charge controller for 24VDC generator?

 

I want to use a 24VDC NATO field generator to charge a battery bank that's
also connected to a Xantrex SW4024+.

The idea is that when grid power is not present, the generator will charge
the battery bank at the same time the Xantrex is drawing on them.

The Xantrex is capable of drawing 200 amps continuous, 350 amps surge.
The generator can supply up to about 70 amps at 28 VDC, adustable 27-31.

As the Xantrex draws down the battery bank, the current demand to the
generator will increase, until the breaker trips. It would be good to have a
charge controller in between the generator and battery bank that limits
current to less than the genny capacity.

Investigation of DC charge controllers, and a discussion with a guy at
Morningstar, suggest that there is no DC charge controller that also has a
current limiting function. This is because the common use of these products
is in conjunction with solar sources that are intrinsically current limited.

There are  some MPPT controllers that limit current. However, there is no
advice available with respect to use with generators. Among the concerns
are:
1. Brush noise might spook the MPPT algorithm.
2. Interaction between the MPPT algorithm and generator load regulation.

Suggestions?

 

I think the answer will be in your generator regulator.  If this circuit is
accessible, you will likely be able to control your exciter current (a few
amps) which in turn will control the generator output (lots of amps).
However, to limit current will require you to regulate down to a much lower
voltage (perhaps down to 20.0V) than the float voltage (~27.0V).  The idea
is to let the battery pick up the excess current while the generator remains
within its capabilities.

Some DC generators control their output by varying the speed of the engine.

Good luck.

 



remains

engine.
Not this one. It is a diesel with a mechanical governor.
Your solution is what I'm stuck with, which is to manually regulate the
generator, using the voltage pot. However, this just postpones the problem
to the point where the battery bank runs down. At the point where the
battery bank discharges to the set voltage, the generator current will
suddenly zoom upwards. The real solution would be an automatic current
regulator.

20 volts is not a good idea, since that corresponds to a dead battery. The
purpose of this genny is to keep it charged.

 


Robert Morein wrote:

Yeah, I've heard of people doing that ;-).


Yep. Personally, I'd like to see you draw 200 amps continuous out of
that 4024. That's a whole lot of juice. Likely, you've got a 250 amp
breaker in your DC disconnect. She'd pop under a continuous load like
that I'd bet, after a few hours, not to mention that the inverter would
probably melt. Yeah, yeah, I know, not on paper. But ten years as a
millwright in a chemical plant, building MCC rooms, trust me, keep a
handful of spare fuses around if you're going to draw down on it like
that.


If it was one of my clients with this problem, I'd say two things:
1) you're using too much electricity.
2) get a bigger battery bank.

Myself, I don't see anything wrong with running the DC generator, set
for, say, 29 volts DC, and just letting it do what it does. I would
imagine that once the battery voltage hit 29 volts, it'd just be
spinning its wheels. Can't see how that would hurt, seeing is that's
what it was designed to do...

DJ

 



The DC disconnect is 400 amps, Xantrex specifies dual 4/0 power leads.

 She'd pop under a continuous load like

I go by the Xantrex time/load curves. 8500 watts for 5 seconds, with a
smooth time derating curve leading to automatic shutdown if the continuous
load exceeds 4000 watts.


 Yeah, yeah, I know, not on paper. But ten years as a

It's already 36 kwh.


If this was a flooded electrolyte battery bank, you'd be right. However, it
is a Hawker AGM Powersafe. They claim a 20 year lifetime, but regardless of
whether that's realistic, they have an extremely stringent charging
requirement: constant voltage only, which means that bulk=float, to within a
temperature compensated 0.1 volts of center value. Provided that is met, the
battery will efficiently recombine the electrolyzed water. The current setup
does this.I can't set the generator point above the float value, or the
battery will rapidly lose water.