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Design

🖥 Hardware

Note

If the output from the battery turns off, it may be because the BMS needs to be reset. This may be done by shorting the P- and B- on the BMS.

I opted to drop support for the ESP8266 for a few reasons:

  • The need to short pin 16 in order to wake the Feather from deeps sleep removes the ability to use button B on the OLED screen.
  • Button A on the OLED screen was tied to the Feather LED.
  • There is no way to determine the method that was uesd to wake the Feather.

⚡ Electronics

On the bq24074, carefully cut the 1.0A jumper and solder the 1.5A to increase the current limit. This needs to be verified that the panel is actually outputting more than 1A.

☀ Panel

📡 BMS

The BMS is really small and can be difficult to mount and use. I thought about hot gluing it to the battery holder and then soldering the wires to it directly. However, I opted against it because I was too afraid that the wires soldered to the P+ and P- and going to the rest of the breadboard would break off easily. Therefore, I soldered pins to the BMS pads and connected it to the system via the breadboard.

I used the standard header pins that come with a lot of the Arduino type boards and just removed the plastic spacer. I then added pre-tinned the pins as well as the BMS pads. I then inserted the pins into the breadboard at the correct locations then clamped the outer pins to the BMS using a set of helping hands. Then soldered the pins to the pads.

🔋 Battery Holder

The battery holder comes wired in series but this design requires that the batteries be wired in parallel. Connect the positive terminals of both batteries together and add a negative lead wire from the end of the holder that connects the two batteries. Insert one of the batteries in reverse where the spring is touching the positive end of the battery.

Warning

Inserting the batteries incorrectly will cause them to short!

🤖 Services

💾 Software

🔌 Circuit

  • Connect the output of the battery to the Vin+ of the INA260.
  • Connect the Vin- of the INA260 to the LIPO pin of the bq24074.
  • Connect the BAT pin of the Feather to the OUT pin of the bq24074.
  • Connect a 1P2T switch to the EN pin of the Feather to ground.
  • Connect a 1P2T switch to the EN pin of the Powerboost and a 10kΩ resistor.
  • Connect pin 33 of the Feather to a node between the Powerboost switch and the 10kΩ resistor.
  • Connect all GND pins together.
  • Connect the SCL and SCA pins of the Feather to the SCL and SCA pins of the INA260.
  • Connect the 3V pin of the Feather to the VCC pin of the INA260.
  • Connect the BMS in between the battery, ground, and V+ of the INA260.

Circuits made with Circuit Diagram

Warning

The li-ion batteries are wired in parallel even though the 2 battery holder come wired in series.

Warning

The JST pins, depending on where they are purchased, may be backwards in terms of the wiring. See this link to determine the correct wiring according to IoT convention.

IoT devices must use batteries wired like this: With the key facing down and the wires toward you, the red is on the right.

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