Solar cell charging and battery chemistry for low power 2.4-3V supply?

Question

I've recently been looking into the possibility of using an amorphous silicon solar cell (like they have in calculators) to supplement my very low power BLE system's current consumption to extend the life.

I've found some cells that can provide 10-20uA of current during daytime (indoors). Since my system is mostly in a low power state, this power supplement is enough to offset some of the total power consumption and greatly extend the battery life.

The voltage supply required for my system can be anywhere between 2.4v and 3v, but probably needs a battery capacity of >750mAh to keep it alive for as long as it needs to be. What would be the best battery chemistry to use in this scenario? And what is the most efficient way to charge the battery(s) while keeping cost to a minimum?

(I've found a solar cell supplier that does a variety of voltages, so that shouldn't be an issue)

Answer 1

20uA charging current and >750mAh equates to a charge rate of 0.00027C or more than 4 years to get a full charge (and that's assuming continuous illumination).

Standard Nicad and NimH batteries are not suitable because they will discharge much faster than the solar panel can charge them.

Low self discharge NIMH (eg. Eneloop) only looses about 10~20% per year, and so should store at least some of the charge. At this low current trickle charging is safe, so you can just connect the solar panel directly across the battery (possibly with a blocking diode in series if the panel has high dark leakage).

A lithium battery would hold the charge better, but needs a complex charging circuit that may waste more current than it saves.

If you really need >750mAh of rechargeable battery then I suspect your average current drain is much higher than the solar panel's output. In that case it it might be better to forget solar charging and just rely on the primary energy source. If that isn't enough then you may want to rethink your design criteria.