A manufacturer of flexible photovoltaic or solar panels with a commitment to provide solar powered mobile (cell) phone chargers and LED lighting in Africa approached us to design and make several versions of high efficient battery charger circuits for different applications.

Our customer's photovoltaic panels produce useful charging current from relatively low incoming light levels so it is important that the circuit has a very low quiescent current drain. Under maximum lighting conditions our circuit also has very low losses compared with several competing products that our customer evaluated.

The remit was to harvest the maximum power from sunlight throughout the day even when the panels were not facing the sun directly.  The mobile phone charger modules would consist of a solar panel, the charger circuit and an internal rechargeable storage battery, generally a three cell NiMH set.  These modules could be carried around or placed in a convenient place including in a moving vehicle to charge the internal battery.  Then, when a phone was plugged into the module, its Li-Ion battery would be charged from the internal storage battery.

Another variation of the phone charger alsom shown below included a booster circuit to supply a constant, higher voltage output for compatibility with a wider range of mobile phones.

This circuit also allowed us to make full use of the internal battery voltage range during its discharge cycle.  All variations protected the internal battery from over-discharge which would shorten its useful life.
It was recognised that these circuits could be abused in the field or adapted for use with a wide variety of solar panels.  Consequently, a lot of thought was put into self protection, and the circuits would survive being connected the wrong way round and being connected into a short circuit.

Following the success of using these designs in the field, the customer returned asking for a higher power charger using an eight cell, nominal 12V NiMH battery for LED lighting in Rwanda.  We worked from the original design to make a version that can take a much larger photovoltaic panel and charge a big enough battery to give reading light for several hours.  For this project, the customer had a limited budget, so we did the circuit and PCB design only and they assembled the boards.  The board layout, using Altium Designer, is shown in the image below.