31 March 2009

Lazy battery pack, an update

Following the previous post, I have done some measurements.

First of all, the DC cable to FT817 does not show unusual impedance. Not the culprit.

Then, since my analog voltage meter is out of service, I have tried a different approach: build a simple XTAL oscillator, supply it with the AA cell and listen to frequency change at different AA cell loads.

Being the human ear faster than any analog instrument, I would be able to detect quick voltage changes.

First doubt: would a standard Colpitts 2N2222A XTAL oscillator work at 1.2V? Yes, it does! As biasing resistors I used 1k5 on the emitter to ground and 10k Vcc to base. Capacitive network in the ballpark of 50-100pF each for a 14 MHz XTAL. Cool! I have just built a XTAL tester that runs from 1.2V up to ... to... 15V and more!

Second doubt: does frequency really change with Vcc variations? And how much? Well, yes, it does! I have not done precise measurements yet, and YMMV, but dropping Vcc from 1.2V to 0.8V the 14 MHz beat on my HF receiver moves 1kHz or so. What?! It still oscillates down to 0.8V! [insert and happy face here]

The resulting circuit has, on one side of the AA cell the oscillator. On the other side I put a selectable resistive load through a normally open pushbutton. The junkbox provided one 1.2ohm and one 0.47ohm 2W resistors, enough for a simple load.

I also hooked up the DVM, and measured across the AA cell (rounded):
  • no load: 1.3V
  • 1.2R load: 1.1V
  • 0.47R load: 0.8V
The beat on the HF receiver confirmed a constant voltage.

With some Ohm's law you can work out that each of these 8 cells has developed an internal series resistance of ca. 0.3ohm. Since my battery pack has 8 cells in series, the pack has an equivalent series resistance of 8 times 0.3 = 2.4ohm.
The FT817 at 5W keydown draws some 2A ... V = RI ... 2.4*2 = 4.8V of drop within the battery pack: no wonder the radio shuts down!

I did a recharge and the situation improved a little, but the pack still does not sustain FT817@5W.

I tried different AA NiMH cells and they do show a lower resistance/lower voltage drop. Strange enough I did not use or abuse that pack. Nevermind, I have learnt few things (highlighted above).