Small espruino device with solar cell?

I would like to create a BLE device which uses Espruino and is powered by a solar panel or cell. It should be as small as possible, not larger than a Puck.js.

Is it possible for a Puck.js or MDBT42Q Breakout Board? What components do you recommend? It should have endless battery lifetime if possible. The board Just advertises and listens for a button, so the power consumption is quite low.

Since both Puck.js an MDBT42Q Breakout Board do not have any battery charging / overcharche protection nor converter device, you have to add that.

Todays solution for the battery is usually a very small 1s LiPo - similar as used in watches - see pictures in conversation about 'Espruino on your watch!'. Since power demands are very low, you get away with the smallest charge controller and converter you can find... or you may not even need a converter, a low-drop voltage regulator - or a diode with sufficient forward voltage drop - can do it.

Less modern solution is using 2 cells of rechargeable NiMH or NiCad batteries. The advantage there is the charging voltage is within the limits of what you want to drive and you need only something limiting the charging. You may even uses mc to control it.

The advantage of managing the charging of the battery with (solar panel) source independently from managing the supply of your mc from the battery gives the freedom to optimize for them power harvesting and storing and the constant generation of power for consumption.

Wed 2019.05.22

To obtain sufficient charge current, especially when the clouds block the sun, or sun low on the horizon, requires quite a large cell, std 60x110mm ~2x4" at 1W and the smallest I've been able to locate 30x60mm ~1x2" at 0.18W with output charge voltage above battery voltage. Simplest circuit, or maybe a voltage doubler or buck step up converter.

P=IE   I=P/E   1/5=200ma
and 0.18/5 = 36ma

All manufacturers recommend as @allObjects points out, NiMH/NiCad and trickle charge as Li/LiPo batteries require a constant voltage/current after startup conditioning FOR THE DURATION of the charge cycle. You need a special board for LiPo charging and a guarantee the sun wont be covered by clouds during that process. Failing this, may/most likely result in fire (see numerous YouTube vids - Google solar LiPo charger or drone quad copter fires) as fluctuating V and I as when sun behind clouds, will change chemical makeup of Li, effectively lowering internal resistance or shorting out the cell.

There are many articles that suggest using a TP4056 module to build a solar charger. Yes, they may work for a few charge cycles, but then you will be calling your insurance company for a pay out.

I'm on the fence post with LiIon as I haven't found a yea article for using the TP4056 for them, or whether trickle charging via solar is feasible.

Lithium Polymer Charging/Discharging & Safety Information

https://www.maxamps.com/lipo-care.php

LiFePO4 would be better, because it's safer, and it's maximum voltage is 3.65V, so you can almost fully charge it, and use it without an additional regulator. Usually more charge cycles compared to LiIon.
Drawback: smaller capacity at 0 charge cycles & couldn't find a small mass produced charger board. But if anybody knows one, drop in a link!

I think given the size of the cell it's not going to provide a bunch of power, so really you want one that provides enough voltage to charge the battery directly, rather than with a step-up converter.

A few things come to mind:

• Use an off the shelf solar power bank like this one - the 5v output may draw too much power to use consistently so you may need to crack it open and connect to the LiPo directly.
  
• As others have said, charging a LiPo from a solar cell is a bit iffy. However if you're using a LiPo with protection circuitry in it then you might be ok, especially if the cell is small so doesn't provide much power. I actually have some devices here where I do exactly that, but I couldn't recommend it for a product.
  
• You could use a Puck.js with a LiPo CR2032 cell, and a trace cut on the board and diode added as I mentioned in your other post (because otherwise the voltage is too high). There's no cell protection circuitry on those so I'd recommend you use a 3.9v zener diode (the closest zener voltage below 4.2v) to stop overcharging. It'll work and be safe, but I'm not convinced about the cell lifetime.
  
• NiMH cells are probably a good bet for this kind of thing as mentioned previously.
  
• You could use a supercapacitor, which should have a very long life. You just need to be sure to choose one that doesn't have a high self-discharge rate. Trying to get one that'll fit in a Puck.js form factor could be difficult.
  
  

However with all the battery based solutions (so not supercapacitor) the batteries have a lifetime - they'll only work for a certain number of charge/discharge cycles or amount of time.

If you just stuck a non-rechargeable Lithium AA cell in the box then that'd provide over 2500mAh and a 10 year life, and with simple advertising would probably outlast most of the other solutions, without needing daylight.

I missed @AkosLukacs post, but LiFePO4 looks good - and it may be ok with being trickle charged via a solar cell even without a charge circuit? you'd have to check the datasheet.

Other potentially more resilient cell chemistries are available too - for instance https://www.sii.co.jp/en/me/datasheets/m­s-rechargeable/ms621fe/

Search for "TP5000 module", ie this . Needs a solar panel with 4 to 7 V output.

Thanks! Funny thing, I probably read all reviews at that site once :)

Sat 2019.06.15

Untested but, have you looked into one of those rechargeable stakeable lawn lamps? They are about ~2" in diameter. Have no clue at what voltage the solar cell is, but the device only contains a single rechargeable AAA battery. This means maybe two would be needed or a voltage doubler circuit. Would contain all the parts needed for this project, but one would have to dismantle to find out. Sounds like fun!!