[RE-wrenches] AGS quiet time

Sun Apr 12 14:59:52 PDT 2026

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 < [ mailto:re-wrenches at lists.re-wrenches.org | 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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