$1 Coin Cell Charger

Sure, coin cells usually last a long time — but do you really want to buy new ones and throw the old ones out? The LiR2032 coin cell is a rechargeable lithium battery, for which you can build a charger at around $1.

The 5 minute hack starts with a TP4056 lithium charging circuit, which is a great DIY board designed to charge high-capacity cells at about 1A. Luckily, it is pretty easy to modify the board to charge lower capacity batteries. It’s just a matter of replacing resistor R4, and a little bit of soldering!TP4056_Schematic

The official TP4056 datasheet even tells you the range of resistors to use (page 3). [Tom] went below that range and found a 33KΩ will result in about a 50mA output — this results in a full (and safe) charge of the LiR2032 in just over an hour.

To finish up the hack he soldered a piece of strip board to the charging header, and attached a coin cell holder. Not bad for a buck. Now what if you added a few resistors and a dip switch too? It’d be pretty easy to make an all in one lithium charger!

33 thoughts on “$1 Coin Cell Charger

  1. Actually the LiR2032 has a capacitance around 40mA and the recommended current for charging safely lithium cells should be 80% or less of their capacity. 35mA is the maximum safe charging current for LiR2032 cells, as indicated on the battery datasheet, therefore 50mA is too high and not safe at all.

      1. This is exactly my point: when dealing with something potentially dangerous we should always trade speed for safety. The user might know nothing about data sheets and should never be forced to check them, therefore one designs the charger for the lowest and safest current. By the way, slow charging also prolongs battery life.

        I also would stay away from anything that pumps 150 mA into a coin cell, if such a charger really exists, unless I’m doing IED research:)

        1. Such a charger is certainly advertised, but I would not be surprised if the specs are wrong. Check out the “Smart Charger” on www dot batteryspace dot com. You are right that you can’t go wrong by charging slower, except that the Sun will soon die and engulf the planet.

    1. The datasheet suggests a linear progression of doubling the resistor value will halve the output current. The formula mentioned is battery current = (1v / resistor value) * 1200. With that formula, 33k would give about 36mA. The only concern I have is the datasheet also mentions a range of +/- 50mA for the two values (1.2k and 2.4k) that they list in the electrical characteristics. That suggests to me that shooting for charge currents in the tens of mA will be dicey at best.

  2. I have that exact same Charger PCB from ebay (china). The vendor didn’t wrote much about the features and I’m still wondering why R6 is not populated. Is it to tinker with resistor values to achieve a different output? If so, is there a Lookup List of some kind?

    1. According to information from the vendor “R5 and R6 control the risky battery temperature range” when pin 1 (TEMP) is connected to a NTC thermistor in the battery pack. Grounding pin 1 (using R6 = 0R) disables the temperature sense function.

  3. That’s neat an all, but you can buy cr2032′s from digikey for $.40 apiece. granted it makes a difference in aggregate but if you want to be sure of it, new is better.

        1. Also importantly, 35-40mAH != 225mAH. It’s a fifth of the capacity, which substantially increases the pain-in-the-ass factor.

  4. and what’s the self discharge of an LiR2032 ?
    I have only one gripe with rechargeable battery’s and that the self discharge.
    found a data sheet that suggests for a 35mAh discharge type it’s still should be at 85% (or more) after that. yeah, maybe with brand new one…
    thank god for eneloop battery’s in the AAA/AA range…

      1. Yeah not particularly amazing. I am designing a low power blinky light thing using an LiR2032 and that is a significant limitation. CR2032 would not work because it needs 3.6V to drive UV LEDs.

  5. While on the topic: I have a 3.7V LiPo with 240mAh.
    As I understand it, this charger has a termination voltage of around 4.2V. Would it be somehow possible to still charge this 3.7V LiPo with this charger by modifying the components?

      1. Thank you for your reply. To get thi straight: The chargerwill not damage the LiPo? Because in the Datasheet it says “Preset 4.2V Charge Voltage” and my LiPo says 3.7V (It is even 3.8V on full load).

  6. How robust is the TP4056?

    Ages ago I bought a bunch of Microchip MCP73831-2 /OT chips for safely recharging lithium cells, but discovered they die too easily if accidentally shortcircuited so my adventures into rechargable lithium cell powered circuits came to a halt, and I want to get back into that area especially as you can buy 10 of those TP4056 boards for about $9 from a Chinese eBay seller.

  7. My RC car days made me just as worried about under-charging a LIPO as charging them. You don’t want to drop them below 3v per cell. I assume these coin cells don’t have built in auto cut offs.

    I guess there must be modules out there which have another two pins for ‘load’, with safety cut off and allow charging with the load connected?

    1. I have recently seen a charger module with a protection circuit on the same board. No luck finding it tho. Protection circuits are available that you can add on.

    2. They certainly don’t have any low voltage cut-off circuitry, I have a couple of torches (one solar charged, another wind-up charged) with dead LIR2032s because the voltage got too low, the torches were very cheap so had no protection circuitry, they just won’t hold charge anymore.

      1. Yes watch out for that, especially if you are just swapping LiR2032 for a nonrechargeable CR2032. Fortunately LiR2032s are affordable enough that you can afford to kill a few.

  8. Does anyone know of a good writeup on rechargeable batteries? Every time something like this pops out it reminds me I know nothig about it (except to avoid the usuall warnings and the stuff that’s on wikipedia)

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