On Thu, 20 Jun 2013 18:45:20 -0700, wrote:

On Fri, 21 Jun 2013 00:01:26 +0100, "Arfa Daily"
wrote:



"Pat" wrote in message
. ..
On Thu, 20 Jun 2013 02:12:21 +0100, "Arfa Daily"
wrote:


If you really *don't* want to
use a single battery solution, then the other simple way to do it, given
that you've now told us that the nav lights are LEDs with an appropriate
internal resistor for 12 volt operation, is to hang the lights across the
full 48 volts, but quadruple the series resistor. Or if you can't easily
get
to the resistor to replace it, add an external one of three times the
value
in series with each light.

Arfa

Interesting idea. I hadn't thought of that. You are correct that I
can't easily get to the internal resistors, but adding external
resistors would be easy enough. However, If I add the appropriate
external resistors to drop the additional 36 volts, I will be creating
a lot of wasted heat. The nav light resistors would be dissipating 2
watts each while the anchor light R would be dissipating over 6 watts.
The spot light R would be dissipating 62 watts! I think I'll stick
with the (switching type) dc-dc converter. Your comments about added
complexity are well taken, though.

Pat


2 Watts is nothing. A 4 or 5 watt wirewound resistor would barely get warm
at 2 watts dissipation, and could be easily encased in a bit of heatshrink
tube in line with the feed cable. Similarly, you could use three equal value
resistors of 4 watts each, all in series and again in heatshrink tubing, for
the higher demand anchor light. With a couple of watts being dissipated in
each resistor, again there would be no issue with excess heat. The 'parking'
spotlight is in use for only a short time, and I would not even bother
trying to do anything with that one. I would just run that from the battery
at the bottom of the stack for the relatively brief period that it was
needed. Try as I might, I really can't come up with a 'practical' reason not
to do that. Even with it pulling a couple of amps, it is not going to have
any significant effect on the performance of the battery stack overall, or
drain that bottom battery by any amount that wouldn't be equivalent to
acceptable variations in battery performance anyway. What is the capacity of
these batteries ? 100 AH apiece ? More ? Even running that light for an hour
would not represent consuming more than say 1.5% of the battery's fully
charged capacity, and if the system cannot cope with that, then I would be
worried anyway, as once those batteries are more than about six months old,
the variation in their capacities - and thus their potential to be
'unbalanced' - will probably exceed that figure anyway ...

Arfa
If it was me I'd try to figure out a way to place the resistors
someplace where any heat they generate could keep something dry. I
wonder, do DC-DC converters emit much RF? I'm really surprised that
the original poster doesn't want to go with the most simple and robust
solution. Especially on a boat. And it would be much easier to carry
and wire in spare resistors than a spare DC-DC converter.
ERS

This is my last post on this subject as we have drifted off topic. My
original post was asking if the experience I had with my cheap $25
dc-dc converter (already purchased) was normal. That question was
very quickly answered as "yes". We have since drifted to how easy it
would be to repair the system out on the water in the dark. In that
situation, I would just jumper the 12 V lights to one of the traction
batteries as Arfa suggested. By the way, the boat is used on a small
inland lake and I could easily return to the dock without any lights
at all by carefully avoiding the one or two fishermen out on the lake
at night. Thanks to everyone who responded. ****End of Thread***