After setting up my battery capacity tester, I’ve decided to do a test on a TrustFire 18650 lithium battery I’ve bought ~1.5 year ago on DealExtreme (url). The battery specs (and label) say it’s a 2400mAh battery, but with cheap chinese batteries, it’s better to measure then believe!
Trustfire 2400mAh battery
The battery i have is not new, but has seen very little use (probably about 4-5 charge/discharge cycles), and has been fully charged (not good, I know) in a small battery box at room temperature for most of that time, so i was expecting some loss in capacity.
I’ve charged the battery to full using a XTAR MP15 battery charger (USB power, very slow, but seems to work well), and started the discharge measurement immediately after the LED on the charger turned green (the battery should be at 95%+ capacity then, and the charger stays in CV mode for some time after that).
And the results? With a cutoff voltage of 3.0 volts (the protection circuit should kick in at about 2.5 volts, but I wasn’t willing to risk overdischarging it), the measured battery capacity was 2.271mAh! Which is great for that price!
So now, on my TODO list: testing AA/AAA batteries, brand name vs. much cheaper, store branded ones.
Recently, I’ve been browsing ebay for random electronics, and I’ve found a cheap ($4.33) battery capacity meter. Since it was below my $5 “I wont impulse-buy it, I’ll think about it”, i immediately ordered it.
It came today, packed in bubble wrap, and contained a (micro) USB powered meter and a 5W 7.5Ohm load/resistor (measured 7.8Ohm at room temperature). On the left side are four terminals, outer two for the resistor and inner two for connecting the battery. In the middle is 4-digit 7-segment display, cutoff voltage adjustment buttons and three display leds, and on the right side is a micro USB connector, suppling power to the circuit. The microcontroller is probably hidden under the LED display.
The meter is rated for 1.5-12V with a maximum load of 3.1A, so I’ve decided to try it out with a 18650 battery which should power a current of about 0.5A through the provided load. I’v also connected the load and the USB power connector. After powering on it showed the voltage of about 3.79V on the LED display.
Voltage display
Using the (+) and (-) keys, you can adjust the cutoff voltage – this is the voltage when the battery is considered “empty”. Usually, most lithium cells have extra protection circuits, which cut off the power, when the cell voltage is too low, to prevent over-discharging – so you have to consider that when setting the voltage limit.
Setting cutoff voltage
Pressing OK starts the discharge process. The display loops between capacity (Ah), current (A) and voltage (V), and shows the current values.
Display while discharging
I’ve verified the values with my multimeter – the voltage was the same on both meters in all three digits, and the current differed only slightly with the last digit (<1mA difference compared to my multimeter).
Warning: the resistor gets HOT (>100°C). I’ll probably replace it with something larger or add some heatsinks to it.
I’ve tweaked the cutoff voltage (so I could see what happens when it’s finished measuring), and the display started flashing rapidly and stayed in the capacity display mode.
Finished measuring
The meter also shows a few error codes, if you mess something up (set up cutoff voltage below the current battery voltage, etc.):
Err1: the battery voltage higher than 15V
Err2: the battery voltage is lower than the stop voltage
Err3: the battery is unable to withstand the load discharge current
Err4: the current is too large (current is more than 3.1A)
Considering the price, the accuracy (within reason of course, my multimeter hasn’t been calibrated in sime time too), and the overinflation of battery capacities in the specifications (eBay sellers, I’m looking at you!), I consider it a nice gadget to have, to test your purchases, before relying on the written spec (eg. “20000mAh” 18650 cell)
