[RE-wrenches] Best Off-Grid Equalize Charging Method

[Walt Ratterman]

Jeff,

I cant offer any documented testing, but I have done a lot of off-grid
systems where the battery banks are large compared to the PV array.
(intentionally - in the hybrid systems with the generator.)

You are absolutely correct that the biggest problem is getting the batteries
charged on a regular basis.

Regarding your question of generator in the morning or evening, we have the
operator inspect the system twice daily - once at sundown and once at sunup.
The state of charge of the battery (I know - we don't really know for sure)
gives the operator a decision of whether or not to run the generator.

Our normal operation is that we let the sun do everything it can do during
the day, and then if the batteries need more, the generator can do this in
the evening.  This results in us getting the biggest bang for the PV buck,
and the biggest potential in fuel savings.  If you run the generator in the
morning, it is normally going to run the full cycle, and then the PV wont
even come on.  (and is hence not even needed......)

Another issue that factors into this is days of autonomy.  We are getting
away from the traditional "3 days of autonomy" recommended in many places.
This is because, as you noted, after you use up your three days of storage,
it is nearly impossible to get the batteries charged back up without running
the generator for increasingly longer times, which the operators are often
not going to do, due to fuel costs.  (And in installations with no generator
(the smaller systems) installations with many days of autonomy simply almost
never get charged back to full after the first few days of clouds, unless
the PV array is oversized AND the users cut way back on their loads.....

Sorry about no documentation....

Take care,

Walt Ratterman
SunEnergy Power International
www.SunEPI.org

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jeff Yago
Sent: Thursday, October 23, 2008 1:55 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Best Off-Grid Equalize Charging Method

I know this has come up before but wanted to see if anything new available. 

Unlike on-grid battery-based systems, off-grid systems require a different
equalize charging strategy to reduce generator run-time and save fuel. The
problem arises when you have a large battery bank and a not so large solar
array. 

You can first run the generator early in the morning at a high bulk rate to
shove as much charge into the battery as possible, then take over charging
with the solar charge controller set for equalize charge and hope you have
enough sun-hours and current to take the generator-charged batteries past
their normal voltage and start gassing, 

OR, 

You can do the reverse and solar charge all day and then at the end of the
day use the generator with the inverter set for equalize charge and continue
charging while hoping this will not require too many hours of fuel usage. 


Of course the size of the solar array verses the size of the battery bank
may favor one method over the other, but are there any actual tests or
studies that make one method always the better choice?

I have found that small arrays have a real hard problem ever getting a large
battery fully charged, let alone provide an equalize voltage for several
hours, yet I have also found most inverters supplied by a generator tend to
back-off the charging current as soon as the battery nears a fully charged
state then tries to stop the generator. I also have found that some
deep-cycle batteries sold for solar applications require charging voltages
and re-charging times that are almost impossible to achieve unless you have
grid power available.

Any documented testing out there?

Thanks,

Jeff Yago




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