Charge the device instead of the battery directly. Measure the output voltage of the charger and use an off the shelf dc-dc converter with the output set to match the chargers voltage and a solar panel that matches the input requirements of the converter.
Charge the device instead of the battery directly. Measure the output voltage of the charger and use an off the shelf dc-dc converter with the output set to match the chargers voltage and a solar panel that matches the input requirements of the converter.
