[RE-wrenches] Cummins DC generator

[R Ray Walters]

The advantage of a DC generator is the generator doesn't have to be sized to handle the AC loads, plus surge, plus charging all at once.
Also, you have isolation for sensitive electronics that might not like a generator's waveform as it flattens and its voltage drops under a surge load.
Third, it is more fuel efficient, both because it doesn't go through as many voltage conversions, but also because the generator can be sized smaller.
Definitely the good old AC gen set into a regular inverter/ charger is easier and lower cost for most applications, so we're just kicking the DC generator idea around a bit. 

Actually I probably wouldn't even worry about 3 stage charging with the genny, as I never try to fully charge a battery that way; its way too inefficient.
I use the genny to get through bad weather, get the batteries up to a bulk voltage, and then let the much more efficient PV do the last 20% of charge or EQ.
I set the genny to a high bulk charge voltage, almost an EQ setting, and then tell the customer to keep the run time down, which isn't hard for them, as they are very aware when the gen set is running and consuming precious fuel. I usually have it start at 40% to50% Battery SOC, and shut off at 70% to80%. That's all bulk charging, so 3 stage charging isn't really used anyway.
Unless you have a really large generator relative to the battery size, and an infinite fuel supply, its pretty hard to radically overcharge a battery. Also its usually in the winter when cells are cold, so thermal runaway is less of a problem as well. 

This is why I don't set up for auto EQ though. If you had a hot summer day, no one home, a small sealed, fully charged battery and then the gen set does a 12 hr EQ every 2 weeks; that would kill some batteries quick. Its just common sense, for instance on GT with Battery backup, I set the charge rate from the grid down to a trickle charge so bad things won't happen to the smallish sealed batteries connected to a 24/7 charging source. Just think what would happen to sealed batteries with a dead cell, and the grid charger was set to full charge rate? Might eventually start a fire.....

R. Walters
ray at solarray.com
Solar Engineer




On Oct 30, 2011, at 11:39 AM, Mark Frye wrote:

> I see. So there is a big difference between using the DC generator. With an AC unit I can run it into the inverter/charger and it will give me three stage charging. With the DC unit, I can really only us it to bulk charge.
>  
> Mark Frye 
> Berkeley Solar Electric Systems 
> 303 Redbud Way 
> Nevada City,  CA 95959 
> (530) 401-8024 
> www.berkeleysolar.com 
>  
> 
> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Drake
> Sent: Sunday, October 30, 2011 9:40 AM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] Cummins DC generator
> 
> Run the DC straight to a breaker in the power center.  Use a voltage sensitive relay to stop the generator when the battery voltage gets high.  
> 
> At 08:53 PM 10/29/2011, you wrote:
>> How would you integrate a dc generator like to Polar Power into a "standard" inverter/charge-battery-pv charge controller system?
>>  
>> Mark Frye 
>> Berkeley Solar Electric Systems 
>> 303 Redbud Way 
>> Nevada City,  CA 95959 
>> (530) 401-8024 
>> www.berkeleysolar.com
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