Fake Battery Warning

       I converted one of my headlights (the one you wear on your head) that used to take 3 - AAA cells and modified it to use a cell phone battery since it is rechargeable and more convenient for me.

       I have one cell that I use for it so I wanted to get more as spares. I found some locally and bought several since it was very cheap. They said it was cheap since it was NOS without packaging.

       After testing the ones I got, they measured from zero (no capacity!) to about 98mAh. The cells are advertised to be 900mAh rated. Check out what I found out after taking one apart.

I bought five.

I took care to buy ones that are not bulging and clean. Hoping for a better chance of getting useable ones.

One was DOA with a high internal resistance. Another measured 3mAH capacity at 200mA discharge current. The others measured 69mAh, 96mAh and 98mAh tops all on the same discharge current.

Some of them have legit looking hologram stickers.

So I tried charging the dead one at 5mA constant current (open circuit voltage at 12V, DON'T TRY THIS AT HOME!) and it shot up 8Volts. At 20mA, it got to 10Volts. This looked like there is no protection circuitry as it is supposed to cut off when over charged.

I placed it inside a fireproof LiPo bag for safety as I know what could happen if it goes wrong. Predictably, the cell puffed up so I took it apart carefully to check the PCB.

With the cell apart, this is the contact side of the PCB.

Ok, looks like a typical protection PCB.

I also found why it had high internal resistance. The burn spot that can be seen behind, on the cell, was a rusted part of the connecting tab which resulted to high resistance and it disintegrated.

But behind it, surprise! The protection circuitry is not populated. The components seen are simply a zero ohm jumper to short the place where the protection mosfet is supposed to be and there is a 47k fixed resistor where a thermistor is supposed to be which is used for detecting when the cell is overheating.

The cells being under spec and then run beyond its limits in the phone and omitting the temperature sensor and protection circuitry is a recipe for disaster when the cell overheats and goes into thermal runaway.

Now it is easy to see why there are more reports of cell phones randomly bursting into flames.

       The other two cells also have 47k reading between the temp and (-) terminal, so I'm betting those have blank PCBs too. They are also the ones that measured 3mAh and 69mAH capacities. The other two measured 110k between temp and (-) but heating them up with a hot air tool did not show any changes so are most probably fixed resistors too.

       Nowadays, it is very difficult to find original ones especially if you are looking for a bargain. Due to the reduced capacity and safety concerns, it is much better to buy guaranteed genuine ones from reputable shops even if it is pricier. An additional cost on the genuine battery is insignificant compared to a burned down house or a loss of life.

       I cannot recommend any method to identify originals to fakes but the only thing I could think of is to measure the resistance between the temp (middle) and negative terminal of the battery. Then heat or cool the cell just enough to see if the resistance changes. If it does, then it means there is a thermistor installed which has a higher probability of a proper protection circuitry installed in the pack.

Page created and copyright R.Quan ©07 Nov 2015.