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

boB at midnitesolar.com boB at midnitesolar.com
Thu Jul 19 19:56:43 PDT 2012 


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_ <mailto: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_ <http://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 <mailto: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> 
>>> [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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