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

Thu Jul 19 21:34:32 PDT 2012

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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