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@positiveenergysolar.com_ <mailto:al...@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, maver...@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
<bteitelb...@aeesolar.com <mailto:bteitelb...@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-boun...@lists.re-wrenches.org
<mailto:re-wrenches-boun...@lists.re-wrenches.org>
[mailto:re-wrenches-boun...@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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