cells are connected in parallel/series to provide 24 volts DC with a capacity of 360 AH. Aren’t they pretty?

Finally, the solar panels. This array is four 100 watt panels wired in series/parallel to provide enough voltage and current to charge the batteries.

It really works! This is the control panel on the amplifier. Notice the output power is shown in the upper left corner, 1512 watts. Id is the current on the drains of the LDMOS finals, 43 amps. It is also nice to notice that under the load of a full kilowatt the inverter is holding the AC at 228 volts. With no load the voltage is 230 volts which means only a 2 volt drop at full output.

A clamp-on ammeter on one lead of the AC shows 9.3 amps of current.

One concern during the design phase was whether the AC connector on the amplifier was heavy enough. It is only a type C13 which is a 10 amp connector. Today’s measurement of 9.3 amps and the fact that the connector stayed perfectly cool to the touch is reassuring.

Next a look at some of the “glue” holding the stages together. The batteries are protected by a BMS. BMS stands for Battery Management System. It’s the big red thing. It will cut off the power if the voltage is too low or too high and it will try to keep the cells balanced. Electrically it goes between the negative terminal of the battery and the negative lead to the inverter. One small red wire goes to each cell to keep track of the state of the cell.

To the left of the BMS is the shunt resistor for a Victron battery monitor. A battery monitor is not required but makes it easy to keep track of the battery’s state-of-charge remotely. The black thing in the upper left corner is a terminal block. A copper strap connects between the terminal block and the shunt resistor. These items together make up the negative leg of the circuit.

In the positive leg there is only one component. That is a 250 amp fuse. Victron specified big 1/0 cable for the inverter DC power, and specified the 250 amp fuse.

One other piece of glue is the AC circuit breaker. A GFCI breaker is connected between the inverter and the amplifier to protect the inverter from overload and to protect humans from electrocution. It is 16 amp 240 volts. Don’t hunt for this unit in the 2020 NEC code. It is not a product designed for the U.S. market. It is designed for the 230 volt standard in Asia and Europe. This project is using European standards rather than U.S. The main difference is this is not “split phase” as in the U.S. which means there is no third wire on the 220 volt circuit. Why is it done this way? Because with the U.S “split phase” wiring a second inverter would be required to get the third wire, approximately doubling the cost. There is no safety or any other reason not to use the European standard. This system is entirely off grid and therefore NEC code is not required. Safety is a big concern and all European standards have been adhered to for safety.

Some observations

Sizing and the capacity of the components is a big part of any design. Today’s numbers provide some insight. The amplifier was drawing 9.3 amps AC at 228 volts. That’s 2120 watts (watts instead of VA because power factor is assumed to be 1 due to automatic PF correction, so watts and VA are equal). The amplifier was producing 1514 rf watts which is an efficiency of 71%. This calculates to 92 amps DC from the batteries (unfortunately no dc reading was taken). Looking next at state-of-charge (SOC), good practice for LifePO4 batteries is to limit depletion to 20 per cent SOC. That is, operate with SOC between 20% and 90% which provides 70% of rated capacity. Usable capacity would therefore be .70 X 360 AH or 252 AH ( or 6678 watts at 26.5 volts). Under a constant load of 92 amps the batteries would be expected to last 2.7 hours. With a 50 per cent duty cycle mode like FT8 the batteries should run at least 5 hours. There are no plans to operate at 1500 watts on FT8. This is just a worse case example.

Concerns would include whether the 4-panel solar array (400 watts) is adequate to maintain this large battery. That would not be the best way to look at it. Consider instead whether the array is adequate to provide typical amplifier operation. If the amplifier is operated two hours a day at 50 per cent duty cycle that would use 92 amps ( or 2438 watts at 26.5 volts). The panels generate about 2 kw on a good day (400 watts X 5 hours of sunshine). That is close to the 2.438 kw of typical usage. Good to go. On cloudy days the batteries are oversized so they will have capacity to operate for more than one day without sunshine. Winter is close and some hard numbers will soon be available. If needed the existing PWM charge controller could be replaced with a MPPT controller for 30 per cent more output. After a few weeks of operation in November the batteries have recharged to 90% every day when there was sunshine. As of late December during the solstice all is still functioning well.

A lot of effort and expense went into this project but it is proving that running a kilowatt from batteries off the grid is possible.

November 22, 2020 – The amplifier has a problem. It is refusing to transmit on all bands except 160 and only puts out 500 watts on 160. This problem started after receiving a message that SWR was too high and the amp should be shut down. The amp was quickly shutdown but has not worked correctly since. It was connected to the A3S antenna on either 15 or 17 meters when the failure occurred. Possibly the amp was putting out too much power for the A3S and a trap shorted out. Apparently the amp’s protection circuits failed. A ticket has been opened with Flexradio but Covid and Thanksgiving have delayed the response.

December 12, 2020 – Amplifier is fixed and back in the shack. Flexradio’s warranty tech support is amazing. They requested a remote test and determined from the results that it needed new finals. The amp was shipped to Austin, repaired at no charge, and shipped back in two weeks time. What a quick turnaround. Kudos to Flexradio.

The finals are MRF1K50H LDMOS transistors each rated at 1.5 kW dissipation. Mouser has these listed for $900 each in single quantities.

Update – June, 2021: The amplifier system made it through the entire winter without any power problems and all worked as designed. To avoid destroying any more final transistors FRStack3 was installed. FRStack reduces the exciters output power when an amplifier is placed in operate mode, protecting the finals. It is performing that function well. Lesson learned.

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