Aligator clips fail

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”.

Aligator clip cables
Aligator clip cables

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).

Boost module without load
Boost module without load

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:

Boost module with load
Boost module with load

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!

Module voltage
Module voltage

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.

Red lead voltage drop
Red lead voltage drop
Black lead voltage drop
Black lead voltage drop

What have I learned today? Always check/verify which cheap Chinese manufacturer to blame, so you don’t blame the wrong one!

Battery testing #1 – TrustFire 2400mAh

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
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.

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)

Copying a file increasing in size

This is a quick hack, for when I used udpxrec (part of udpxy) to record an IPTV stream to an mpeg file, but instead of saving it to a network share (to watch it with some delay on my OpenElec box), i saved it to a local drive (by mistake). So here was a file, gradually increasing in size, which i wanted on my network drive (to start watching before the actual show/recording is finished). cp of course wont work, since it stops when it detects the end of file (does not detect new data being added, and wait for it), so you need to use something else.

When you think of a file with data being appended at the end, the first thought is “tail -f” (-f = follow and print the data being appended). Since tail only prints the last few lines (or bytes), you need to set it to output from the beginning with the “-c +0” (output bytes starting at the zero-th byte). I also pipe it through pv to follow the progress and copy rate.

So the command is:

tail -f -c +0 /path/source.mpg | pv > /destinationpath/destination.mpg

 

Playing with a cheap ($10) logic analyzer

A long time ago I ordered (eBay) and recieved a very cheap logic analyzer (it came out to be a Saleae clone). I havent really played with it enough, but I decided to test it.

It connects to PC via mini(!)-USB cable, and has 8 (+2 gnd) pins on the opposite end. The label marks it as a 24MHz 8 channel analyzer. I’ve connected it to a USB->RS232 adapter (from an old mini/nano Arduino kit).

Logic connection

I expected it needed some weird Chinese software, but it worked with original software from Saleae (which is nice).

Logic1
Screenshot with data already captured

Capturing data is also very easy – the only two options are sample rate and data capture duration.

Data capture options
Data capture options

After the capture timer is over, the program shows logical levels on a time graph (first screenshot) – it does not seem to autodetect the protocol used. After selecting the protocol and protocol options (mainly the correct pin/s) it also decodes the actual data (“test123” in this case).

Supported protocol decoders
Supported protocol decoders
Async serial options
Async serial options

As it can be seen on the first screenshot, it correctly decoded the data sent via the RS232 adapter.

For $10, it’s a great toy. It probably has some timing/sync issues, and the casing isn’t that great, but for that little money, who cares!