[Milsurplus] Running mobile on lead-acid batteries

[Dave Maples]

All: I have some comments on this.  They are kind of long; read if you see
fit.

1. A normal car battery is not designed for long-term large currents.  It's
designed for short-term peaks in the hundreds of amperes.  A deep-cycle
battery is designed for the kind of service Hue is describing.  Both types
of batteries fall under the heading of valve-regulated lead-acid (VRLA)
batteries, if they are typical modern batteries.  Let's assume we substitute
for the car battery a 170 ampere-hour (AH) VRLA battery similar to what we
use in our cell sites (I work for one of the cellular carriers).

2. VRLA batteries are considered to be discharged when the per-cell voltage
falls below 1.75 volts per cell, or (1.75*6)=10.5 VDC for a typical 12V
battery.  If you run them below that voltage, you will definitely remove
between some and a lot of the long-term life from them; you can also ruin
them on the first such discharge.  Our cell sites disconnect the load from
the batteries at the 1.75 V/cell point; our adjustable powerplants are
actually set to disconnect at about 1.83 V/cell.  The extra voltage above
the minimum increases the long-term life by a good bit (which, if you have
hundreds of thousands of batteries to maintain, becomes very important).

3. That 170-AH rating assumes that the battery will be discharged in 8 hours
to 1.75V/cell.  To hit the 8-hour figure the load current has to be 170/8 or
about 21 amperes.  If the load draws more current that that which the 8-hour
capacity is based on (~21 amperes in this example), the battery will provide
less total power.  Hue's load will draw 55A.  Using that figure and matching
it against the 170-AH rating on the battery, a straight-line discharge of 55
amperes would theoretically discharge the battery in just over 3 hours (3 hr
* 55 A = 165 AH).  In reality, however, we'd have to derate that by probably
25% or so due to the higher discharge current, so we could probably count on
getting a little over 2 hours from the 170-AH battery.

4. The discharge current is going to vary in unusual ways as the battery
voltage falls.  As the battery voltage falls, the filament current will not
necessarily fall linearly (because as the tubes cool their filament
resistance will fall).  If we are using dynamotors to run the transmitter,
they are not going to decrease current linearly either.  It's likely but not
certain that as the input voltage falls the overall current drain is
probably going to go UP rather than down.  This will cause the battery to
discharge even faster, making it less likely that we will get much beyond
the 2-hour rating with this battery (might not even make it that far, if the
dynamotors draw a lot of excess current at lower voltages).

It's an intriguing problem.

Dave WB4FUR

-----Original Message-----
From: milsurplus-bounces at mailman.qth.net
[mailto:milsurplus-bounces at mailman.qth.net]On Behalf Of Hue Miller
Sent: Wednesday, April 25, 2007 12:48 AM
To: Milsurplus; 'Glowbug list'
Subject: [Milsurplus] Running mobile on lead-acid batteries


Let's imagine a liaison transmitter, BC-375 or ART-13, running off
a car battery. A good sized car battery seems to have about 150 AH.
I am wondering how long this radio would run key-down from the
battery. This drain i am thinking about is about 50-60 amps. Of course,
the radio will not run down to zero volts; i think it would maybe cut out
around somewhere in 7-9 volts. While the input power to the final
would tend to go down as the square of the ratio to full high B+, when
the heater voltage is varied downward at the same time, the input
power will fall off even more rapidly. Anyone have experience with
such a operating situation, or does a military manual you know of,
address such situation? Appreciate any input!  Tnx-  Hue Miller
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