12 September 2014

Mind cheap technology around Li-ion batteries

I was given a LED torch that included both 12V DC and AC chargers, and a 14440 Li-ion battery.

The AC charger circuit exposed.
The AC charger was faulty at the DC plug towards the torch. Discarded right away. A further inspection at a later time revealed a properly designed DC supply.

The 12 Vdc charger exposed.
The cigarette lighter 12V DC charger worked, so I used it to top-up the battery right inside the torch. Since the charge was not coming to an end, I interrupted it and measured the battery voltage: 4.7 V !! What?!?? A Li-ion charger is supposed to be produce no more than 4.2 V(*). Else ... it can catch fire.
I opened up the cigarette socket charger and found a 78L05 voltage regulator and an open-circuit voltage of 5 V. That thing is simply d a n g e r o u s !

12-to-5 V circuitry. The large resistor limits dissipation on the linear regulator, while the LED is connected across the 12V input with a 10k series resistor.

Now, either the cable from the dangerous 12 V charger will be transplanted to the AC charger, or I will add a series diode to drop the voltage to a safe(r) amount.

But, how about the remaining 99.999(9)% of world population that will buy similar products and never question their technical properties, thus trusting the producer? Either the product will fail early (most likely), so they'll blame Chinese manufacturing, or get hurt (heat, fire) without anyone to complain to. I am aware there is no solution, at least in the Country where I live.

(*) If you are in a hurry, a Li-ion battery can be charged at a constant voltage of maximum 4.2 Volts per cell. Do keep an eye on it and measure stand-alone cell voltage every 30 minutes. Disclaimer: do it at your own full risk.