Rechargeable coin cell batteries are great for all your small projects. They look exactly like regular coin-cell batteries, but in a shocking turn of events you can recharge these little guys. They can put out a reasonable amount of current, and they’re small. Just what you need for your Arduino smart watch, or whatever else the kids are doing these days.
But if these batteries are rechargeable, you need a charger. That’s where [Jon]’s entry for the Hackaday Prize comes in handy. It’s a small, cheap charger for LIR2032 and other rechargeable batteries comes in. It’s barely larger than the battery itself, and it plugs right into a USB port. How this isn’t a product already, we’ll never know.
The circuit on this coin cell charger is built from an MCP73831, a nice single cell, lithium ion and lithium polymer charge management controller. In the standard, ‘I only need to read the first page of the datasheet’ configuration, this chip can put 500 mA into a battery. Standard rechargeable coin cells only have a capacity of 40 mAh, so you’ve got plenty of headroom at 1C.
The total cost for this project was under $8 for three boards, and a BOM cost of $2 for one. That’s fourteen bucks for three of them, if you know how to solder, compared to a standard, off-the-shelf charger for about $20. Building this is cheaper than buying the equivalent product. It’s unbelievable, but true.
Look for lir2032 tofind rechargeable coin cells.Note max recommended charge is 0.5C (35mA)
Also be aware that the voltage can be as high as 4.2v,so could be bad news for 3.3v parts expecting a normal LiMno2 coin cell. Max possible discharge current is also a lot higher than non-rechargeable cells -several hundred mA so smoke is possible.
They also have no over-discharge protection, so if you run them flat they’re no longer rechargeable.
How bad is the self-discharge rate on those cells?
IIRC bmw keyfobs use a panasonc cell part number VL2020, i dont know the specs but have had spare keys sat in a draw for years and still work fine if your looking for something long lasting they are all over ebay and not to expensive.
I havnt seen them go much higher than about 3.3v though so a different chemistry to the lir2032 .
What I would like to see is a LiFePO4 version of the CR2032. The operating voltage range nicely coincides with that of 3.3V logic chips.
Could cheat and use the 0.6 to 0.7 voltage drop of a ordinary power diode to drop the voltage.
The esp8266 goes from 3.6 and down to 1.7 volt, and 4.2 minus 0.6 equals 3.6.
“How this isn’t a product already, we’ll never know.”
It is, I’m afraid. Google turned up at least three variants available on a popular auction site.
Not that it takes away from the design itself – it’s tidy.
I like it.
Now can we have a QI chargeable 2032 battery version,so I can charge my keys and garage remote
I find it hard to believe I actually remembered this from 6 years back, but maybe I somehow actually did.
https://hackaday.com/2012/03/28/coin-cell-battery-charger/ not saying ‘same’ just ‘relevant’ :)
hmmm… but the one from this article is round and the other one is rectangular-isch… so there is a huge difference.
Anyway, both projects seem fine to me, and whether or not this is done already isn’t really an issue I guess. What we really should ask ourselves is if the battery itself is the best choice for the application. If you need to make it smaller just for the looks of it BUT therefore need to the battery once a week… I wouldn’t say that the design is a good design… or is it. Though we live in a time were we find it completely normal to charge our watch every evening, while in the past it would last for 2 years on a single coin cell.
Anyway, replacing the normal battery by a rechargeable one makes battery lifetime only worse as you need to swap it even more, though at a lower operational cost in the long run. That is if the battery cover doesn’t wear out and you need to hold it together with tape… ahh… the horror…
Ain’t nothing wrong with “relevant”! Thanks.
Prosecutor to Defendant (in Witness Chair): “You think you’re so smart, tell me this, ‘What has 4 legs, a trunk, is gray and lives in Africa?’ ”
Defendant’s Lawyer: “Your Honor! That is irrelevant!”
Prosecutor (turning to Defendant’s Lawyer): “That is correct!”
Just made one of these. First time soldering SMD components, and it looks pretty rough, but it works a treat! Thanks for the clear instructions, components list and your gerber files.