Adjustable voltage booster?

[Glenn Ashmore]

I have lucked up on a Ecotech high output emergency vehicle alternator for
the boat project. This hyper expensive alternator puts out full output at
very low RPM which makes it very attractive for charging my bank of four L16
batteries. The problem is it is designed to output a constant 14.2V to
power all the lights and equipment a big fire truck needs. That is fine for
bulk and absorption charging but can fry the batteries once they get closer
to full charge. This alternator is rather unique. It has one section that
is permanent magnets and a second that is a field coil. When the magnets
can't supply the required current the field coil kicks in. When the magnets
output to much current the field coil is reversed to reduce output.
Obviously I can't use a normal external "smart" regulator so I need to find
another way to reduce the output voltage.

It does have a voltage sense line that goes to the battery which with some
experimentation I found that if you increase the voltage on that line the
alternator is fooled into reducing the output by about the same amount.

I need a circuit that I can switch into the sense line to boost the voltage
by about 1V so the alternator will drop down to a safe 13.2V when I am
motoring for a long time.

Any suggestions?.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

 

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I have lucked up on a Ecotech high output emergency vehicle alternator for
the boat project. This hyper expensive alternator puts out full output at
very low RPM which makes it very attractive for charging my bank of four L16
batteries. The problem is it is designed to output a constant 14.2V to
power all the lights and equipment a big fire truck needs. That is fine for
bulk and absorption charging but can fry the batteries once they get closer
to full charge. This alternator is rather unique. It has one section that
is permanent magnets and a second that is a field coil. When the magnets
can't supply the required current the field coil kicks in. When the magnets
output to much current the field coil is reversed to reduce output.
Obviously I can't use a normal external "smart" regulator so I need to find
another way to reduce the output voltage.

It does have a voltage sense line that goes to the battery which with some
experimentation I found that if you increase the voltage on that line the
alternator is fooled into reducing the output by about the same amount.

I need a circuit that I can switch into the sense line to boost the voltage
by about 1V so the alternator will drop down to a safe 13.2V when I am
motoring for a long time.

Any suggestions?.

It would take some research but there are lots of circuits for
doubling a DC voltage. Most of the simpler one take a square wave
oscillator like a 555 and just hang a doubler and tilter on the
output. Not good for lots of power but you probably don't need a lot
to massage the alternator's regulator's sense line

Once you have a reasonably stable 24 VDC (doesn't need regulation) a
simple comparator comparing the battery against a stable reference
voltage which then outputs a higher than 14 volts to the sense line.
The comparator would become the regulator for all purposes.

Simple in concept and gets around the problem of creating a stable
isolated 1 volt you would tack onto the battery/sense line.

The only thing is you'd need to know the current requirement of the
sense line. Since this sounds like a fairly sophisticated alternator,
the controls are probably all electronic and low current.

 

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Rethinking my earlier response . . . There's no reason the 1 volt
really has to be stable. If a DC/DC converter derives its power from
the battery its own output will track the battery voltage.

If you used a simple two transistor push-pull oscillator on a ferrite
core you could just wind it for the V out you need - or a little
higher so you have some adjustment range, and float that on top of the
battery voltage