Another cheap gadget: battery capacity meter

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
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
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
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
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)

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