[RE-wrenches] AGS quiet time

Sun Apr 12 15:14:26 PDT 2026

Yep, totally agree. However, then you start facing the issue of SOC drift
if you can't get them up to 100% regularly.

Damned if you do, damned if you don't.

Right-sizing all components is hard while staying within reasonable
budgets. I guess that's why we get paid the big bucks.

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 6:03 PM Bill P. Battagin via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> Jason,
>
>                 I couldn't agree with you more about over-sizing the
> battery when Off-grid and open loop.  In my opinion this is THE solution.
> Yes there's the extra cost of the greater capacity battery however it
> solves the generator angst issue (both customer and installer).  There is
> the added upfront cost of the battery, maybe 30-35%? but over time much of
> that cost will be recovered from lower genny fuel consumption, repairs and
> maintenance, plus i'm guessing the L-ion pack will last longer if its not
> worked as hard and discharged as deeply.  Obviously its a plus to also have
> the benefit of more power autonomy it the sun ain't shining.
>
>                Overall in the long run, maybe it does come out a little
> more expensive, but its a better more reliable system with less problems.
> We're more worried about the gas in the generator going bad due to lack of
> use than BMS shutdown. Is bad gas angst a thing?
>
> Taylorsville Bill's  $.02 worth,
>
> *Feather River Solar Electric*
>
> *Bill Battagin, owner*
>
> *4291 Nelson St. (Freight)*
>
> *5575 Genesee Rd. (USPS, UPS)*
>
> *Taylorsville, CA  95983*
>
> *530-284-7849, 258-1641(cell)*
>
> *CA. C10 Lic # 874049**Solar Powered since 1982*
>
>
> ------------------------------
> *From: *"Jason Szumlanski via RE-wrenches" <
> re-wrenches at lists.re-wrenches.org>
> *To: *"RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> *Cc: *"Jason Szumlanski" <jason at floridasolardesigngroup.com>, "Zeke
> Yewdall" <zeke at darkforestsolar.com>
> *Sent: *Sunday, April 12, 2026 2:28:40 PM
> *Subject: *Re: [RE-wrenches] AGS quiet time
>
> 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).
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