[RE-wrenches] AGS quiet time

[Zeke Yewdall]

A recent thread included Quiet Time in the list of non-included features
with newer inverters.  And I too have had customers asking for this. But,
in many cases, I find that quiet time is not actually a useable feature for
many current off grid houses with lithium batteries, and this is why.

With lead acid battery banks, we typically designed for the generator to
start around 50% SOC.  But, occasionally we could discharge down to 20% SOC
without damaging the batteries long term.  And, typical off grid houses
were usually designed with fewer nighttime loads, compared to nowadays.
 So, if the battery bank hit 50% SOC at 9pm, but quiet times extended till
8am, we still usually had plenty of reserve left to wait till 8am before
starting the generator without any additional failures or battery damage.

But a lot of current off grid systems with lithium batteries are sizing for
a lower days of autonomy (because of the higher cost of lithium batteries,
though that's changing now), and deeper regular depth of discharges.
Generator start is often set at 20 or 30% SOC instead of 50%.  If we are
planning for regularly discharging below 50% SOC overnight, which I do
commonly see, there's not nearly so much reserve capacity to wait for a
generator start.  If we hit 20% SOC at 9pm, with a smaller battery bank
relative to the load, and a higher nighttime load from more things
remaining on all night that we never used to have (routers, internet,
heating system controls/pumps/etc), then the chances of us making it till
8am to wait for the generator start is a lot lower -- we risk a BMS
shutdown of the battery sometimes betwen 0% and 10%, before 8am, which
depending on the particular equipment can be a difficult situation to
recover from.  Many AIO's will only black start batteries from solar, not
generator, and most (though not all) older charge controllers and inverters
will not black start batteries from either solar or generator.

So, because we are using deeper discharges, using more energy overnight,
and waiting longer before a SOC based generator start, we often don't have
the option of waiting 4 or 8 hours or such to start the generator.  It
needs to start very soon after the call for it, or we run into further
problems.

What would be ideal is a system that looks at SOC before quite times would
start, and if it's not sufficient to make it through a typical night
because it's been a cloudy day, starts and runs the generator a bit before
quite times start, rather than hoping to make it to morning before running
it.  An experienced off grid owner can do this automatically, topping the
batteries off before going into the night if they are too low.  But
unexperienced owners or ones who just don't watch their battery SOC all the
time, will not.  Perhaps we'll come up with AI driving controls that watch
the typical batery use patterns and look at the weather forecast, and can
do this, just like the experienced off grid operator does.

Zeke


-- 
Zeke Yewdall
PV Engineer
NABCEP #031508-89
zeke at darkforestsolar.com
303-523-3592
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