Re: Discharging a wireless USB keyboard for long-term storage

Can I do it without extracting that stupid hidden rechargeable battery? Say, via the exposed USB Type-C port.

I know how to play with 1.5V dry, non-rechargeables AA battery! The spark is fun! :)

I am not worrying but fire risk as a result of self ignition and spontaneous combusion.

I don't wanna trust others, nor spending money.

If the keyboard has a lithium ion cell then storing for extended period above 80% state of charge is indeed dangerous, likewise under 20% state of charge (or more particularly below approx 3.2volts) risks permanent damage. If the keyboard is able to report state of charge then best bet is to run it down to somewhere between those limits before storing.

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.

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.

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.

Paul

I think the current state of the keyboard, is a safe state.

I don't sit here worrying about the single cell 3.7V battery inside each of my Bluetooth speakers. I do not hunger to short them out :-) I know what size of resistor to use, and I would also selected grabbers suited to the conductors present in there. But there's just no need to drain it.

The battery should not be easily removable, a spring-loaded battery holder should not be present in there. The cell should be hard wired, it could use welded-tabs for example. Such connection methods ensure glitch-free power delivery.

Paul

It's still better do discharge all juice.

So I cannot do it via the exposed USB-C port, nor the USB-C plug of the keyboard cable?

I can always buy a new one. Don't worry about the life of that stupid toy (the wireless keyboard).

Should I register my wireless keyboard with the police and get a license plate for it? That belongs to another thread about regulating rechargeable batteries. :)

If the design is intended to power other circuitry, it can only discharge the cell down to 3.0V or so.

You haven't opened it up yet, to see what kind of a mess is in there. With a keyboard PCB, the battery and any other circuits, would be off to the side of the key matrix. You should be able to see things well, see how accessible the battery is, and so on.

Paul

Can I trick the circuitry (circuit board) into believing that charging is actually discharging using the exposed USB-C port or the keyboard's cable? :)

As I said in another branch of threads, I would have to first unplug all the Cherry keycaps, Cherry MX switches, unscrew then pile the plastic edges of the caseing just to open the plastic casing!!

That's a lot of time, mood and patience! :)