I am coming into possession of hundreds of 18650 batteries. They are used, but initial checks indicate over 90% of them are good batteries. I would like to discharge, charge, and measure capacity of them while logging their values.

I'm trying to find a cost-effective solution to measure as many as I can at one time, but I'm not coming up with much. Are there any designs for what I'm wanting? Preferably open source.

The cells are 18650 Sony VT5.

  • 1
    \$\begingroup\$ Wolf at secondlifestorage.com best theory is to test IR. Low IR has very high correlation with good remaining capacity. \$\endgroup\$
    – winny
    Oct 18, 2019 at 20:29
  • \$\begingroup\$ Consider making the question more applicable to future visitors by asking "How can I evaluate the condition of used lithium ion batteries?" The particular cell size is a detail that may affect the construction of a test apparatus, but not the theory in general. \$\endgroup\$
    – JYelton
    Oct 18, 2019 at 20:42
  • \$\begingroup\$ I've updated the title accordingly. I figure I can test IR when I'm discharging the cells, and comparing before and after voltage. But I suppose if IR is enough, that would speed up my testing time greatly. Thank you \$\endgroup\$
    – Jeff
    Oct 18, 2019 at 23:17
  • \$\begingroup\$ You can make a bulk charger that is as simple as a resistor from a say 4.0V supply to each cell. 4V is low enough to allow floating at charge terminate and the resistor sets acceptable Imax. These will take a long time to fully charge as Ichg assymptotes to zero as Vbat approaches Vchg. You also need to precheck that cells have voltage above Vabsmin - notionally and often a cell below a certain voltage cannot be recovered and is dangeous to charge. | You can make a CC charger with Vmax = say 4V but that adds complexity. At that point using many TP4056 modules at well under $1 each makes sense. \$\endgroup\$
    – Russell McMahon
    Oct 19, 2019 at 0:15
  • \$\begingroup\$ Discharge test can be timed at constamt current to some Vmin, or even at constant R. || You can but Chinese LiIon cycling testers at lowish cost. [AliExpress / Banggood /ebay?] eh Here $2 free shipping discharge tester. \$\endgroup\$
    – Russell McMahon
    Oct 19, 2019 at 0:16

1 Answer 1


I had exact same problem before and I have simple solution for it, it's simple and cost effective but takes a lot of time.

I used one arduino uno and 16x2 lcd display and few power resistors, Generally lithium batteries have maximum 4.2 V and can supply about 0.5-1A. You can use 5 1 ohm 1 watt resistors to discharge the battery and connect a n channel MOSFET for start&stop the circuit.

Arduino's analog pin will read battery's voltage level and if it drop to 3.5 V or 3V , it'll stop the discharge. Energy stored in battery can be calculated by current*time and you can calculate current by voltage/5ohm .

It takes 4-8 hours to measure the battery . You can lower the resistors to make it faster but it'll damage the battery and faster you discharge, lower the capacity.

I have schematics in somewhere but if you want to use it, I can add schematics and code

Also you can measure the basic capacity by voltage levels , if fully charged battery's voltage is 4.2v . It's probably good condition but bellow 3.8V goes into trash .

  • \$\begingroup\$ This is pretty much what I've ended up doing. I could buy a device to multiple cells at once, but for the time being I modified an old board of mine with a bq25890h and measured charge and discharge. Also putting a 5ohm load on it and calculating IR with voltage with/without load. \$\endgroup\$
    – Jeff
    Oct 26, 2019 at 1:30
  • \$\begingroup\$ You can use arduino and measure 5 cells at the same time . Also I will improve this design by adding multiple discharge resistors . If you want the results add your mail \$\endgroup\$
    – Mordecai
    Oct 26, 2019 at 5:49

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