[RE-wrenches] Small battery bank vs too large array

Fri Jul 20 07:11:34 PDT 2012

Thank you Phil,

That is the solution.  Your wealth of knowledge has proven itself again.

Drake

At 12:34 AM 7/20/2012, you wrote:
>OutBack implemented Global Charge Control in the FM charge controllers
>a while back, based on the input of the good folk at SELF who worked
>to power a number of hospitals and clinics in Haiti after the
>earthquake.  In some of those systems, the PV array was based on the
>typical running consumption of the hospital, in the 30 - 100 kW if I
>remember right, and the battery bank was relatively modest. However,
>on the weekends and holidays without the normal AC loads the PV input
>was considerably more than the battery could absorb without
>destructive heating, something like a C2 or C5 rate.
>
>Global Charge Control is implemented using the MATE3, a FN-DC and FM
>charge controllers set to GT mode.  You set a high charge current
>limit in the M3, and then it monitors the charge current going to the
>batteries.  In normal operating mode the controllers stay in wide open
>mode, harvesting as much power as possible.  However if the loads drop
>and the total current from the charging sources begin exceed the
>global charge limit, the system compensates and the controllers back
>off to prevent sending too much current to the batteries.
>
>In today's world of PV modules being cheaper than diesel, we're seeing
>a lot more systems that can use this tool.
>
>Hope this helps,
>
>Phil
>
>
>
>On Thu, Jul 19, 2012 at 7:56 PM, boB at midnitesolar.com
><boB at midnitesolar.com> wrote:
> >
> >
> > I didn't see that Brian T had the same idea until after I sent that email
> > off to Allan...
> > boB
> >
> >
> >
> >
> >
> > On 7/19/2012 7:53 PM, Allan Sindelar wrote:
> >
> > Wrenches,
> > I forwarded Brian's post on to Robin at Midnite, as I thought it was an
> > interesting idea. Below is his response, as well as boB's.
> > Allan
> >
> > Allan Sindelar
> > Allan at positiveenergysolar.com
> > NABCEP Certified Photovoltaic Installer
> > NABCEP Certified Technical Sales Professional
> > New Mexico EE98J Journeyman Electrician
> > Founder and Chief Technology Officer
> > Positive Energy, Inc.
> > 3209 Richards Lane (note new address)
> > Santa Fe, New Mexico 87507
> > 505 424-1112
> > www.positiveenergysolar.com
> >
> > Allan, It is simpler than what is being suggested. The reason they want to
> > limit to 20 amps is because the battery doesn’t need anymore than that. The
> > controller will automatically limit the charge current as the battery gets
> > full. If a large load is turned on, the controller will try to refill the
> > battery up to its capacity. The 80 amps will quickly be reduced because the
> > voltage will rise to the point where the charge tapers off.
> >
> > There is nothing else that needs to be done. If the problem is that the
> > battery bank is too small for a big controller, the best answer is to get
> > more batteries. A 80 amp charger into a 200 amp hour battery is going to
> > raise the battery voltage so quick, it will not affect the battery at all.
> > By the way, discharging a battery at 60 or 80 amps is probably going to do
> > damage to a small battery also. We do have an input on the Classic that
> > could probably be programmed to do as requested. That input feature has yet
> > to be implemented. I’m sure we will have discussions about this when the
> > time comes to write the input code. Maybe this feature will be designed in,
> > but it doesn’t sound like it is a very good 
> feature to spend a bunch of time
> > on. After all, the main problem is that the battery bank is just too darned
> > small.
> >
> > Bob, Tom and Ryan do you have any comments on the subject?
> >
> > Thanks,
> >
> > Robin
> >
> >
> >
> > One idea I had in mind was to have an option, in software, to limit the
> > current into the battery,
> > when the charging current goes above some set threshold.  The controller
> > would have to get its information from
> > the battery monitor over the network.
> >
> > If it is a grid tie system and grid is there and GT inverter is selling,
> > then no problem...  The controller
> > can work at its full output.  If grid or loads go away, then the CC will
> > know and it can throttle back at
> > that time.
> >
> > We don't have a battery monitor yet, but we 
> will have one.  This will be one
> > of the settings as well as
> > Re-Bulk based on state of charge, ending amps and those types of things.
> >
> > boB
> >
> > On 7/19/2012 8:34 PM, maverick at mavericksolar.com wrote:
> >
> > I say it is waste of time.
> >
> > 1. AGM batteries can take the high current and you are right, the absorb
> > voltage is reached and the absorb current is 
> tapered rather quickly. Current
> > generation charge controllers are rather fast at the transitions. I have a
> > bunch of data from a system with a PentaMetric that shows the battery bank
> > going to absorb voltage at grid tie, during 
> cloud events, but only for a few
> > seconds at a time.
> >
> > 2. A properly designed GTBB system should cover the connected loads for 24
> > hours of each sunny day, at a minimum. Keep in mind, off grid systems are
> > designed for that, and 3 days + of backup, etc.
> >
> > 3. I would say, based on my experience, the minimum battery bank should be
> > 400Ah. I personally try to set it at 600Ah (48V). It is a backup system
> > after all. But the key is the customer's expectations...who are they going
> > to call after the lights go out?
> >
> >
> > Thank you,
> >
> > Maverick
> >
> >
> > Maverick Brown
> > BSEET, NABCEP Certified Solar PV Installer ®
> > President & CEO
> > Maverick Solar Enterprises, Inc.
> > Office:     512-919-4493
> > Cell:        512-460-9825
> >
> > Sent from my HondaJet!
> >
> > On Jul 19, 2012, at 6:35 PM, Brian Teitelbaum <bteitelbaum at aeesolar.com>
> > wrote:
> >
> > Drake,
> >
> >
> >
> > I’ve been hounding a couple of the charge 
> controller manufactures about this
> > issue, but so far I just haven’t seen a light bulb go off in their heads,
> > but I’ll keep trying, and maybe this is a better forum to do it.
> >
> >
> >
> > MPPT controllers can be adjusted to current limit at amperage values below
> > their rating, but if you do that, you are also limiting the output of the
> > array in general, and the amount of PV power available to run the loads
> > directly from the array (through the inverter). Not the best use of
> > available PV power.
> >
> >
> >
> > Say you have an 80A controller and a 200AH 
> battery. With current technology,
> > you have two choices:
> >
> >
> >
> > Let the controller operate at 80A. If you 
> have loads to draw off some of the
> > current (or sell to the grid), great, but if not you could be seeing a
> > charge rate of C/2.5, which would be pretty hard on a sealed 200AH battery,
> > to say the least. Granted, the battery voltage would rise pretty quickly,
> > and the controller would start to taper off, but it would still see high
> > currents especially if the absorption time is set long. Not a happy
> > scenario.
> >
> >
> >
> > Or, you can set the current limiting on the controller to 20A for a C/10
> > charge rate. But if you had loads drawing 60A, you would be pulling that
> > additional 40A from the battery and not using the array’s full power. Also
> > not a happy scenario.
> >
> >
> >
> > What we need is a controller that can read the signal from a shunt at the
> > battery, and use that as the basis of current limiting control.
> >
> >
> >
> > For example, if we have an array that can produce 80A of current, but we
> > want to limit the battery to 20A of charge, there would be 60A of potential
> > current there to run loads without drawing on the battery. If there are no
> > loads running, the controller should current limit at 20A (reading from a
> > shunt), but if loads are turned on, the controller should be able to let
> > more current through while still limiting the 
> battery to 20A. When loads are
> > shut off, the controller should go back to a 20A limit.
> >
> >
> >
> > This doesn’t all have to happen very quickly 
> as a battery can take a heavier
> > charge for a short period of time, but I think that this would be a major
> > improvement of controller function.
> >
> >
> >
> > Of course, if you are grid-tied you can sell all the excess power, but if
> > the grid goes down, or you are off-grid
.?
> >
> >
> >
> > Because of the low cost of PV and the high 
> cost of batteries these days, I’m
> > seeing more and more requests for large arrays with smaller batteries. I
> > also think that PV is now cheap enough to 
> allow for oversizing of arrays for
> > better battery charging on cloudy days, which can reduce generator run
> > times. We need smarter controllers.
> >
> >
> >
> > What say ye, charge controller gurus?
> >
> >
> >
> > Brian Teitelbaum
> >
> > AEE Solar
> >
> >
> >
> > From: re-wrenches-bounces at lists.re-wrenches.org
> > [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Drake
> > Sent: Thursday, July 19, 2012 2:21 PM
> > To: RE-wrenches
> > Subject: [RE-wrenches] Small battery bank vs too large array
> >
> >
> >
> > Hello Wrenches,
> >
> > Where can I get a device that will measure current through a shunt and
> > create a signal to trigger a relay?
> >
> > We want to be able to use a 2 kW array with 
> four, 200 AH sealed batteries on
> > an Outback system.  2 kW of PV would be too 
> much amperage for the batteries.
> > The idea is to open relays to disengage strings in conditions of high
> > current to the batteries.
> >
> > The reason for this is to create backup systems where power will be
> > abundantly available when the sun shines. The system will normally connect
> > to the grid, except during outages.  In 
> normal charging conditions the power
> > will go straight to the grid.  When the grid is down, power will be
> > available for loads and battery charging, but batteries will be protected
> > from overcharge?
> >
> > Any suggestions on ways to accomplish this are welcome!
> >
> > Thanks
> >
> > Drake
> >
> >
> > Drake Chamberlin
> > Athens Electric LLC
> > OH License 44810
> > CO License 3773
> > NABCEP Certified Solar PV Installer
> > 740-448-7328
> > http://athens-electric.com/
> >
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