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

Thu Jul 19 19:34:24 PDT 2012

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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