I been playing a lot with Li-ion batteries (NCR18650Bs to be exact) and I have a question regarding how to test their maximum capabilities and generate my own conclusions. If you take a look at this post or their datasheet (http://onvaping.com/battery-safety-and-ohms-law/) they state that their max continous discharge is about 2C or 6.8Amps. This is fine and dandy but how can test this? I bought one of these (http://www.hobbyking.com/hobbyking/store/__5548__IMAX_B6_50W_5A_Charger_Discharger_1_6_Cells_GENUINE_.html) but it only discharges Li-ion batteries at 1C which is not what I need. Is there a charger like this one that discharges at higher Cs (2C, 3C or more)? If not, how could I approach these tests? Any help is appreciated. Thanks.
Your charger can only discharge at a maximum of 1 Amp, which for a 3200mAh battery is 1A/3.2Ah = 0.3C. To discharge at 1C you need to draw 3.2A.
Theoretically to get a 1C discharge you need a 3.2A constant current sink, but a resistor that draws ~3.2A on average is close enough. At 3.5V (expected mid-point voltage) the required resistance is 3.5V/3.2A = 1.1Ω. But getting exactly 1C isn't vitally important, so 1.0Ω or 1.2Ω would also do the job.
The resistor could dissipate up to 15W so it will get hot, and should be a wirewound type rated for 25~30W. If you can't get one that large then wire two or more smaller resistors in parallel or series to get the required values, eg. 2 x 2.2Ω 15W in parallel = 1.1Ω 30W.
You will need an ammeter which can accurately measure up to 4A with low voltage drop. Most digital multimeters can do 10A, though cheap units may have poor quality leads. Replacing the leads with your own plugs and wiring will help to ensure a good connection.
To avoid damaging the cell, Stop discharging when the voltage goes below 3V (there is negligible usable capacity remaining after this point).
To get accurate results you need to record the current at several points during the discharge (eg. one reading per minute), then integrate current x time to get capacity.
C ratings are determined by manufacturers and dictate what amperage the cell can safely and effectively be used continuously. Some specifications that are likely considered are temperature and capacity. As these limits are pushed capacity diminishes significantly.If you want to test your own 18650 batteries find a power resistor. These are generally ceramic and good at dissipating heat, not like the typical resistors you may have used. You can also connect to a heavy load if you cant find a high enough watt rated resistor. Next, youll need a good software program and an Rs232 capable ammeter capable of measuring and logging the data.You can then develop your own methods to compare capacity.
These tests should never be left unattended. You should closely monitor the temperature of the cell! In most cases, you should just refer to the specs of the cell as provided by the manufacturer.
If you really want to test their maximum discharge capabilities you will have to have an ammeter and a specially designed consumer that is adjustable.
Given that the max discharge rate can vary a lot (between 1A and even 30A+ for power tools INRs) you will have to have a pretty large interval of adjustment.
The most simple way to so is get something off the market that supports variable discharge rates up to 30A, but you could build something yourself. High power resistors and a switching system should be the minimum needed. Basically, you build big board of high power resistors or high power adjustable ones and start testing.
You can start to test with half of the current stated in the official cell specifications and go from there. After exceeding the stated current, the cell will start to get hot. When it starts to get hot it means you are at it's design limit. A temperature sensor can help with this.
So the minimum things you should have are: ammeter, high power adjustable resistor and temperature sensor. For good precision, build 2 separate circuits: one for values between 0-5A and one between 5 and 50A.