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Batteries, solar and portable power
Running a station off-grid.
Running a station off-grid.
Running a station away from the mains means running it from batteries, charged from a solar panel, a vehicle or a generator. The questions to answer are: how much energy does the station use, how much can the battery supply, and how will it be recharged?
A battery's size is its capacity in amp-hours (Ah). Divide the usable amp-hours by the average current to get hours:
usable Ah ÷ average current (A) = hoursAverage current mixes the small receive current with the large transmit current by how often you transmit. A QRP radio at 5 W on a 20 Ah lead-acid battery can run for many hours; a 100 W radio at 25% transmit time will drain the same battery quickly. These are estimates: SSB speech draws less than the peak figure, while digital modes and FM draw near peak all the time.
| Sealed lead-acid | Lithium iron phosphate (LiFePO4) | |
|---|---|---|
| Weight | heavy | much lighter for the same usable energy |
| Usable capacity | about half, for long life | most of it, with a built-in protection board |
| Voltage as it discharges | sags steadily | stays nearly flat, then drops quickly |
| Charging | simple charger | needs a charger or controller made for it |
Both are nominally 12 V and fall within what most radios accept (check the manual). A lead-acid battery that is nearly flat may sag too low under a transmit peak, so the radio's output drops, which is the voltage-drop problem again; see Shack power supplies. See Batteries for the chemistry.
A silicon cell makes about 0.5 V, so a "12 V" panel is many cells in series, with an open-circuit voltage well above 12 V. Connected directly to a battery it could overcharge it, so a charge controller sits between them. A series diode stops the battery discharging through the panel at night.
A panel's rated watts are for full sun at the best angle; real output is usually lower. A rough sizing check is panel watts ÷ about 14 V for the maximum charging current. Partial shade can cut a panel's output sharply.