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

Fri Jul 20 10:23:03 PDT 2012

Phil,

Thanks for the very thorough explanation of the Global Charge Control. That setup would certainly provide the level of control that I was getting at.

I can see how the FN-DC could do that since it utilizes multiple shunts and can therefore use discrete amperage readings to provide better charge controlling functions. This will be very useful, not only for the type of system that you describe in Haiti, but for off-grid systems employing large PV arrays to provide adequate charging in foggy or overcast weather. With PV at a buck-a-Watt, this is starting to become a viable way to reduce battery size and cost without resorting to running a generator. PV lasts substantially longer than batteries, so is a better investment in my opinion

Brian Teitelbaum
AEE Solar

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Phil Undercuffler
Sent: Thursday, July 19, 2012 9:35 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Small battery bank vs too large array

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 (r) 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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