Sizing of a Solar Charging System for a QRP Beacon

Posted on Friday, April 7th, 2017 at 9:18 pm in

When you run a small low power beacon like the
Ultimate3S from QRP-Labs it is attractive to consider
powering it from batteries with a solar panel.
But how long will it last?

A number of things are conspiring to make our lives as amateur radio experimenters difficult. On the one hand the cheap Chinese imports make it very easy to add small but useful gadgets to your shack. On the other hand technology itself is rapidly moving towards smaller and less power-hungry devices causing your stuff to e outdated two days after you buy it!
At the confluence of these two streams sits low power beacons: Small devices that can send out a radio signal and only sniffs at the fuel tank. The latest amateur party balloon experiments are a case in point: With a total payload measured in grammes they circumnavigate the world several times over.
But what about the more common homebuilt beacons? Let us take for example the Ultimate3S beacon kit that was constructed by several South African amateurs: Can this beacon be powered from a solar panel? Let us investigate further.

How deep is your pocket?
The first and most important factor to take into account is that almost nothing is impossible if you spend enough money. You can buy incredibly efficient systems with enormous battery capacities at a price that will bring tears to your bank manager’s eyes.
I don’t operate at those altitudes, my oxygen supply will be cut off very quickly by my Minister of Home Affairs. I’m going to limit my spending to items that the average hobbyist can afford.
Thus, it is spoken, that we will be limiting the most expensive part of the discussion – the batteries – to standard 12 volt 7 amp-hour (gelcel) batteries. I want to do this because my experience is that the batteries are the items that require the most money and the most maintenance. This might not produce the most optimized technical design, but it will certainly optimise my chances of survival when I get home.

However, for the technically inclined I have a challenge near the end of this article – please participate!

How much is enough?
In order for you to size your solar system for your beacon correctly, you need to note the power rating of the beacon and all its support components that will be drawing power from the system.
For example, let us take the popular QRP-Labs Ultimate3S beacon to see how one might calculate the correct size of the solar system. This example system is configured with:
• no receiver slots and no 3G or network connection
• a standard GPS for timekeeping
• no relays for band-hopping
• a standard Si3531A frequency synthesizer
Measuring the beacon together with the GPS it draws around 150mA from the 5v supply in receive mode (standby as some would call it). During transmit it jumps to 350mA or more. However we are only transmitting for small periods at a time and let’s say we are running WSPR with one transmission every 20 minutes, i.e. 2 minutes out of every 20 minutes or around 10% of the time.
This is easily calculated to be:
• In Standby mode about 2 Watt for about 90% of the time; and
• In Transmit mode about 4.7 Watt for about 10% of the time.
Without being too scientific we can estimate an average consumption of about 200mA .
Now a very basic assumption is that I want to operate around the clock 24×7 and thus:
Before you kill me for using the wrong numbers, remember I am drawing current from a 12v battery through a linear regulator (LM7805), thus 12v x .2A = 2.4W. We can easily round this up to 60 watt-hours per day.

If you fitted the OCXO (Oven-Controlled-Crystal-Oscillator) to gain better frequency stability you must add at least 200mA to that number, which doubles your demand. Quite frankly I would not recommend that for a solar powered system.

Sizing of the Inverter / Converter / Regulator
Reducing the 12 volt battery line to the 5 volt line required by the beacon, we often take the easy way out with a small 3-pin linear regulator a la LM7805.
The more modern solution might be to use a switching inverter or regulator (12V → 5V or 3.7V → 5V), but that comes accompanied by its own set of negative attributes such as noise which cannot simply be ignored.
Assuming your peak power drawn is only 350mA during transmit, your inverter / converter / regulator needs to be able to deliver this power without overheating. This translates loosely to heat in the quantum of:
7V X .35A = 2.45 WATT!
Yes, a heatsink is recommended.
Note: The inverter/converter must always be bigger than the maximum peak power demand.

Sizing of the solar PV array (solar panels)
For the beacon alone, excluding other losses, we will use approximately 57.6 Watt-hours per day. We need to generate more energy than we use to stay ahead of a flat battery, so …
The charging hours effectively available per day can vary between 4.5 hours (Cape Town) and 6 hours (Polokwane) per day and let us therefore use 5 hours as a good estimating number.
If we increase the 57.6 Watt-hours per day to say 100 Watt-hours (to allow for some extra charging when really-really needed), the amount of panels we will need is:
So the minimum panel size we need is a 20 Watter at 12 volts. More thoughts about the voltage later.

Rating of the Batteries
Normally it is preferable to design a solar system for higher voltages because that reduces the cable and system losses which could be as much as 15% more in a 12V system. However in this case a 12 V configuration should be fine.

Also remember that we also do not want to cycle our batteries more than 50% deep, or even less if we can. To be honest, these cheap 12V/7Ah batteries shouldn’t be discharged to less than 80% of their rated capacity otherwise their lifetime is drastically reduced.
If we agree to 50%, the batteries that we need must be in excess of 100 Watt-hour x 2. (times 2, to reduce the battery cycle to 50%). For 80% we need five times the rated capacity!
So we need at least a 200 Watt-hour battery bank. This is the smallest bank of batteries you can go for in this application. You also know the average use is about 100 Watt-hour per day, so if you want additional capacity – say for a rainy day – you need an additional 100 Watt-hour of battery bank.
But let us not digress. Let us get back to 1 day capacity. For our 12V system we need
100 WATT-HOUR / 12V = 8.33 AMP-HOUR

That is much more that the capacity of one battery. If we use standard 7 Amp-hour batteries, that would mean two batteries. Anything less could leave you powerless on a cloudy day.
In fact, we could digress here into a discussion of two batteries in parallel, or two in series (24V) but sufficient to say that the 24V option is likely to be a more efficient approach.
Also remember that if you change anything in the configuration, for example add additional bands, you must reconsider the above calculations.

Rating of the Regulator (MPPT)
A MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank(s). They have become very popular lately, due largely to their ability to protect and extend battery life while transferring maximum energy.
The MPPT solar regulator is designed according to the output current rating to the battery, in other words it is aligned to the battery characteristics. Don’t buy a lighter version thinking you are going to use less.

We know we have 20W of panels (independent of the configuration). If we choose a 24V battery bank then 200W/24V = 8.3A MPPT. If we choose a 12V system, this would regulate 200W/12 = 16.6A MPPT. This may yet be another reason why you would want to consider a 24V (two batteries in series) system.

The Complete Solar Power System
From the previous assumptions and subsequent calculations, this is an example of a practical system:
• 1 x 12 V to 5 V step-down regulator at 1 A
• 1 x 20 W Solar Panels at 12 V
• 2 x 12 V / 7 Amp-hour batteries
• 1 x 16A MMPT Regulator
Be aware that this is only a guideline and your mileage may vary. For example if you are not too concerned with battery life, you could quite happily operate with one battery.

The above discussion is very generic and there are many things that can go wrong or can go right, depending.
The battery should be protected from discharging too deeply, but at the same time the beacon must be protected from the power cycling up-and-down too often. It could freeze upon startup.
Any switching power supply will influence the quality of the signal and may even prevent the beacon from being spotted completely if not sufficiently dampened.
The LCD display can be switched off completely to save a few milliamps and thereby reducing power demands.
Adding bandhopping, more bands or more frequent transmissions may severely reduce battery life as calculated above.

The Real Challenge
I mentioned earlier that I have a challenge for the more technically inclined amateurs.
It is my considered opinion that the system we discussed above doesn’t make very good use of all the resources we have at our disposal. I’m convinced that some optimisation isn’t only possible, but also easy to achieve.

For example, I’m wondering if the latest lithium batteries running 3.6 V or 7.2 V or even 11.2 V (as used in model aeroplanes) isn’t going to be a much better reservoir.
I’m also wondering if the up-and-down of voltages, from the solar panel to the battery to the beacon, cannot be done in fewer steps, or even eliminated completely?
So here’s the challenge: Please send me your ideas (including eBay links if possible) for a better and more efficient system. The best design will be featured in the next issue of RadioZS.

Until next time,

73s de ZR6LU Leon Uys from Johannesburg 0825735580