[RE-wrenches] AGS quiet time

[Jason Szumlanski]

The issue I have with open loop and LFP when off-grid is that you have to
set the AGS start voltage pretty high on sites where there is sometimes a
large load relative to battery capacity. The voltage curve drops off too
fast around 20% to start the generator sometimes before the BMS shuts
down. You can't use the full capacity of the battery because you're faced
with starting the generator at higher than necessary voltages. That takes
away one of the huge advantages of LFP over lead-acid: more usable capacity.

That said, I have sites with huge batteries and low loads that work great
with open loop because the batteries rarely approach deep discharge, and
when they do, there is enough time to "catch" the steep part of the curve.

In a perfect world, everyone would play nice and agree on communication
standards, or at least just let the dang inverter read the SOC and that's
it.


Jason Szumlanski
Principal Solar Designer | Florida Solar Design Group
NABCEP Certified Solar Professional (PVIP)
Florida State Certified Solar Contractor CVC56956
Florida Certified Electrical Contractor EC13013208




On Sun, Apr 12, 2026 at 5:21 PM Zeke Yewdall via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> I used to be a firm believer in only operating lithium batteries in open
> loop, because trying to do closed loop with most requipment was such a
> disaster of communications tinkering at best.  I would still try to operate
> the AGS with SOC based controls as much as possible, but still have a
> voltage catch in case the SOC drifted off from reality.  Outback would let
> you have both at the same time.  But still no comms with the batteries.
>
> But, if you can have the batteries and inverter from the same
> manufacturer, which is more and more possible, so you know closed loop
> communications are going to work, that is much nicer than trying to
> determine voltage based start and stop points on a lithium battery.   So
> I'm coming around to using closed loop communications since starting using
> matched equipment.
>
> I get not taking the SOC all the way to 100% with the generator to leave
> some room for solar.  But I think it depends on your climate.  If you
> have some winter sun and can still get to 100% regularly, I think that can
> make sense.  But if you have no winter sun (like where I am now -- typical
> PSH well under 1 for Dec and Jan -- I've measured as low as 0.25PSH for the
> month before --  then I am still a believer in getting the batteries to
> 100% regularly (once every week or two), even if that takes a generator.
> Otherwise the SOC indicated by the BMS can drift from reality.  Some BMS's
> will recalibrate their SOC if they get a voltage reading at rest at an SOC
> lower than 100%, but many BMS's will only recalibrate the SOC calculation
> when they reach 100% (same as ah counting SOC meters that have to reach
> 100% to recalibrate/reset).
> _______________________________________________
> List sponsored by Redwood Alliance
>
> Pay optional member dues here: http://re-wrenches.org
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Change listserver email address & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> There are two list archives for searching. When one doesn't work, try the
> other:
> https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> http://www.re-wrenches.org/etiquette.htm
>
> Check out or update participant bios:
> http://www.members.re-wrenches.org
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20260412/9167675f/attachment.htm>