Why the beak-like shaped V, then the wiggling, while recharging alkaline at I=ct - Page 1
Forum section: electronics / projects

author: RoGeorge
author_rank: Super Contributor
timestamp: August 12, 2023, 09:36:37 am
content: Got a new toy last week, found out about 'gnuplot'.Tried it on an older Python logger I've wrote for DP832.Was fooling around plotting the V logged from a Rigol DP832 (triple power supply) while recharging non-rechargeable alkaline batteries. I know alkalines can leak and not worth recharging, did it out of curiosity, only to see some plots made with no extra effort from the old python2 (text) logger.DP832 has 3 independent voltage source channels, each of the 3 can report their own U, I and P, so 9 values in total (measuring resolution is 0.1mV and 0.1mA, P is calculated). The logger retrieves U, I and P for each channel, adds a timestamp, then appends the 9 values as a new text line in a CSV file, in this format:Code:YYYY-MM-DD,HH:MM:SS,CH1(V),CH1(A),CH1(W),CH2(V),CH2(A),CH2(W),CH3(V),CH3(A),CH3(W)2023-08-11,20:21:41,1.4579,0.0192,0.028,1.4619,0.0184,0.027,3.4624,0.0192,0.0662023-08-11,20:21:42,1.4580,0.0192,0.028,1.4619,0.0183,0.027,3.4623,0.0192,0.0662023-08-11,20:21:43,1.4579,0.0192,0.028,1.4618,0.0183,0.027,3.4626,0.0192,0.0662023-08-11,20:21:44,1.4567,0.0192,0.028,1.4619,0.0183,0.027,3.4626,0.0191,0.0662023-08-11,20:21:45,1.4561,0.0192,0.028,1.4619,0.0184,0.027,3.4624,0.0192,0.0662023-08-11,20:21:46,1.4558,0.0192,0.028,1.4619,0.0184,0.027,3.4626,0.0192,0.0662023-08-11,20:21:47,1.4556,0.0192,0.028,1.4618,0.0183,0.027,3.4621,0.0191,0.066...I've put on the CH1 a used Panasonic AA alkaline, on CH2 a used Energizer, and on CH3 two series AAA Duracell, also used, in order to see how much they can recharge.Tried various charging currents 0, 1mA, 3mA, 5mA, 10mA, 20mA, 50mA, 100mA. Then went back and forth between the last 3 values a couple of times. Each tried value was kept constant for a few minutes, then changed with another constant current, and so on.1. - It was intriguing to see a sudden drop in the charging voltage with one of the batteries, as seen in the annotated plot. What can cause that?2. - Another unexpected thing to see was that, while the charging current was kept constant, the voltage on the battery was rather "jumpy", seen in the last part of the second attachment, which is the continuation of the annotated attachment while the batteries were left to charge over night at 100mA constant current. I've checked and it's not because of bad contacts, the voltage is like that from inside the battery. Why so noisy when charging?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2023, 12:59:40 pm
content: Basically, why they can't be recharged (generally speaking). I would imagine it's a combination of hydrogen gas bubbling up, plus zinc particles (and the hydroxide mush surrounding them) jiggering around creating random conductive pathways through the cell as they go. Motion can also be produced by the zinc growing (probably with dendritic growth?) and Zn(OH)2 consuming.If I had to guess, the Energizer was near full already, so had reasonably good connectivity between zinc particles?Kinda like lead-acid, you over-discharge it and/or let it sit, sulfation occurs, whole sections of the cell become passivated and capacity drops. Think of that, but occurring in real time as zinc particles are consumed, and then only partially reformed; but worse than that, it's not a connected grid, it's a paste subject to free motion (on a microscopic scale, perhaps; it's a pretty thick paste, to be fair) so random connections form and break all the while.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: August 13, 2023, 10:28:47 am
content: Indeed, I remember reading the alkalines doesn't have any mechanism of reabsorbing the gasses during an eventual recharge. For the fun of it, abused a few more alkalines:Duracell AA- tried recharging two of them that were flat dead (one was showing 7.56mV, and the other 181.11mV without load)- when charged, the voltage started to raise at 100mA (that lead to about 1.6 ... 1.8V), but after a few hours I've tried if they can stand charging at 200mA, which made their voltage rise to 2.1 ... 2.3V, they started to warm, and after an hour or two they popped at 5 minutes apart- if they pop, the sound is strong enough to hear and easy to distinguish in a quiet room. Some two drops of whitsh-grey goo dropped from the minus side of the battery. Didn't spill, just dripped. When fresh, the spill can be easily washed under a water jet. After washing, left them outside over night, and found them in the morning with crystalized bubbles on the negative plate.The charged Duracell AA still measure more than 1.6V each, after starting from less than 0.1V and popping.Side note, the AAA (also Duracell) were much sturdy against popping. They were able to take more than 200mA and withstand 2.5V each for a couple of hours, and get quite hot without popping. I suspect the sturdiness of AAA is bigger for leaking, too, not only for popping them. I've seen plenty of AA leaked alkalines, but not that many leaked AAA.On the contrary, the other two AA charged slower (without popping them, at max 100mA, the Panasonic and the Energizer), after charging and letting them disconnected over night, they show about the same voltage as before charging, some 1.4V.Popped ones apparently charged 100%, the still sealed one seems they didn't charged much, go figure. Used batteries are the strangest parts. Last time I've found a used Li-Ion that deflated itself and since then developed some vacuum inside its plastic pouch.