I am trying to make a Solar Powered Weather Station. I'm using the Arduino Fio, with a BMP180 and a DHT22 as the sensors. There are sleep modes activated with the Fio and the Xbee. It seems like the battery capacity requirements, after measuring, around about 664 mAh per day.

The Fio has a built in LiPo charger (MCP73831/2). Is this 3.7V 2000mAh battery safe to plug into the Fio as its battery source? Here is the datasheet from the link just provided. I'm not an engineer and I want to make sure that this is a safe device.

Once determining the correct battery, I will like to pair it with a solar panel.

Thanks for the help

  • 664mA is a lot. That's 28mA on average. I'd expect that number from running those components together, without any sleep/standby. You should be able to get that value way lower. I'm not sure the MCP73831/2 is designed to handle solar panels. You'd have to get a solar panel with an open circuit voltage that's below the 7V maximum of the MCP73831/2. I think you'd have to add some extra component to connect the solar panel to the battery. The MCP73831/2 is more for charging via USB. – Gerben May 23 '19 at 15:58
  • So, I think there was an issue with the previous meter I was using. I returned it and got a new one which seems to be working more as expected. The weather station is still using more than I thought. The usage per hour, when using the new meter, is around 11 mAh / hour which is about 264 mAh per day. I thought, from my calculations, it was going to be way less, around 76 mAh /day. I think I trust the meter more than my calculations. – noodle Jun 3 '19 at 5:16
  • How long is it awake vs. asleep? What is the sleep current? And what is the awake current? – Gerben Jun 3 '19 at 14:09
  • It's asleep for 60 sec and the meter says it draws about .01 A (my calcs say it should draw .38 mA). It then turns on the Fio for 3.4 sec and the Xbee for a little longer than 2 sec. When its on the meter says it draws .06A (my calcs say that it should draw 51mA). – noodle Jun 4 '19 at 4:57
  • So 80% of the battery is drained during sleeping. Doesn't your meter have a option to measure mA or even µA? Manual range? That sleep current should be way lower. I'd try removing all components, and see what the sleep current if for only the Fio. Then add the DHT, and see how much the sleep current changes. That way you can find the culprit. – Gerben Jun 4 '19 at 13:08

You have a LiPo charger module. It's meant to charge single LiPo cells. It is therefore, the intended safe use that you can connect any single LiPo cell to it. All single cell LiPo batteries are nominally 3.7V. The only real question might be -- does the charging circuit try to charge too fast?

A 2000mAh pouch LiPo can safely be charged at 1X the capacity. So a maximum current of 2000mA for an hour in this case. The battery management chip has a maximum charge current of 500mA, so there's no way it can reach over 2000mAh of current while charging.

The datasheet for the battery management chip shows a R_PROG resistor that basically sets the maximum charge current. Section 5.1.2 shows the formula for charge current regulation at:

I_REG [mA] = 1000 [V] / R_PROG [kΩ]

The schematic for the Fio shows they have used a 2k ohm resistor.

Thus: I_REG [mA] = 1000 [V] / 2 [kΩ] = 500 [mA] (the maximum the chip can operate)

  • Thanks Jose. I'm going to buy this 850mAh battery (sparkfun.com/products/13854). Its a bit smaller (make me feel better safetywise) and should last 3 days without sunlight based on the new meter. I think this 2W Solar Panel (voltaicsystems.com/2-watt-panel) may be good match. The Fio uses 3.7-7V from the miniUSB and this PV supplies 6.5V. It can provide 340mA which may charge the 850mAh battery in 2.5 hours with good sun. Do I need to worry about over charging or battery voltage regulation? How can I ensure that the battery will always have a safe voltage? Many thanks. – noodle Jun 3 '19 at 6:54

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