The right picture shows another three cell charger with a boost converter to provide a higher, constant voltage output from the battery to directly interface with many common mobile (cell) phones.

The 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 end of charge point was determined by a precision reference to prolong battery life. Because these were prototypes, it was also important to protect the circuit from damage in case they were connected the wrong way round.

The picture above shows a later design to charge an eight cell, nominal 12V NiMH battery for LED lighting.