John G0FSP described the handy unit in Sprat 75, which he uses to power his HW9 with a 5 AH lead acid gel battery and can monitor the charging current, in addition to the battery volts.
Monitoring the charging current assists in aligning the panel with the sun, whilst in the key down mode, battery volts give an indication of the operation time left in the gel battery.
John's panel was rated at at 225mA maximum. Incidentally, solar panels are available at a discount for G QRP Clubmembers from GW4IED, who attends many UK rallies.
In the circuit, the 10v zener achieves the required suppressed voltage scale. R1 and 2 depend on the meter used, a 5mA one being at hand for the prototype. RV1 and 2 are 20 turn 1k0 PCB mounting types. Adjustment of these allows for easy calibration of the meter and can compensate for any discrepancy in values of R1 and 2. D1 prevents current flowing from the battery to the solar panel, whilst the fuse protects against any short circuit.