Forklift batteries! They're often available cheap, used from scrap yards; sometimes with much more life in them than you'd expect. They might just need more electrolyte. They're designed to be durable under harsh uses. They're often separated cells for easy maintenance and that makes it possible to hack them into different configurations.
Hacking high energy DC systems is perhaps a little more responsibility and risk than was desired tho.
You should not need $16k in lithium batteries for critical load backup of a water pump. You can get 5kwh rack batteries for $2k that will last 20 years in stand by applications [1] which should run a well pump for about 5 days under typical usage (30 minutes per day), if you spent $16k you would have 40kwh of battery capacity enough to run everything including the A/C in your typical home for a full day.
Not sure what the permitting / venting issue would be with VRLA (sealed lead acid) as they are used in UPS's everywhere and only vent gas under extreme overcharge conditions. However if I where getting new backup batteries I would definitely go with LFP due to less space used and extremely long cycle life while being very affordable, they also have much more usable capacity than a lead acid and a much flatter voltage curve. I replaced my lead acid in my RV and got 4 times the capacity in the same space at half the weight and I should never have to replace them, they will probably outlive the RV.
The biggest failure mode with both solar systems and battery systems is the electronics components of the solar controller / inverter and battery BMS. The mosfets or capacitors or digital controls seems to fail much sooner than the solar panels or lithium battery cells. Not sure if its just using cheap components to save money or what, but it definitely helps to oversize the controller, inverter and battery BMS's. Would be nice to see GaN or SiC FETs and components in residential solar equipment.
Nine trouble free, no maintenance years on the same lead acid batteries! Sounds like he unreasonably expected every part of his system to run flawlessly until the end of time. That solar controller has been hard at work every day for nine years. It is unfortunate that most of not all parts of his system are manufactured outside the country but that's true for his cars and computers too.
I’m surprised the regulations forbid lead-acid in favour of lithium.
Lead-acid can emit hydrogen during charging which is easy to get rid of with good ventilation. Worst-case scenario it produces a single explosion, hopefully in a utility room with not too much risk to life.
Lithium in contrast will give you very hot, unextinguishable fire.
VRLA lead acid batteries should not vent unless overcharged, they are used in UPS's everywhere, not sure what the permitting issue would be.
Lithium Iron batteries which are what's typically used in stationary backup are extremely safe and do not burn as hot compared to more unstable high energy density chemistries like NMC typically used in mobile applications. They are also relatively inexpensive and have extremely long lifespans if not used in extremely hot environments, we are talking 20 years to 80% capacity in a stand-by application.
So, let's say that the whole stuff (before solar cells go below a meaningful percentage of efficiency) will last in total 27 years[0], that is three cycles of 9 years.
It is likely that he will need in the next 18 years another charge controller and another set of batteries, so between:
35+2+4+2+4=47 thousands/27=1,740 US$/year
and
35+2+16+2+16=71 thousand/27=2,630 US$/year
I believe that the issue about the lead batteries is due to them being "in a cabinet", most probably they can be relocated or however the cabinet made ventilated for far less than the difference between lead and lithium batteries.
Still, even if lead batteries can be used, it does look like expensive.
[0] and it has to be seen how much it will be the cost for the disposal of the panels
I recently lost a panel to squirrels chewing the wires. Solar is low-maintenance but not zero. Having used a generator also I will take the solar every time.
Hacking high energy DC systems is perhaps a little more responsibility and risk than was desired tho.