From Newsgroup: sci.electronics.repair
On Sun, 6/28/2026 12:11 AM, Mr. Man-wai Chang wrote:
On 6/28/2026 3:30 AM, druck wrote:
DO NOT try to quickly discharge Lithium batteries, either let it
discharge naturally, or take to a place which disposes of Lithium
batteries safely.
USB-C's increased power capability is from being able to use higher
voltages, and not increasing current over thin wires. It has nothing to
do with discharging.
So there is absolutely no way to quickly discharge rechargeable batteries so as to return them to safe state?
I will open a thread targetting only rechargeable batteries including ones made out of lithium.
The "recommended" discharge interval for lithium energy storage, might
be on the order of two hours.
Yes, people routinely beat the piss out of them, by running model cars
above the current limit, and all this does is drastically cut the capacity.
Heating the cell itself above say, 50C, is not good for it. This is one reason we don't abuse them. The electrolyte is flammable. Some "tapes" used
in the construction of cells, begin to dissolve in the electrolyte at
high temperatures. In automotive applications, there is an active cooling system in the automobile to keep the 3.7V cells below temperatures
like that.
A dead short across Lithium is no more clever/desirable, than carrying out
your experiment on an automotive battery.
An automotive battery, can make 150 amps when the voltage across the
terminals drops to 9.5V (I measured this in my driveway). That's 1425 watts being drawn by the car starter motor, and if you did that with a
resistor instead of a starter motor, the resistor would be
glowing red hot. That's the power level of your kitchen toaster.
The Lithium battery has a mAh rating, like maybe 3600 milliamp/hours.
I could draw 1800 ma (1.8 amps) for each hour of my two hour period.
That's 6.7 watts, so I would be using a ceramic resistor (because
of the heat). 3.7V over 1.8A would be approximately 2 ohms. I would
purchase a 2.2 ohm 25 watt resistor. Next, I want the precision grabbers
as I do not know what exposed conductor I can access inside the keyboard.
The battery could be the welded tab type. Alligator clips fall off
randomly and can be irritating for a job like this.
Now, I personally, would not be in a rush like that. I could use
a higher resistance value for my resistor (the discharge interval
would be correspondingly longer). In terms of hobbyist
fabrication, I need.
1 resistor 22 ohms (1 watt or higher, I've got a couple ceramic ones)
2 pieces of wire
2 precision grabbers with the spring steel wire jaws (not alligator clips, which will slip)
You want a grabber that won't slip off. The Pomona ones can deform with abuse, and you might want a two or three hook grabber, depending on what conductor
you can easily access. Alligator clips leave exposed metal when they fall off, and the alligator clip could touch some part of circuit you were not intending to touch.
https://www.digikey.com/en/products/filter/test-leads/grabbers-hooks/620
That list of three items needs to be soldered up, the solder joints
covered with polyolefin shrink wrap (to prevent accidental contact
with the specimen).
Now, I'm ready for my discharge experiment. I could discharge to 3V.
https://www.ersaelectronics.com/blog/18650-battery-voltage-a-complete-guide?srsltid=AfmBOoqhb1UAhmBMGZpBGDWE7biCOFn1K23lLtMUDansB5SSw37EGdIO
4.2V Fully charged, the "stress-limited" voltage (when you stop charging, it does not stay there)
3.7V The settled, long term voltage during the two hour discharge
3.0V The knee, a good place to stop the discharge
0.0V A single cell 3.7V rechargeable *can* be discharged this low,
but it may affect the long term 3600mAh rating. Second grade
Chinese cells from no-name shops, are 800mAh (despite what it
says on the sleeve), but this makes no difference to the
charge and discharge rules, only the runtime is affected).
As a hobbyist, you need to know how to measure voltage and current,
to monitor how the discharge operation is going. The BMS on battery
packs, it checks whether the pack is below <number-of-cells * 3.0V>
and a MOSFET in the BMS can cut off current flow to prevent discharge below that level.
If you absolutely want the cell at 0.0V, you can do it. Leave your
22 ohm resistor on there all week, and it'll be flat. And the resistor
will no longer be hot.
3.7V +---------+ If you use 2.2 ohms instead, resistor gets *hot*
| | Convenient form factor is a white ceramic power resistor.
----- 22 ohms
--- 1 watt or preferably higher wattage
| |
+---------+
Examples of ceramic resistors. I use some of these for testing ATX power supplies.
https://www.aliexpress.com/item/1005007544570307.html
Paul
--- Synchronet 3.22a-Linux NewsLink 1.2