DC Generators

mcnutt13579 · February 2015

I operate a generator service. Almost everybody who has off grid solar around here has a supplemental generator and most are sized that it gets some good run time. Outback FX systems are by far the most popular here. I am not an expert on them because we have some good solar electric guys who take care of that end. But the customers and solar guys turn to me for generator advice.

One thing that has intrigued me are the DC output generators from Kohler and Polar Power. They dump DC power directly to the batteries. I believe that when the FX commands generator start it expects to see AC voltage on the AC in terminals. Can this be changed to work with DC input to the batteries directly?

Also, I recently learned that the Flex charge controller has aux terminals that can be used for generator. Maybe this is an option?

CALLD · February 2015

I don't have a back-up generator because I have access to a (mostly reliable) grid, but seeing everyone else on here talk about generators has always interested me.

I cannot see how a "DC" generator would actually work unless it had a built-in charge controller. For a so-called DC generator to be of much use it would have to have all the options built into it that a solar charge controller has so that it could be used to charge any type of off-grid battery correctly. Because of that I would imagine most off gridders have a normal AC generator that charges the batteries via an inverter with a built in charger or via a seperate battery charger. If I ever did get hold of a DC generator I would perhaps try to run it through my MPPT charger controller at night only. My charger controller has a function that prevents it from passing more than it's rated current so an over-sized generator would not harm it provided the DC voltage was not more than 150volts.

jonr · February 2015

Unfortunately, even a 120V AC generator produces voltage that peaks around 170 volts. But I agree that rectified AC fed into the right MPPT charge controller might work.

Vic · February 2015

Hi mcnutt..,

Many of the DC Gensets appear to have battery voltage sensing, for auto-start. Some may have some output voltage adjustability. Many seem to be pre-set to min-Bulk voltages.
http://www.kohlerpower.com/onlinecatalog/pdf/g4213.pdf

Yes the better Charge Controllers like the OB MX-60 and FMs, and the Classic CC from MidNite all have Aux outputs that can be used to start a genset. These will lack the niceties of limiting OverCrank, etc.

In the past have used a Transformer/Rectifier/Cap bank to drive a spare MPPT CC as a DC charger. However, designers of main-stream MPPT CCs council caotion, as PV modules are inherently current-limited, and external power supplies are often not limited. It was recommended that a resistor be placed in series with the DC + input to the CC, to allow proper current-limiting by the CC.

For a basic DC supply, a transformer at the AC input is a great safety addition, and allows choosing the Vin to the CC.

FWIW, Vic

gww1 · February 2015

I do say for myself that if you had enough solar normally and where just going to use a generator during bulk, dc would be much easier. If I had a dc generator that 60 volts is all it put out, I could leave it on till it run out of gas without worring too much about overcharging my battery. I wish I had one. I would hook it strait to the battery, not worry about a transfer switch or charge regulation or anything. what could be easier for something that is only used during extended cloudy periods.
gww

DC Generators

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