[RE-wrenches] PV Assist

William Miller william at millersolar.com
Fri Aug 16 10:53:01 PDT 2013


Friends:
 
We have been receiving a flurry of requests for design of off-grid systems
where the loads are enormous, relative to normal off-grid requirements.  It
is obvious that the size of PV and battery arrays to power these loads is
beyond what is practical.  We have dubbed these systems Generator/battery
with PV assist, or PV assist for short.
 
These systems will cycle batteries multiple times per day.  It is my
understanding that the extra battery cycels will shorten battery life.
Since generator run is expected, we are willing to increase generator run
time in order to prolong battery life.
 
The parameters I suggest are an aggressive load start and a very high
battery start parameter.  This will prevent deep discharge of the battery
bank.  I know shallow discharge is not the norm, but I don't believe shallow
cycling is a problem as long is it as at the top of the voltage window, i.e.
the batteries achieve absorption at least once per day.
 
We know a load start ciuld result in an abbreviated run time, curtailing
absorption period, but we are assuming there will be a battery votage start
at least once per day allowing full absorption.
 
Has anyone else considered these issues, and what conclusions did you make
 
I am looking forward to a spirited discussion as usual.  I throuroughlyenjoy
and beneift from them, even thought the advice is usually conflilcting.
Thanks in advance.
 
William Miller
 
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