![]() The Tesla Powerwall inspired me to use the 18650 Li-Ion cells. I have decided to build a 6S40P battery with 22.2V. My goal is to build a battery that can store around 2KWh. ![]() For the battery type, I chose 18650 lithium-ion cells. I discarded all cells over 80mOhm.I’ve built a huge field battery, which will be charged via photovoltaic. ![]() Three cells were under 4.10V, discarded them.įor IR measurement, most cells gained 6-10mOhm IR. Most of the cells were 4.15V-4.16V, very good, they kept charge really well Now I started the battery mounting, picked the first 280 cells. Ok, after over a year I selected the cells to mount the first battery, 280 cells, using a round-robin pick. After one month I measured all cells again and discarded all cells 75mOhm.Īnd left them still for one year. ![]() After doing a CDC cycle (full charge-discharge-recharge) on each cell (using Liitokala 500 chargers) I measured each cell for voltage and IR (using a YR1030+) discarder all exaggerated heaters and cells which didn't charge at all Discarded all dented/damaged cells/with rust signs re-wrapped cells with damaged cover The procedure I used last year to select "good" cells was: A reference for the testing flowchart is here: So last year I followed all the good practices for testing and selecting cells, discarding the bad ones. Last year I tested all my cells (a total of 1120, around 250 recycled from used notebook batteries, the others slowly bought from the famous "dutchman" and part bought from a good italian supplier also present here in the forum -spam spam spam!). So I did all my testing before mounting it and I can tell you about the parameters I used (after learning here in the forum! Thx guys!). I have a measurement for you: last week I mounted my first 14s20p battery, using second hand cells bought last year. And its also depending on the SoC: the smaller the SoC the less the voltage drop. This only holds for cells in a resting state, so at least one month after charging. If they loose more than 100 mV in 2.3 months i concider them bad and wouldn't use them in a diy pack. Very good cells loose less than 1.2 mV/year - so this is less than 0,005%/month - nearly not measurable. P.S.: The Spread on voltage drop after resting is very big. Good cells normaly show below 50 mR or less, some might be good up to 70 mR. You can lookup the 'normal IR' on some charts here in the forum. The 'normal' IR is depending on the cell type (high drain or not e.g.). Use an IR-meter with 1kHz test frequency. If the voltage stabilizes around the last reading - fine - if they still loose >1% toss them.įor a final judgement the internal resistance is also very important. You can let them rest for another month or 2 and measure again. If they loose more voltage in one month i wouldn't recommend to use them any more - they are self-dischargers! and for not demanding purpose you can go up to 80 or 100 mV. Measure the voltage again: If they loose less than 1% of their initial voltage they are good to go (around 40 mV) - up to 50 mV might be o.k. Leave the cells untouched for one month, resting in a cool place (10-20 deg. Measure the voltage 24 h after full charge (rest voltage).
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