Connecting a fully charged 18650 lithium cell in parallel with a fully discharged one. How much current flows between them?
By Julian
Youtuber, shed dweller, solar charge controller aficionado
Good morning all…
Youtuber, shed dweller, solar charge controller aficionado
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I have those same batteries, it came inside a flashlight. They have VERY high internal resistance. Don't try this with a high drain lithium ion battery.
How many 18650 Button top cells can we connect in parallel? I'm panning to add together 3 or 4 of these 18650 in parallel in one of those holders in order to give longer run time in my "under the hood work light" that I use in my garage. This light is fixed on the top of my workbench so I could just attach the cell holder right next to the light and use the existing wiring from the cell inside the light to the new cell holder. The run time at the moment is miserable, just around 2 hours with 1x 18650 2000mah in max setting. Then six hours on the charger.
If the dead cell is zero volts then it will be seen as a short and will draw as much current as the charged cell has. As the voltage difference between the two cells become equal the current will drop.
Internal resistance (IR) is related to SOC (state of charge). A cell will display different IR values according to its state of charge. Temperature also affects IR. Therefore you should let the cell stabilize and cool down after charging/discharging for at least one hour prior to measuring IR.
Thank You, Julian. You know your shit. No one can say different without lying and/or coveting.
Oscilloscope? No. But what I'd add to those ammeters is a couple of header pins that could be read during initialization and taken in one state to indicate that the arduino would also be sending data out serially, for logging and analysis. It would ba a trivial hardware mod, and not very much software would need to be changed as well, just one conditional test in the initialization code.
Hi Julian. How could I charge two 18650 battery's in parallel (initially connected in parallel at exactly the same voltage so no damage caused)?
I would like to double the capacity but don't want to remove the individual batteries to charge. Would like to just plug in to a TP4056 to charge rather than remove the batteries.
Thanks.
I remember when you posted this but I don't remember have you tried some good cells
Then you answer it at the end
The batteries on my electric bike are 0.8 milliohms.
Tried this a while ago. Was surprised by how much the contact resistance between the cell and the holder matters. See, for cheap holders the springs/contacts are made of some low quality steel, which is a poor conductor. As it heats up, it looses its elasticity which means it pushes less on the battery tab, creating an even worse contact. This overall becomes a current limiting mechanism.
I noticed this trying to use a pump from AA rechargeable. The thing went down in power after ~10 seconds and stayed there for the whole ~15 minutes the battery lasted.
Nothing because ultrafire
We call this "top balancing" the cells. Many top balancing modules have current limiters set on the board eg. 50mA.