Move to rechargable 3.7v batteries

[Yohannfra]

For now each device is powered with 3xAA alkaline batteries, maybe a 3.7v lipo rechargeable battery would be better.

Pro :

• rechargeable
• open the road to arduino pro mini 3.3v (Logic level shifter will be needed for the neopixel)
• makes all the boxes smaller and lighter

Cons:

• Need to recharge the devices
• Lipo batteries are more expensive than AA alkaline batteries

[s0210310]

Just ordered basic parts, after assembly I'll help design a 3.7V system using commonly available parts. I was thinking of:

1. 3.7V 18650 cell + holder, something smaller to accommodate same casing?
2. USB-C charge controller, TP4056?
3. boost converter to bring li-ion voltage to 5V, need to find a widely available module.

Any help brainstorming would be appreciated.

.

[Yohannfra]

Great to hear, thanks for your help !

Why would you need a boost converter ? The 18650 has a nominal tension of 3.7v which is enough if we switch to Arduino pro mini or something else (I'm trying with ESP32 and STM32).

The 18650 is a great idea I didn't know it so thanks for it.
For the casing I made some experiments and it 's possible and quite easy (if we go with the 18650) to embed the holder directly in the 3d printed casing (like here).

I have some tp4056 with micro usb, can they charge the 18650 ? If yes I'll go with it.

[s0210310]

So a Li-ion cell will have a voltage of 4.2 when fully charged and 3.0V when fully discharged. I'm not sure how well the voltage reg of ESP will handle that wide of a swing (when compared to 3.3V it needs). Need to check on it, I'm not very familiar with ESP32. For Arduino Nano though we'd need a boost converter to get it up to 5V.
I've used the TP4056 with micro USB in a project, it works well. It has both over charge and over-discharge protection plus a very nice charge algorithm. Load needs to be disconnected though when charging.
Nice idea for integrating battery holder into the case!
So I think the questions we need to answer are:

1. How many 18650 cells do we need? Depends on power consumption, 3.7V*2500mAh would give one cell about 9.2 Wh, which I think is more than plenty.
   1.1 If weight is an issue, we can go with 18350 batteries, with about 900mAh capacity but greatly reduced form factor and weight.
2. Do we need a voltage regulator? Depends if development continues with 3.3V or 5V controllers and how tolerant the controllers are of voltage variation.
3. Do we need a battery low indicator? The tp4056 has a full charged and charging SMD LEDs, adding a way to indicate unit needs charging would be a nice to have.
   3.1Adapt the casing to make use of tp4056 indicator LEDs + batt low LED.
4. Adapting the cases to include 18650 cell.

[Yohannfra]

1. one celle is enough yes
2. Well I don't think this is something I can answer now, I will certainly move the project to something else than Arduino (maybe ESP32, STM32L152)
3. Maybe we could directly use the leds of the TP4056, by placing it in a certain way we could see the light by transparency through the 3d printed box.
4. Yes but not for now :)