[RE-wrenches] AGS quiet time

Mon Apr 13 02:01:55 PDT 2026

Could it be that the EV SOC is somewhat faked by the manufacturers because
they know the mass market will not stand for "SOC drift" as an excuse?

My wife had a Saab in the 2000s that would go to "0 miles remaining" but
still keep running for over 50 miles. Ask me how I know. :)

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 Mon, Apr 13, 2026 at 12:36 AM Bill Battagin via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> Seems kinda funny that the EV manus can pretty well nail the SOC of their
> battery packs for reliable readout of range left and percent of charge
> (SOC).  We've been driving EVs (3 varieties) for 8 years now and this has
> been our experience. This now being done with LiFePO4 batteries in EVs and
> it's not very often that an EV battery get charged up to 100%.
>
>             Unless I'm missing something it seems our industry-wide SOC
> monitoring is not as sophisticated, until it is we're using voltage, with
> its limitations, for AGS. Is it just a matter of having the best coulomb counter?...uh
> oh, now I'm drifting out of my field.
>
>             I would love to see more webinars from our manufacturers that
> get more into the weeds about the guts about how LiFePO4 batteries with
> their BMS's interact with our systems especially off-grid, our main market.
>
> Bill
>
> Feather River Solar Electric
> Bill Battagin, owner
> 4291 Nelson St. (Shipping)
> 5575 Genesee Rd. (USPS, UPS)
> Taylorsville, CA  95983
> 530-284-7849, 258-1641(cell)
> CA. C10 Lic # 874049
> Solar Powered since 1982
> Home of the Sunny Side Up
>
>
> On 4/12/2026 3:14 PM, Jason Szumlanski wrote:
>
> 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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