First experiences with Li-ion cells

I have been curious for a while about Lithium based rechargable batteries, and how I could get to play with them without investing too much money. Yesterday I was given three identical exhausted HP laptop batteries, marked to be 14.4V 4400 mAh, Li-ion. Not having a way to try a recharge, a disassemble was strictly necessary.

Those batteries even have 5 LEDs that show the charge level, and of course they were reported as dead. Once open I was presented a series of two elements in parallel, 4 each: 4s2p, and a lot of electronics. 

Meanwhile I had read something about these batteries at batteryuniversity dot com and I learned that:

• to protect cells from overdischarge, then internal circuitry disables the cell, resulting in 0V across the poles; the cell can be reactivated with a charge current
• Li-ion cells can be recharged with a constant voltage not higher than 4.20V, with a high current, even equal to C [cell's capacity], for the right amount of time

Each of my eight cells, left uncharged for an indefinite amount of time, measured 0V. So far so good. I threw together a 4.04V 2A voltage source and, while monitoring current (DVM in the picture) and voltage (analog voltmeter, not in the picture), I started charging each cell one-by-one for 30-45 minutes: in my opinion this is a safe time that does not pose the risk of (literally) blowing the cell. YMMV.

First cell went fine. After the initial 1.5A spike, it charged at 400 mA (and decreasing, 310mA on the DVM at picture time). After 45 minutes it had reached 3.8V and held it for hours with a slight decrease to 3.7V. Looks good.

Second cell was a surprise, since it initially behaved as the first but then went short circuit! Since it was unattended, I found a pretty warm regulator and cell when I checked in 10 minutes. Current was 3.5A. Maybe this cell was the faulty one in this battery pack?

Lesson learned: add a (resettable) fuse in line so that cells can be left unattended and, if they go short circuit, nothing blows or melts.

Third cell was better too. I will continue with the 45 minutes cycle to see if I can revive these cells. Having 3x8 = 24 potential Li-ion cells for free is interesting, but most important I can learn something new.

23cm biquad and coax

After a long a careful theoretical planning, I moved the 23cm RX-only biquad antenna in a more permanent location, not obstructing the view out of the balcony.
Since the antenna is now closer to the coax entrance into the shack, I could shorten the cable: what a better chance to take a couple of measurements?

After fixing the antenna in place I fired up SDRsharp and tuned the local 23cm beacon. Its carrier was peaking -40dB (relative). Then I cut at least 4 metres off the coax cable (unknown 75 ohm), re-soldered the TV-plug at the shack end and measured again: carrier now peaks at -35 dB (relative). That's about 5 dB S/N improvement.

Lacking proper instrumentation I cannot certify the gain is due just to the shorter coax, or to any other factor like: impedance match (remember my antenna R+jX was never measured), better coax-to-connector(s) junction, ... In any case apparently now I have 5 more dB of RX "power" for the next 23 cm event.

I am still after a simple method to remotely turn my antenna over a 90° range (max). I have few ideas but they are mechanically too complex for my time and tools.

UV-3R and external power sources

When a charged battery is inserted into an UV-3R, the radio powers up automatically. This doesn't occur when the battery is inserted, radio is OFF and the charger is plugged: the radio begins charging the battery.

The good news (for me) is that the radio (UV-3R mkI) powers up automatically also when there is no battery inserted and an external power source is supplied.

I am planning to install an UV-3R in the car to access the local UHF repeater. I don't need to change frequencies or touch the radio controls: just wear the headphone/hands-free and talk. This way I will get the radio up and running together with the car.

In order to reduce 12V of the car battery to about 4V I will use a recently acquired DC-DC switching step-down module (variable voltage, 2 amps) and throw everything into a small box under driver's or passenger's seat.

And REMEMBER: on UV-3R tip is NEGATIVE!!!

New Italian prefixes

Those of you that spend time on the air will, sooner or later, notice an increase in IUnxyz stations.
While it is not a brand new prefix, since it has been used in the past for special/contest stations, "IU" has been allocated for new callsigns once the "IZ" series runs out of letters.

Greet these HAMs with a warm welcome!

A container for 23cm biquad antenna

There is a bit of local activity on 23cm and the RTLSDR dongle allows me to receive that HAM band. An improvised indoor dipole brought in few interesting signals, increasing my interest for 1296 MHz. I have already built a biquad antenna, which needs a (cheap!) housing before being installed outside.

My biquad size is about 20x30x6 cm and I have had troubles locating suitable plastic containers in the kitchen department of local supermarkets.

During a visit to IKEA lower floor I spotted the SAMLA series of plastic transparent containers. The 11 litres one is large enough to host my biquad (38x28x14 cm) and it costs 1+1.75 = 2.75€ (lid+box).

The antenna will go on the internal side of the cover, which is easier to work on and cheaper in case the experiment fails. In that case the SAMLA container will be repurposed in the house.

RTLUSB in VirtualBox virtual machine

Months ago I was advocating the creation and distribution of a Linux virtual machine image with pre-installed software for RTLSDR work (drivers and end user applications). The release of Zadig and SDR# for MS Windows has quickly obsoleted my idea. But how about running the whole SDR thing within a virtual machine? Would the "virtual USB" software layer be fast enough to pass through RTLSDR IQ samples?

A test within the virtual machine is quickly done with the rtl_test.exe utility distributed with RelWithDebInfo.zip file (google). My setup:

• host computer i5 processor Win7 64bit
• guest computer WinXP 32bit
• R820T dongle

I have tried running rtl_test.exe at several sampling rates and always got lost bytes, even at the lowest value of 250 kSPS! And the CPU usage was not impacted at all, it remained low/idle.

My conclusion: unless you accept data loss in your RTLSDR decodes, a VirtualBox virtual machine does not guarantee enough "USB bandwidth" for successful reception within the VM itself.

SDR on a netbook

Since sdr# runs fine on the 2004 laptop (Centrino CPU), I wanted to test the RTLSDR hardware on the eeePC 901 with a dual-core Atom N270 processor. That is not an hyperfast processor, but the netbook is small and it has a solid-state drive (SSD, as opposed to "spinning" hard disk or higher capacity).

First test involved using ADSBsharp to upload data to the common hub. This piece of software runs the RTLSDR dongle at 2048 Msamples/sec, and the machine was coping with the load, delivering as high as 120 decoded packets per second.

Then I tried latest sdr# development release and, despite the additional load for the display, the eeePC 901 behaved exactly as the older laptop. At 250 ksamples/s (total bandwidth of 250 kHz) there is CPU to spare.

Sure, the screen is tiny, but for a portable operation the netbook is small and solid. Note: I forced the "Super performance" mode, which apparently consists in a 5% overclock. With a reduced sample rate the CPU can be slowed down (to be tested).