I agree with Allan and Dan, and would also add that modeling the
performance in some type of software would also be important.
Both PV Design Pro (now apparently obsolete) and HOMER allow you to set
on and off set points based on SOC. You can literally see how the
battery is cycled to predict its life, and see how many shut downs
different settings will get you. You can also see how often you reach
full charge.
Also Solar Converters in Canada makes a nice adjustable LVD that is low
cost. AEE sells them. I've used them many times for DC only systems.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 12/30/2013 7:32 PM, Exeltech wrote:
Eric,
Allan's recommendations on both points are excellent, and spot-on.
I would add the following suggestions for your controller and system if budget
and circumstances permit:
1) Use a MPPT model controller to enhance the recharge rate under
less-than-ideal conditions. The PV is already undersized for the battery you
indicated (below).
2) If you get the job .. install the batteries in a location that will keep
them as cool as possible. Heat is just as detrimental to battery life as is
failure to fully charge.
3) Temperature-compensated charging will enhance battery life. However, the
75W PV vs. 120 A-H battery allows for only a C/20 recharge rate at best. This
is not conducive to full recharge on a regular basis unless the loads are very
small and are lightly used. If the budget allows for opportunity to double the
PV wattage .. it would be wise to do so.
4) A simple "stop-light" style LED voltage display won't consume much power,
but can go a long way toward providing a visual indicator for state-of-charge, especially
if it's adjustable, and can be be biased toward a higher SOC (in essence, make it fib a
little, and indicate a low SOC sooner than than is actually the case). Worth considering
as an option .. but easily omitted to reduce cost and/or simplify the system.
I wish you success on winning the project.
Regards,
Dan
--------------------------------------------
On Mon, 12/30/13, Allan Sindelar <al...@positiveenergysolar.com> wrote:
Subject: Re: [RE-wrenches] Higher LVR to force full re-charge in small OGPV
systems
To: e...@solarnexusinternational.com, "RE-wrenches"
<re-wrenches@lists.re-wrenches.org>
Date: Monday, December 30, 2013, 6:48 PM
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
al...@positiveenergysolar.com
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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