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

Fri Jul 20 18:28:46 PDT 2012

Hi all, I just got a correction from Darren - global charge control is in
the Mate3, and therefore works with any FM or MX controller that has the GT
mode. It uses the current sense from the FN-DC and the GT mode signal to
control the output of the controllers.

Sorry for any confusion

Phil
On Jul 20, 2012 4:11 PM, "Chris Mason" <cometenergysystems at gmail.com> wrote:

> Phil,
> Good explanation of the feature. When did GCC make it into the firmware,
> and can the older ones be updated? It sounds like a perfect solution for
> some of my installations.
>
> On Fri, Jul 20, 2012 at 12:34 AM, Phil Undercuffler <solarphil at gmail.com>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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>
>
> --
> Chris Mason
> President, Comet Systems Ltd
> www.cometenergysystems.com
> Cell: 264.235.5670
> Skype: netconcepts
>
>
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