[usa2003ardf] Batteries

Fri Aug 8 15:46:15 CDT 2003

Concerning batteries and heavy boxes, someone mentioned using alkaline 
batteries in their boxes, so I made this little trade study using data from 
the Energizer website (http://data.energizer.com/).  I couldn't quickly find 
a manufacturer's site for lead-acid weights, but I found a table for some 
generic sealed lead acid batteries.

Here's the data  (copy and paste into an editor, then change to courier 
type...):

Battery          Cap Ah    V     No.  W-hrs   grams/   grams/  Watt/hrs

                                12V           cell      12V     per kg

Gel Cell         17       12     1    204.0   5900    5900      35

Gel Cell          7       12     1     84.0   2600    2600      32

Gel Cell          4       12     1     48.0   1800    1800      27

Gel Cell          2       12     1     24.0    900     900      27

Energizer e2 D   18.9      1.5   8    226.8    141.9  1135.2   200

Eveready Alk D   17        1.5   8    204.0    141.9  1135.2   180

Energizer e2 AA   3.135    1.5   8     37.6     23     184     204

Eveready Alk AA   2.707    1.5   8     32.5     23     184     177

Energizer e2 C    9.185    1.5   8    110.2     66.2   529.6   208

Eveready Alk C    7.935    1.5   8     95.2     66.2   529.6   180

I was really surprised that the D-size Alkalines had so much Capacity.

A few comments.  I settled on Watt-hours because I figured it would provide 
a convenient figure-of-merit:  If you know your transmitter power in Watts, 
you can multiply a factor for conversion loss (I have no idea what that 
factor should be), and then the duty cycle (usually 1/5), and then the 
number of hours of operation you need.

Say you've got a 2 W transmitter and you want it to run for 6 hours:

2W*1/5 (duty cycle)* 6 hours= 12/5 = 2.4 W-hrs.

Apply some factor for conversion loss: 2.4w-hrs times 2 = 4.8 W-hrs.

Can it possibly be that small?  Maybe it would be better to measure the amp 
draw and use Amp-hours.  Does anyone have some real-world data?

I haven't looked at the economics of using throw-away batteries, but my 
limited experience with lead-acid gel cells is that they're a hassle.  On 
the other hand, 8 Ds or Cs at Walmart is probably 10 bucks or so, and 8 AAs 
about $5 or 6.  So five transmitters worth would be $50 for Ds and Cs and 
$25-30 for AAs.  That could really add up over time.

Would it make sense to have two or three of these for the more remote 
transmitters, and use more economical, but heavier lead-acids at the 
easy-to-reach transmitters?

To reduce the weight, look at the whole system and try to improve 
everything.  For batteries, you could reduce the power requirement (lower 
power out) or reduce the voltage required by both the radio and the 
controller (or use separate batteries for the controller, if it needs more 
voltage than the radio).  Embedded Research has/had a $20 DC-to-DC converter 
that makes 12v from as little as 2V: http://www.embres.com/ (click eps-1 
link). It looks like they have chip availability problems.

This might also be a reason to not use MCW on FM.  You could reduce the 
weight of the batteries because a keyed carrier uses less power (if it 
actually does...).  MOE is 25 dits long, and the on time is 16 dits, so 
that's 64% compared to 100% for MCW.  Not bad...

I saw a plastic surplus ammo container recently, but I'm not sure it wasn't 
heavier than a .30 cal steel container.  Can anyone think of an alternate 
container that is suitable?  4-6 inch PVC?  The lightest watertight 
enclosure I can think of right now is a clear plastic container like JIF 
peanut butter comes in.  I'd probably built everything onto the lid so the 
guts would come out.  This might be a little too fragile.

Jerry, that sounds like a great device.  Thinking purely about weight for a 
moment, I recommend you look at the power requirements and at the best 
battery for the job, either rechargeable or throw-away, and look at a design 
with only one band transmit and what batteries and enclosure that takes, and 
at a design with two bands and batteries to match.  What is the penalty of 
having two bands?  Having two transmitters does seem to make a lot of sense. 
  If the battery penalty were too much and you chose the smaller 
one-transmit-at-a-time option, you could still have an external battery for 
when you wanted two band transmit.

Apparently the best current rechargeable is lithium-ion.  Solar Car 
competitions consider these equivalent batteries:

165 kg of sealed Pb-acid
100 kg of NiCad
60 kg of NiMH
30 kg of Li Ion
30 kg of Li Ion Polymer/Li Ion Alloy (polymer)

(from http://www.formulasun.org/asc/tech/asc2003regs03-03-13.pdf)  I'm just 
doing math in my head here, but I think the alkalines would be beat Li Ion 
(if you don't mind the one-use thing).

Of course, it's probably a bad idea to pack it into too small a box, because 
then it's harder to modify and it becomes its own problem.  A little space 
might be a smart design choice.

Matthew Robbins
AA9YH

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