On 1/4/2011 1:09 PM, PeterD wrote:
On Tue, 04 Jan 2011 12:09:44 -0700, D Yuniskis
wrote:

On 1/4/2011 11:08 AM, klem kedidelhopper wrote:
I have a 24 volt military truck that I use for snow plowing. In the
past I've had trouble finding 6000 series 24 volt sealed beam lamps
for the plow frame.. And when I did I have found them to be very
expensive. So I came up with a circuit using a DPDT switch in which
both the high and low circuit lamp filaments are wired in series when
power is applied. I was very careful to initially select two lamps
that drew exactly the same current when tested on my bench power
supply. The arrangement has worked flawlessly for the past few years.
The other night I had occaision to look at the lights from outside the
truck while it was running. Charging voltage on a 24 volt system can
run as high as 28 volts on a typical 24 volt system. Although I didn't
try to measure the voltage accross each lamp, I noticed that one lamp
is slightly brighter than the other on the low circuit. I'm assuming
that filament is getting weaker and thinner and will likely fail
shortly. And of course when that happens I'll lose the low circuit. I
can rematch two more lamps but before I do the thought occurred to me
if there was any practical easy way the equalise the voltage accross
both lamps in order to compensate for any minute differences in
filament resistance. I think that the total current, (two lamps in
series) is about 5.0 amps. Thanks for any suggestions. Lenny

Is the *battery* on said vehicle a *single* "24V" battery?
I.e., if, instead, it is two 12V batteries in series
(which is what the 24V battery would look like internally),
then why not run another conductor to each "lamp assembly"
(3 wires instead of 2) and run each filament off of its
intended 12V?

One should never tap off the bottom battery in a two battery 24V
system, it will result in improper charging of the batteries.

That depends on the actual charger configuration -- hence my
asking if it was a 24V monoblock or a pair of 12V monoblocks.

Given that this is the *military*, I am assuming that they
have deep enough pockets to create their own standards
(e.g., 400Hz power). As such, if they opted for two 12V
monoblocks in the vehicle, I would assume they would have
also designed a charging system that *treated* them as
two 12V monoblocks and not a "24V battery" -- since the
added cost of charging them as two monoblocks would easily
be outweighed by the increased "battery" life that obtains
from that since imbalances between cells are a (the?) primary
cause of battery failure (presumably the military wouldn't
want to have to lug around spare batteries if they could make
the ones they had last longer -- esp. in a war zone). Cells
will *try* to equalize with overcharging in short strings
but, as the string gets longer, battery life goes to hell
(since the "good" cells get overstressed trying to "coax"
the bad ones back in line; and, as a cell gets bad, it gets
bad-er!)

But, I've never peeked under the hood of a military vehicle
so can't tell if:
- 1 monoblock or two
- three terminals to the battery or just two
- is the *entire* system running at 24V (or just parts that
draw higher currents)

Two solutions are either do what the OP does (two in series) or use a
voltage regulator capable of handling the current (not an efficient
solution). You can get 24 volt headlights on eBay if you want them, as
well... There are other solutions too, such as getting housings that
handle replaceable bulbs, which may be available cheaper as a 24 volt
unit. (Not sure on costs there).

Just go series, don't worry about minor light output differences, but
realize that if one goes out, both will go dark! Always carry two
spares with you.