[RE-wrenches] Higher LVR to force full re-charge in small OGPV systems

[Eric Youngren]

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<http://solarnexusinternational.com/images/documents/shortcuttofailure_ghtcpublished.pdf>,
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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