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What is going on with this cell?
#11
(08-10-2020, 02:19 AM)gauss163 Wrote: ^^^ Is the IR on those much higher than others?
Not sure I have a good way to test that. The one in my OP definitely does, as you can see the extreme difference under a 2C load. The rest I haven't paid attention to voltage drop. 

Is a 0.3V drop in two days not enough of a tell? The rest settled at a 0.1V drop under the same conditions.
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#12
0.3V drop in a few days is quite a bit, yeah.

Reading resistance in this case could be just as easy as doing a simple comparison. Let's say you have cheap DMM that doesn't do great on accurate Ohm readings. But by comparing the readings to known good cells, we can infer if the one cell is bad.
Example:
Known good cells are reading with cheap DMM in Ohm: 100-Ohm - We know that generally 100-Ohm is pretty high.
Known good 2nd cell reading: 90-Ohm - Still seems high
Known good 3rd cell reading: 108-Ohm - Again, high.
Ok, now we have a base line. The DMM is possibly reading about 60-Ohms too high; this is a guess, but we are comparing with known good cells, so we can infer that the values are much lower

Possible bad cell: 160-Ohm
We see here that this one is reading MUCH higher than the others. We don't know the exact amount, but we can get a good idea it's worse than the others by a lot.
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#13
I may be missing the obvious. I though in terms of battery testing when we mention internal resistance measurements it's by a fancy device that does AC impedance testing on the battery. If it's as simple as a DMM on resistance mode, I have several expensive ones. What should I expect to read on a good cell vs bad?

NVM, just tried with all my DMMs, none will give a ohm reading on any of these headways. So I guess I'm confused by your instruction.
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#14
A DMM Ohm reading is a general reading. It's not 100% accurate. But it does give you a good idea of what's going on. Kinda like putting your ear to someones chest to hear their heartbeat as opposed to using a stethoscope.

A standard DMM should be able to do an IR/Ohm test on any battery, as long as the voltage isn't too high (probably don't want to test it on a 48V system, for example).


Here's 2 examples of a DMM. You would use one of the settings circled in Yellow. For cells, on the meter on the left, you'd use 200 setting, this is for reading up to about 900-Ohm. If the meter doesn't register, drop down to the 20 reading and see if it reads then.
The one on the right is auto-ranging.
Also note that the Red probe must be inserted in the correct sensor port. The port will be labeled VΩmA

After setting the DMM into this setting, take the probes and touch to the cells terminals. Orientation doesn't really matter for this test.

[retracted]
Use the following two posts as examples.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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#15
Unfortunately the above suggestion will not work, i.e. the ohms function on a multimeter cannot be used to measure the internal resistance (IR) of a battery (attempting to do so might even damage some multimeters).

Instead, one simple method is to employ a resistor and the voltage function, e.g. as described here or here.

To learn more about the various methods of measuring internal resistance see the papers I cite here.
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#16
Yeah have to agree, a typical DMM ohms range won't measure a battery cells IR.
Typical DMM are only designed to measure resistance of unpowered passive devices, not cells (or circuits, etc) with voltage on them.

In addition to the links gauss163 provided, the OP could also try this fairly simple method I posted which is step load DC IR using two resistors.
https://secondlifestorage.com/showthread...5#pid66405
...beware: a bit of math is needed :-)
Running off solar, DIY & electronics fan :-)
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#17
Thats what i thought, thanks guys. I have a digital load with a step program function, so I may try to do that.

But not really sure anyone answered my question. DO I need to do that to know for sure these are bad cells? Is 0.3V drop in 2 days not enough of an indication they have a self discharge issue?
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#18
Yes, there is likely high self-discharge, but similar big drops can occur when they have very high IR and the charge termination current is not lowered enough in compensation, e.g. see the graphs here.

Your cell may be suffering from both. What was your charge termination current?
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#19
Ahh, I see why that was asked. All the cells were charged using a bench top power supply, and I let them sit at CV of 3.65V for about 15 extra minutes once current fell below 5-10mA before taking them off to make sure they absorbed everything they could.
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#20
Ah, with such a low termination current you charged them more fully than most CC/CV chargers, so that likely means that self-discharge is the primary culprit, probably due to an internal short.
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