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Charging 6600 mAh Li-ion battery with 3.5W solar panel

Home Forums Mayfly Data Logger Charging 6600 mAh Li-ion battery with 3.5W solar panel

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    • #12244

        I have a stupid question for Shannon or anyone who knows Mayfly better than me.
        I have a 3.5W 6W solar panel from Voltaic. I am charging 3.7V 6600mAh Li-ion battery with it.
        Voltaic specs say that the panel can output peak current of 550mA. This brings my battery charging
        time to 12 hours. Now what I would like know is, how do I make sure that the battery is fully charged other
        than counting hours. The Mayfly documentation says,
        “A yellow LED will light up whenever there is sufficient sunlight to charge the battery.”
        do I assume that the battery is charged if this LED is off? Is there another LED that I should look for?
        I do have a way of monitoring the battery voltage in my program but I would like to know if the battery
        is fully charged before I turn the Mayfly on.
        Thank you for your help.

      • #12245

          And on the related note,
          am I correct in assuming that solar panel will charge the battery even if I leave Mayfly in off mode?
          In other words, I don’t need to turn on the Mayfly and then wait for the battery to be charged over the entire day. Is that correct?

        • #12246
          Shannon Hicks

            On most of our Mayfly sensor stations, We use the 3.5W panel from Voltaic, and it’ll charge a brand new battery just a few hours in full sun. When you buy a new Lipo, it should already be at least 50% charged, so you won’t be starting from zero. And you should avoid totally discharge a battery because that will damage it and shorten it’s lifespan or kill it entirely. So there should never be a time where you need to charge a fully-depleted Lipo. If this happens often then you need to change your circuit or increase your battery capacity. Our datalogger stations use sleeping routines, sensor switching, and other smart power management tricks to allow them to run for weeks even without sunlight, so on a normal day they need about a half hour of sun to charge the battery after being in the dark overnight.

            The charge regulator on the Mayfly circuit board has a maximum output of 500mA, so even if you put a large 6-watt panel on the Mayfly, it’s still not going to charge it any faster since you’re limited by the 500mA charge regulator. But in areas with lots of shade, using a large shaded 6-watt panel can give you the same results as using a smaller panel in full sun.

            The yellow LED on the Mayfly indicates that the battery is actively being charged. Once the battery is fully charged, that LED goes out and there’s no other indication that the battery is full. The Mayfly can sample the input Lipo battery voltage on analog input A6 so you can print it to the serial monitor or write it to the card or whatever. There’s an example sketch on how to do that here: https://envirodiy.org/mayfly/software/battery-measurement/ If you’re needing to know Lipo battery voltages often and when not connected to a Mayfly, then there are small Lipo voltmeters you can get, just be sure to observe polarity since many of them have reversed pins on the JST-2.0 jacks.

            And yes, the Mayfly will charge your battery even if the power switch is turned off. You can also charge a Lipo battery by connecting the microUSB cable of the Mayfly to your computer or just use a regular phone charger.

          • #12252

              That was very helpful and informative reply. I did see that the yellow LED doesn’t quite go out but faints (equivalent of ‘off’) after charging
              the battery using microUSB cable. It took almost 12 hours which is what I originally figured. For anyone who comes across this thread in future, a word of advice: choose your battery wisely. You may not need a very big battery.

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