In some Chinese shopping sites you will find LiIon batteries with fantastic capacities. You can find 18650-type batteries from UltraFire with a capacity of almost 10Ah.
That doesn’t sound too good to be true? It is.
Let’s do some fact checking first and look at the UltraFire web site. You will see that the UltraFire 18650 LiIon batteries have a capacity of 2500mAh. Here it is clear that these batteries must be fakes. Can somebody build a battery of the same size with almost 4 times the capacity? No! This just isn’t possible. These batteries are fakes and the capacity isn’t specified correctly!
But do they provide at least the same capacity? Let’s check. The setup for the capacity check is simple. The battery will be fully charged and then discharges with a constant current of 500mAh (this value had been chosen to limit the stress on the batterie, but also make sure that the measurement doesn’t take too long). You should not discharge a LiIon cell below 2.7V, therefore this is the point where the battery is “fully discharged”. Now you only have to measure how long it takes from full charge until the cell voltage is down to 2.7V. Then the capacity is very easy to calculate as time x current.
This measurement is easy for me as my electronic load that is used for power supply tests has a battery mode that not only turns off the load at a given end voltage, but also automatically calculates the capacity.
You can setup a measurement like this even with a simple resistor, but the current won’t be constant. However, for a rough approximation, you can just assume that the cell voltage is constant at 3.7V the whole time. The real capacity will be a bit lower. Just make sure you turn off the current when the cell voltage is at 2.7V. Fully discharging the cell will most likely damage it and make it unusable.
Now comes the funny part. I did not expect that the capacity is even close to the specific value, but have a look at these values of this “9800mAh” battery:
Really? Around 1100mAh? That’s 11% of the advertised capacity. It also means that this cell performs worse than standard NiMh cells. It is hard to understand how you can build a LiIon battery that performs that bad.
If you wonder, why the display shows 3.143V: the battery voltage recovers when turning off the load. However, if you turn on the load again, it will drop again to 2.7V very fast.
I’ve tested two 18650 batteries. The perform both similarly bad.
I’ve also tested two 16340 batteries that are advertised as “2300mAh”. Their real capacity was also around 10-11% of the advertised capacity (230-240mAh).
While there are some cheap offers of LiIon batteries on Chinese shopping sites, don’t even try these things. They’re not worth any money. Also note that this test doens’t say anything about real UltraFire batteries. I will try to find some original batteries and perform tests with them in the future.