Eric,
Yes, one of the classic problems with small systems and less
educated users. Just a small contribution: it would seem to me
that in this case the LVD setting is as key as the LVR. What if
the LVD was set as high as possible - say, 12.0 V with sealed
batteries, along with a 13.8 - 14.0 V LVR. The system would still
be run until it shuts off, but the battery remains at a relatively
high SOC. The next day the system will return to operation, but
with a much higher range of SOC. The users will still experience
shutdown, and will over time become familiar with when to expect
it and how to live within its capacity, but the battery life will
be protected.
This approach seems counter-intuitive, but the more I think about
it the more sense it makes. The system would also return to
operation sooner following cloudy weather - in fact, it would
provide a minimal amount during each of the cloudy evenings.
Is the C12 still made? It would still seem ideal if so: bulk,
float, LVD and LVR, all adjustable, with removable knobs and a
15-minute reserve button after initial LVD shutdown. Rock-solid
reliable, too, in my experience.
I'd be interested in what approach you select. This intrigues me.
Allan
Allan Sindelar
[email protected]
NABCEP Certified PV
Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder, Positive Energy, Inc.
A
Certified B CorporationTM
3209
Richards Lane
Santa Fe, New Mexico 87507
505 424-1112 office 780-2738 cell
www.positiveenergysolar.com
On 12/30/2013 5:00 PM, Eric Youngren wrote:
Hi Wrenches,
We are bidding on a project to provide several hundred small
(75Wpv, 120AH battery) 12V DC off-grid solar home systems for
rural villages in West Africa. The project planners have
requested that the systems be designed and built to provide 7
years of battery life. That seems crazy optimistic to me but
I'm trying to design the systems that will give the batteries
a fighting chance of lasting that long.
In
my
experience with these types of installations, the system
users have little or no understanding of how to properly
manage their batteries and usually no metering or SOC
indication to help them even if they knew what to look for.
So, the default control strategy becomes: run the loads until
the Low Voltage Disconnect (LVD) turns them off, then wait
until the Sun returns and the voltage rises to the Low Voltage
Reconnect (LVR) setpoint (around 12.5V is a common default) ,
upon which point the cycle repeats, with the result that the
battery bounces between LVD and LVR, almost never reaches a
full SOC, and the batteries are lucky to survive for maybe two
years. After that the system will provide a little power
during the day while the Sun is shining but the batteries will
quickly crash below the LVD after the sun goes down.
So, we want to offer a controller with a high LVR setting
that will ensure the batteries reach a full recharge after
each and every LVD incident. I know the C-12 has an
adjustable LVR setpoint and I see one Chinese brand (Manson)
that can do it. Does anyone have any other suggestions of
small (<20A) PWM controllers that can be adjusted to not
reconnect until 14V or so? Price is going to be a factor
in this project so they need to be low cost.
Any other suggestions? What would you use for the optimal
LVD and LVR in this situation? These are rural household,
DC only systems with primarily lighting and small device
charging loads. I know that this strategy will mean that
they might not have any access to battery power for up to a
few days if they hit the LVD during the rainy season and there
is not enough sunshine to get the battery charged in a single
day. My thinking is that experience will provide good
feedback that will encourage them to conserve and manage their
batteries to avoid the LVD situation as much as possible.
That's the idea, anyway. I'd appreciate any advice from you
all. Thanks!
Wishing you all a happy and productive 2014!
Best energy,
Eric
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