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

Jason Szumlanski jason at fafcosolar.com
Wed Sep 5 06:11:22 PDT 2012


This thread is a little stale, but I have an application to bring it back
to life.

Has anyone designed a solar powered go-kart? There is a high school
challenge here sponsored by the local University. This seems like an
application that could use a large array with a small battery to keep
weight down. I'm considering a 69aH-85aH AGM battery range @ 48V with three
SunPower 327W modules wired in series with a Midnite 250 controller.

Any advice would be appreciated, especially regarding:

   - Safety
   - Monitoring
   - Maximizing endurance (ties into monitoring)


Jason Szumlanski
Fafco Solar



On Thu, Jul 19, 2012 at 8:02 PM, <toddcory at finestplanet.com> wrote:

> what is the point of having battery backup if is is so small as to not
> really be able to back anything up (except when the sun is shining)?
>
>
>
> this seems like a system design error rather than a charge controller
> problem.
>
>
>
> my .02
>
>
>
> todd
>
>
>
>
>
>
>
>
>
> On Thursday, July 19, 2012 4:35pm, "Brian Teitelbaum" <
> bteitelbaum at aeesolar.com> said:
>
>  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/
>
>
>
> Sent from Finest Planet WebMail.
>
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