On Nov 18, 2:46*am, Grumpy The Mule wrote:
Howdy,

The saturation voltage will probably be less than a volt,
likely only a few tenths of a volt. *When the vehicle is
running the output voltage could be too high. *A transistor
that comfortably handles the ten amps is going to be something
like a 2N2955 or 2N3055 there are curves on the data sheets
that tell the story. As the current souce runs out of compliance
and the current drops, the output voltage rises because the
saturation voltage decreases.

There is an easy fix if you use a PNP pass transistor. *Clamp
the base at 13.6V plus a base emitter drop, say 0.8V, the output
can't rise higher than the float voltage.

Better yet, use a TL431 to sense the battery voltage to end the
constant current charge by reducing the bias and enter float mode.
(The TL431 is a very handy device and cheap. *Check the app notes
and I'm sure you'll find many other uses for it.) *Then either NPN
or PNP will work, choose your poison.

I modified a junky old eight amp Kmart charger like this with a
PNP device for charging the six volt battery in my tractor.

You'll need some sort of short circuit protection. *During a short
the ten amps times the supply voltage will be dissipated in the pass
device. *Could be a fuse or breaker or active protection. *I left
this out of my first attempt and the pass device didn't like being
shorted for even a moment. *Luckily I tested this before putting it
all back together rather than finding out later on in a snow storm.
The existing thermal breaker was much too slow.

LEDs would be a huge improvement.

73,
Grumpy

JIMMIE wrote in news:1fa6ec49-6eab-4cb6-8eeb-
:



I think I am going to try charging it through a 10 amp constant
current source. If it works the way I am thinking as the battery nears
full charge the pass transistor should saturate applying nearly full
generator voltage across the battery topping off the battery. Im
thinking(hoping) that when this occours the voltage drop across the
transistor may be just enough to yield nearly the ideal float voltage
for the batteries.
The batteries normally power my heater and lights for my popup camper
and a couple of radios that are mostly *used for rx. With only an
initial charge from the battey charger before leaving home I can camp
about 4 days. With some additioal charging I am hoping to extend it
another 2 oor 3 days. Since our trips usually include a few hours a
day of driving I thought I would use this oppurtunity to top off the
batteries. I have leaned against the idea of just running the engine
for that purpose. The incandescent bulbs I am using in the camper are
the main draw on the batteries perhaps going to LEDs would help.- Hide quoted text -

- Show quoted text -

Saturation voltage my oh my. A simple DC to DC convertor on the gate
of a FET of like 4 milli ohm so no more Vsat. I've done it with the
UC3906 which is designed to charge lead acid. All you need is a source
that'll will handle all that current. All the tricks I've seen done
with lead acid charging constant current will fawk up ur battery
unless you know when to shut it off like the UC3906 does.
Maybe 3 TL431's as sensors and you can do it. But the UC3906 can
sense a shorted cell and will not charge the battery if there is a
shorted cell.

73 OM
n8zu