Recently I’ve found a cheap USB boost converter (small device that turns a lower voltage (~1V-5V) source to a regulated 5V output – so you can charge your USB devices from a range of batteries with different (lower) voltages).
I’ve also received a pack of cables, with aligator clips on both ends, some time ago, so I’ve decided to use them instead of soldering/”improvising”.
For input power, I’ve used a TrustFire 18650 battery in a holder, and the aligator clip cables to connect the boost converter. On the output, I’ve also connected a USB power meter to measure the output voltage and current (I wasn’t sure if it was the 500mA or 1A module).
The voltage was 4.94 volts (somewhat low, but with such a small load, it could be the voltage regulator issue), and zero exit current. Everything is OK!
I’ve connected my powerbank to the USB port (to charge it, and check the current), and the situation got bad:
Current was around 120mA (well below useful), and the output voltage was 4.68V (also too low). So ok, it’s a <$1 module, from eBay, probably no quality control whatsoever.. But just in case, let’s check the battery. I’ve connected a small 7segment LED display voltmeter to the boost module (under the clips), and nothing. The voltmeter requires ~3V to turn light up, so either it’s not working at all, or the voltage is too low. So I’ve used a proper multimeter, and measured the voltage of only 0.588 volts at the boost module! The batteries have protection circuits, so this isn’t a battery issue, since protection steps in at around 2.5 volts and shuts down the power completly. Voltage at the battery was around 3.8V when under load. Then I’ve touched the aligator clip cable with my hand (by accident), and it was warm to the touch – so here’s the problem!
Next thing, I’ve measured the voltage drop on the cables, and was surprised by the results (well, not really that much, since the cables heating up quite a bit). The voltage drop on the red cable was 1.657 volts and 1.378 volts on the black cable. So, on a simple circuit with ~200mA of current, i was losing around 3 volts (~80% of the voltage) on the connecting cables.
What have I learned today? Always check/verify which cheap Chinese manufacturer to blame, so you don’t blame the wrong one!