If your fully charged batteries won't supply enough current you could
I put ammeters on my chargers and raise the voltage until I see the
My small MIG welder outputs (IIRC) 18V to 24V DC at no load. It might
jump start but it wouldn't charge. That takes around 28V for good 24V
batteries, over 30V for old ones. The simpler automatic chargers won't
go that high. If they did they'd ruin new batteries, thus they let old
but salvageable batteries appear dead.
I built a 12V/24V battery charger from a 13A Powerstat, 100A bridge
rectifier and the transformer out of a "50A" buzz box welder from a
yard sale. It will continuously (1/2 hour) deliver about half the 20%
duty cycle current rating, with the hottest detectable spot on the
transformer not exceeding 60C, without the box cover and fan.
It wasn't meant to be a jumpstarter, rather a 12V / 24V fast charger
and general purpose tinkering power supply for projects that exceed my
30V 5A lab supply's limits.
At 25A as the batteries charge their voltage rises without much
decrease in current, so it won't taper charge like a purpose-designed
battery charger unless I watch it and turn down the Powerstat, or run
it through a solar panel controller that limits battery charging
I've ordered this to make it a more suitable voltage- and
current-controlled lab supply:
Batteries will still get the (protected) rectifier output directly,
avoiding problems with reverse polarity and overvoltage, such as if
the AC power fails with a battery on the output.
Between the inherent reactance of the welder transformer and the 0.2
Farad output cap the ripple is only about a Volt at 20A, however the
voltage droops too much as the current increases, as expected from a
"constant current" welder transformer.
The bottom line is that you can build a powerful battery charger from
an arc welder transformer but you need an autotransformer to control
Hmm ... most Army trucks have enough room so you could add a
second battery. Use two 12V ones in series. This will probably get
you higher current during starting, too. Just needs a bit of welding
and bolting to make an extra battery shelf.
My MGAs (both 1500 (1956 I think) and 1600 MK II (1962 I think))
used two 6V batteries -- one on each side of the driveshaft mounted
behind the driver's and passenger's seats, and easily accessed via a
removable panel with 1/4 turn DZeuss fasteners. The two were in series
(of course, to get 12V from 6V batteries) with a jumper arching over the
driveshaft to connect the positive of one to the negative of the
Anyway -- this with two 12V batteries should keep you going
until 24V batteries become more common and more affordable. I do hear
that this is the way things are going.)
At work a number of years ago, we had a 24V battery with a
molded-in carrying strap on one of the workbenches, and someone
"borrowed" it to start his car. He was amazed at how fast it started,
and had not realized that the battery was 24V instead of 12V. Good
thing that he did not turn on headlights or the radio during the
In series for 48 volt operation, or in parallel (I would guess
with diodes to steer the charging and discharge currents so one does not
draw excessive current from the other. Or perhaps dual electrical
systems, on for normal operation, and one to run com gear and the like?
O.K. Since my experience with a military 24V battery, I assumed
that most were so equipped, rather than two 12V ones in series.
An interesting read, though somewhat dated, and with a fairly
narrow application focus (fire-fighting trucks).
Among other things, it does not mention the newer alternators
(which were beginning to appear) which have solid-state regulators built
inside the alternator -- which makes the switching a bit simpler for a
dual system (24V for the original stuff and a separate 12V for the added
And another approach would be a 24V to 12V switching regulator
these days for the 12V loads.
But -- I gathered that the OP had a truck with a 24V battery
instead of two 12V ones -- thus his difficulty in finding affordable
I was assigned an M 109 type Deuce-N-A-Half and learned to do the
routine maintenance on it but I can't visualize the battery
configuration beyond it being 24V. I think someone said it could take
two commercial 12V batteries if necessary, like if we bought one
surplus or Supply was out of batteries. That was common in Europe when
everything was going to Vietnam. J. C. Whitney kept our Jeeps running,
often with 'performance' parts which only made them deadlier on the
winding back roads that BMWs were created for. The Jeep we had
permanently towed a water tank trailer to keep the leaky radiator
topped up. After driving the truck flat-out at 55 MPH on the German
Autobahns I lost interest in owning one.
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