Reply To: Dying battery in winter solved by 3.5 W solar panel

I’m not sure where you saw something that said the Mayfly only supports a 2.5W panel, because that’s not accurate. The only limiting thing is the voltage that the panel produces — the Mayfly only works with panels with an output voltage of 6 volts. If you use a solar panel with a 9v or 12v output, you will damage the Mayfly.

The reason for the voltage limitation is the MCP73831 charge controller chip on the Mayfly board. It can’t handle input voltage above 6 volts. It also has a maximum useful input current of 500ma, so if you do the math (Ohm’s law) you can calculate that a 3-watt panel will provide 500ma of current. So for sunny locations, any panel that supplies more than 3 watts is overkill. If you are in a shady location, or in areas that don’t get at least an hour of direct sunlight, then using a larger panel like the 3.5-watt (https://www.voltaicsystems.com/3-5-watt-panel), 6-watt (https://www.voltaicsystems.com/6-watt-panel), or even 9-watt (https://www.voltaicsystems.com/9-watt-panel) might be useful. The 9-watt panel puts out 3 times more current than the Mayfly can use (1500ma) but if your logger is really shaded, then the efficiency of the panel drops significantly, but even at 33% output, it would give same power as a 3W panel in full sunlight, and the maximum useful current the Mayfly will accept.

We usually use the 2-watt panel (https://www.voltaicsystems.com/2-watt-panel) in almost all of our locations because they are sunny enough to keep the battery charged. But in a few areas we use the 3.5-watt and have had great results, so we’ve never needed to use anything higher than that.

If you’ve already got a 12-volt deep cycle battery and separate 12v panel and charge controller, then it probably makes more sense to power your Mayfly board from the 12v battery and not use a 3.7v LiPo to power the Mayfly, and then you don’t have to use a separate 6v panel just for the Mayfly. To power the Mayfly from 12v, you have to use a 12v-to-5v adapter like this one (https://www.amazon.com/HitCar-Inverter-Converter-Recorder-Straight/dp/B00U2DGKOK) and plug it into the microUSB port on the Mayfly. There is an external battery header (unpopulated) on the Mayfly (version 0.3 and 0.4 boards), but it has a maximum input voltage of 5v. The upcoming Mayfly version 0.5 will be able to accept a connection directly 12v batteries, but for the older boards, you’ll need to use the 12v-to-5v adapter. It’s easiest to use the ones that have the microUSB connector on them, but you could also use one with bare wires and connect that to the external battery header using a polarized 2-pin connector with 0.1″ pitch.