Solar panel for a hotspot: an off-grid node in the field

A hotspot is a tiny computer, and it doesn't care where its 5 volts come from: a wall socket in your apartment or a battery charged by the sun during the day. The second approach opens up an interesting possibility — placing a DMRhub network node where there is no mains power at all: a cabin with no electricity, a tower out of town, a sealed box on a garage roof, or a portable setup. In this article we'll walk through the whole off-grid power chain: from the panel to the 5-volt output for the Raspberry Pi, how to estimate the battery capacity for a night and a cloudy day, and where in this chain you must not cut corners on safety.

What an off-grid node is made of

The basic off-grid power layout is always the same; only the ratings change. Energy travels through a chain of four links:

1. Solar panel — converts light into direct current. Its voltage “floats” depending on the amount of light.
2. Charge controller — the main regulator. It takes the unstable panel voltage and gently charges the battery without letting it overcharge.
3. Battery — the buffer. It stores energy during the day and gives it back at night and in cloudy weather.
4. 5 V converter — steps the battery voltage (usually ~12 V or ~3.7 V) down to the stable 5 V the Raspberry Pi needs.

You can't power the Pi straight from the panel: when a cloud rolls in, the voltage sags, and the Pi reboots or ends up with a corrupted memory card. The battery is exactly what decouples the “moody” sun from a computer that is “nervous” about its power supply.

How much a hotspot consumes

The good news: an MMDVM hotspot is one of the most frugal consumers in the ham radio shack. The modem itself (the MMDVM board) draws almost nothing; the bulk of the consumption is the single-board computer.

• A Raspberry Pi Zero 2 W at idle pulls roughly 100–180 mA at 5 V, that is ~0.5–0.9 W; under load — up to 500 mA, about 2.5 W (per CNX Software measurements).
• Accounting for the MMDVM board, Wi-Fi and converter efficiency, it's realistic to budget ~3–5 W of average consumption for the node — and that's the working figure for the calculation.

Sizing the reserve for the night and cloudy weather

Battery energy is conveniently counted in watt-hours (Wh). It's simple: power in watts × hours of operation.

How much you need to store

Say the node draws 4 W on average. Over 24 hours that's:

4 W × 24 h = 96 Wh/day

The battery has to survive the dark hours. In summer the night is short, but you should plan for the worst case: two or three cloudy days in a row, when the panel barely charges. If we want two full days of autonomy without sun:

96 Wh × 2 = 192 Wh of usable capacity

What battery that translates to

A Li-ion pack of 18650 cells must not be discharged to zero — a healthy margin for service life keeps 20–30% in reserve. So the nominal capacity needs to be about 1.4 times the calculated figure — around 270 Wh. Converted:

• 12 V lead-acid (AGM): 270 Wh / 12 V ≈ 22 Ah. Cheap, but heavy and afraid of deep discharge.
• Li-ion on 18650: a typical cell is ~10–12 Wh, so you need roughly 24–28 cells in the pack. Lighter and longer-lived, but it requires a BMS.
• LiFePO4: the best choice for a buffer — thousands of cycles, safer than ordinary Li-ion, and it handles discharge calmly.

What panel you need

Over a single day of daylight the panel has to replace the daily consumption plus top up the buffer. A rough rule of thumb for temperate latitudes is to count on a “solar” equivalent of 3–4 hours per day (less in winter, more in summer). To cover 96 Wh:

96 Wh / 4 h ≈ 24 W of peak power — the minimum

In practice people take 1.5–2× more (clouds, dirt, a non-optimal angle), that is a panel of 40–60 W. The margin isn't overcaution, it's the norm: real output is almost always below the rated figure.

PWM or MPPT: which controller

The charge controller comes in two types, and this is the main fork in terms of money and efficiency.

PWM — simpler and cheaper

A PWM controller essentially “clamps” the panel voltage to the battery voltage through a fast switch. The real efficiency is around 74–81% by various measurements. It's suitable when the panel voltage is close to the battery voltage and the system is small. For a modest 4 W node it's a perfectly workable and budget-friendly option.

MPPT — more efficient and more expensive

An MPPT controller “decouples” the panel and battery voltages and continuously hunts for the maximum power point. Efficiency reaches 94–98%, which yields 15–30% more energy from the same panel (data from Victron, Renogy). The gain is especially noticeable in the cold and in winter. If the node runs year-round and every watt-hour counts, MPPT pays for itself.

Safety: what you must not skimp on

An off-grid node sits unattended for months, often in a closed box. Two safety components are mandatory here.

• BMS — mandatory for any Li-ion/LiFePO4 pack. It disconnects the cells on overcharge, deep discharge, overload and short circuit, and it also balances the cells.
• Fuse — on the positive lead right at the battery. Rated just above the operating current. It's the last line of defense if something shorts beyond the BMS (for example, a chafed wire).
• Sealed box and ventilation — the node lives outdoors. Moisture kills the controller and the contacts; a sealed enclosure with cable glands is mandatory. Li-ion dislikes frost while charging — for winter that's a separate issue (charging below 0 °C is harmful).

Putting it together step by step

1. Measure the real consumption of your hotspot with a USB power meter over 24 hours.
2. Compute the daily consumption in Wh and multiply by the number of days of autonomy (2–3).
3. Pick a battery with a ~1.4× margin (a reserve against under-discharge) — preferably LiFePO4.
4. Take a panel 1.5–2× the calculated minimum for your region.
5. Choose the controller: PWM for summer/budget, MPPT for year-round.
6. Assemble the chain: panel → controller → battery (with BMS and fuse) → DC-DC 5 V → Pi.
7. Bring the node online through a hotspot-behind-NAT if you head into the field with mobile internet and no public IP.