Cheap solar panels and NiMh batteries
Ruben Jönssonemail (remove spam text)
I have done a small board based on a PIC24FJ64GA004, a MRF24J40 2.4GHz radio and a one-wire interface. The PIC wakes up about once every 5th minutes, takes a temperature reading from a string of temperature sensors on the one-wire interface and sends the measurement results to a coordinator over the radio..
The unit is powered with 2 AA batteries and I expect these to last about one year.
The other day when I was at the store I saw a set of solar powered garden lights at a very reasonable price. They are supposed to be charged by the sun during the day and light a LED during the night. So I picked it up just to see how it was working, hoping to be able to use the solar panels as power supply for my radio unit. It turns out that the solar panel is directly connected to a small 1.2V, 80mAh NiMh battery. The battery is then connected to a a small circuit board via an on/off switch. The board has a LED and some circuitry under a blob, probably sensing the voltage from the solar panel to determine if it is day or night.
My powersupply is made to work with 2 cells so I took the NiMh batteries and the solar panels from 2 units and connected these in series. Still with each solar panel directly connected across each battery. I then connected the 2 NiMh batteries to my circuit instead of the 2 AA alkaline batteries.
I have placed the unit indoors behind a window where the solar panels are exposed to sunlight (through the window) a couple of hours every morning (if the sun is shining). When I measure the battery voltage in the morning before the sunlight hits the panels, the voltage is about 2.4-2.6V over the two batteries and after a couple of hours charging by the sunlight, the voltage is as high as 2.75V. The circuit is working very well and has been for about a week now. The 80mA batteries should be able to keep the unit working for a couple of days even without any charging.
Now, to my question: Is it safe to just connect the small solar panels directly across the NiMh batteries just like this (as it also was in the original circuit)?
The fully charged voltage of 2.75V seems a little high for the batteries. Could this reduce the lifetime for the batteries?
AB Liros Electronic
200 39 Malmö Sweden
Tel +46 40142078
See also: massmind.org/techref/power/batterys.htm?key=batteries