Solar Panel and Charge Controller for Portable Ham Radio
A solar panel is the quietest way to keep a portable station running for a weekend, and it fits well with the way most amateurs operate away from home: a few hours of listening, a few short bursts of transmitting and a battery that has to last until the next day. A panel alone does not charge a radio battery safely, though. You need a charge controller between the panel and the battery, a battery of the right size and wiring that handles the current. This guide shows how to estimate how much power you really need, how a PWM controller differs from an MPPT controller, how to wire the system in the right order and how to check that the controller does not put noise on the bands you want to use.
Start With an Energy Budget
The right size of panel and battery depends on the energy you use, not on the power of the transmitter alone. Energy is power multiplied by time, and for a battery it is convenient to count watt-hours. A radio that draws 1.5 A while receiving at 13.8 V uses about 20 watts, so four hours of listening costs about 80 Wh. The same radio at 20 A for a few minutes of transmitting uses roughly 276 watts during those minutes. The duty cycle decides the result: SSB and CW transmit only a small part of the time, but FT8 and other digital modes can transmit for up to half of the periods. For the operating side of digital modes, see our article on FT8 operating etiquette and power.
Here is a worked example with assumptions that you should replace with your own numbers. Suppose the station is a 100 W radio run at 50 W, receiving for 5 hours and transmitting for a total of 30 minutes per day. Receive at 1.5 A and 13.8 V gives about 104 Wh. Transmit at about 10 A for half an hour gives about 69 Wh. With a laptop and small accessories of 30 Wh, the daily total is roughly 200 Wh. A 12 V battery has to hold more than that, since you should not drain it completely, and the panel has to put back 200 Wh in the hours of good light.
Sizing the Panel
The nameplate rating of a panel, say 50 W, is the output in strong, direct light at the test conditions. In real conditions, with the sun at an angle, a little haze and heat in the cells, you usually get less than the nameplate, and on a cloudy day only a small fraction. A reasonable planning approach is to count the equivalent of a few hours of full output per day and to leave a generous margin. With the example above, 200 Wh divided by a planning figure of 4 hours gives 50 W, and a margin for losses in the controller and battery pushes the choice to 80 to 100 W. If your trips are short, a smaller panel and a bigger battery are a valid alternative, since the battery carries you through the night and the panel recovers part of the charge in the daytime.
For a Field Day or a weekend expedition with a high-duty digital station, check the discussion in Field Day planning for power over 24 hours. There, the energy budget and the choice of battery type are treated at the level of the whole event. For a day trip with a QRP radio, a folding 20 W panel and a small battery are often enough.
Why You Need a Charge Controller
A panel marked as “12 V” usually has an open-circuit voltage well above 12 V, because it must be able to push current into a battery that is rising toward 14 V or more. If you connect it directly, the battery can be overcharged. The charge controller limits the rate at which current is added to the battery and protects it against overcharging, and many also prevent a deep discharge. Even a small panel can damage a battery on a sunny week without such a control, and a battery that is charged wrongly can also be a safety risk.
PWM or MPPT?
Two designs are on the market. A pulse-width modulation (PWM) controller connects the panel to the battery and switches the connection rapidly to hold the voltage at the charging target. In effect, the panel works at the battery voltage. A maximum power point tracking (MPPT) controller works differently: as the article on MPPT explains, it is a technique to maximise the energy that is extracted from a variable source such as a solar panel as conditions change. It converts the panel’s higher voltage and lower current into the battery’s lower voltage and higher current, so that the panel can operate at its most efficient point.
| Feature | PWM controller | MPPT controller |
|---|---|---|
| Price | Low | Higher |
| Panel matching | Panel voltage must be close to the battery voltage (a “12 V” panel for a 12 V battery) | Accepts a wider range of panel voltages, including higher-voltage panels |
| Efficiency in cold or partial light | Lower, because it cannot choose the best operating point | Higher, especially when the panel is cold or the battery is low |
| Size and weight | Small and light | Larger, with a heat sink in bigger models |
| RF noise risk | Present, depends on the design | Present, and the switching converter is a typical source |
| Best for | Small panels, short trips, low budget | Larger panels, cold weather, long cables, maximum energy |
For a 20 W folding panel, a PWM controller is a sensible choice. For a 100 W panel that you use for a weekend, the extra energy from an MPPT controller is usually worth the price, and the difference grows with the size of the array. The Wikipedia article on charge controllers also notes that a controller with MPPT frees the designer from matching the panel voltage to the battery voltage, which is useful when you buy a panel for another purpose.
Battery Chemistry Settings
Most controllers have a battery-type setting. Lead-acid batteries, whether flooded, AGM or gel, need a charging profile with absorption and float stages. Lithium iron phosphate batteries need a different profile and usually a lower float or none at all, and some also have their own protection circuit which can disconnect the load abruptly. Use the profile that matches your battery, and read the battery maker’s recommendation. If the controller has only lead-acid profiles, check that the voltage limits suit your lithium battery before you connect it. Our guide to battery power options for portable ham radio lists the types and their strengths.
Wiring Order and Safety
- Connect the battery to the controller first, through a fuse, so that the controller can detect the system voltage. Follow the maker’s instructions, because some units require this order to avoid damage.
- Connect the panel next, with the panel covered or in shade while you work, since a panel in sunlight is a live source.
- Connect the load (radio and accessories) to the load terminals or directly to the battery through a fuse, as your controller design allows. Many amateurs run the radio from the battery, not through the controller, to avoid a current limit.
- Fuse the battery lead close to the battery and size the cable for the current, as described in the wiring basics for 12 V systems. Use polarised connectors such as the Powerpoles described in our Anderson Powerpole guide.
- Disconnect in reverse order when you pack: panel first, then load, then battery.
Keep the panel cable away from sharp edges and from the antenna feedline, and tie the cable down so that the wind does not drag the panel. Panels are glass or laminate and are not designed for hard knocks, so protect the corners.
Keeping Controller Noise Off the Bands
Any switching device can radiate on the HF bands, and a solar controller is a switching device. The noise can appear as a comb of lines that move as the panel voltage changes, so it varies during the day. Before you rely on the system, test it. Connect the radio to its normal antenna, tune to a quiet part of the band and watch the S-meter or the waterfall while the controller is charging in full light. Then disconnect the panel and compare. If the noise rises, try these steps in order: move the controller and its cables away from the antenna and the radio, add a clamp-on ferrite choke to the panel cable and to the battery cable close to the controller, and finally try a different controller. The principles of choking are in the guide to common mode chokes. A simple solution in the field is to charge the battery in the daytime while you are not operating, and to disconnect the controller when you transmit and receive on the quiet bands.
A Simple Example System
To make the numbers concrete, consider a typical weekend kit for a 100 W radio run at moderate power. It consists of a 100 W folding panel, an MPPT controller rated for at least 10 A, a 12 V battery of around 30 Ah, a fuse near the battery and a short cable with polarised connectors to the radio. In good sun the panel can supply the daily budget calculated above in a few hours, and the battery carries the station through the evening. On an overcast day the same panel may deliver only a fraction, and you accept shorter operating periods or a lower transmit duty. Nothing in this arrangement is complicated, but each part has to be rated for the current it carries, which is why the fuse and cable size deserve as much attention as the panel.
When you plan a trip, run the arithmetic again for the worst case you can imagine. If the budget fails on a cloudy day, either add a second battery, bring a smaller radio for that weekend, or reduce transmitting time. A reserve of one full day of battery capacity is a comfortable margin, and it is much cheaper than a larger panel.
Practical Tips From the Field
- Point the panel at the sun and adjust it every couple of hours, since the angle matters more than the cleanliness of the glass.
- Avoid partial shade. A small shadow from a mast or a branch on part of a panel can cut its output sharply, because the cells in a string are connected in series.
- Use a meter. A small DC meter or a controller display tells you the charge current and the battery voltage, so you can see whether the system is keeping up.
- Carry a spare fuse and a short jumper lead for emergencies, and a little tape for the cables.
- Do not leave a battery in the sun on a hot day. Heat shortens its life and may be dangerous for some chemistries.
The Bottom Line
A solar panel with a proper charge controller and a battery of the right size is a clean and quiet source of power for portable operation. Start from an energy budget in watt-hours, choose a panel that covers it with a margin, and add a PWM controller for small setups or an MPPT controller for larger ones. Set the battery type correctly, connect the battery before the panel, fuse the leads, and test for noise on your bands before you depend on the system. With those steps, the sun will keep your radio running for as long as you want to stay on the air.
Frequently Asked Questions
Do I need a charge controller with a solar panel for ham radio?
Yes. A controller limits the charging current and protects the battery from overcharging. Without one, a panel can overcharge a battery and damage it.
Is MPPT better than PWM for a portable station?
MPPT extracts more energy, especially from larger panels and in cold or partial light, but it costs more and is bigger. For a small folding panel, PWM is often sufficient.
How big a solar panel do I need?
Calculate your daily energy use in watt-hours, divide by the equivalent hours of full sun you expect, and add a margin for losses. A day of QRP operation may need only 20 W, while a high-duty digital station needs much more.
Can a solar charge controller cause noise on HF?
Yes, any switching controller can radiate. Test it with your radio, keep cables short and away from the antenna, and add ferrite chokes if you hear a rise in noise.
In what order should I connect the battery and the panel?
Usually the battery first, through a fuse, and then the panel. Follow the manual of your controller, and reverse the order when you disconnect.
Can I charge a lithium battery with any solar controller?
Only if the controller has a suitable lithium profile or you can set the voltages to match the battery maker’s specification. A lead-acid profile may not be correct for lithium iron phosphate.