Yes, it's cheaper to buy your own cells, cheaper yet if you get seconds or derated used cells.
https://www.youtube.com/watch?v=E4pN4DVPOcY

The price I gave is an estimate  for a good priced, prebuilt 100AH 12V battery. 

Will Prouse keeps reviewing cells, BMS units, prebuilt batteries, rack mount setups.
https://www.youtube.com/c/WillProwse

While a young enthusiast, and thus optimistic on new technology reliability and longevity, his reviews and material are technically competent.
 

Yep, designed and build my own hyperbaric chamber and tried that 20 years ago. It  gave me a nasty MS flare up that lasted a few months.  About 10% with MS react very badly to hyperbaric oxygen, according to some UK research. Most find it helpful to some degree.

It was made from a 90 psi horizontal water pressure tank with an extra end cap. It had 8  1 inch by 4 inch light openings on top,  with 1/2 polycarbonate "windows" fitted inside, and dual outside or inside controls for operation.  I could sit and turn around in it.   It had an oval door which sealed via gasket with even the slightest pressure.  It was made from an extra stamped end piece I bought with the tank from an in-state manufacturer.

I sold the chamber to an MD in Texas who wanted to try it for her husband.  She was pretty thrilled to get a chamber tested to 4 atmospheres for (then) under $3.

Thanks for the kind thoughts.
Alas, I have memory and cognitive impairment from MS and epilepsy, so technical writing is just plain misery.  Can't remember the previous paragraph so can't hold onto context, etc.  Same issue (worse) with software.  Makes technical projects very hard and stressful.

So I'm doing a whole lot less, plus seeing quacks and doctors,  trying new treatments without any success, which is hard on morale and the wallet.

If you're changing the bridge rectifier pre-emptively (a wise suggestion, 38ac), then consider cleaning up your the high frequency EMI by adding 4 capacitors to the bridge. The use of the piggyback spade connectors works well, or you can solder them to the base of each connector.  One on each side. 

This is the type of capacitor you want, with the part number on Digikey:

CMPPAC310V104K10S5

It's a thin film 310VAC rated capacitor, 0.1 uF.  About $0.30 US.  Similar spec parts should be available around the world.

I can't get the page to load or I'd give you link.  Digikey seems to be overloaded this afternoon.

This is highly effective in reducing radio frequency noise from the ST line output.  Check it with an AM radio (old regenerative tuner type with static sound between stations) before and after and you'll agree.

Yes, you won't know what you've got until you check it out and also take a look at the waveform, and see if you have issues with your various intended loads. (Check your AC voltage using a true RMS voltage mulitmeter at a range of loads.)  Many ST's are being sold that are wildly high voltage, and thus will likely require an AVR or a large dropping resistor in the harmonic to bridge rectifier circuit.   If you intend on using it for non-electronically regulated lighting, you will have less Listerflicker by using an AVR using the AC ouput as excitation instead of the harmonic , and this will also help with "harmonic hump" distortions if present. 

Some LED lights are electronically regulated with a small AC to DC switch mode power supply. They will get rid of the Listerflicker unless it's extremely bad.  The stock harmonic regulation amplifies Listerflicker, and during compression stroke, the harmonic output will drop so low that the AVR won't help, since it only limits the excitation and can't amplify it.

My neighbor has one on his ST-3.  His is using the harmonic for excitation, regulated by the AVR, as the stock unit voltage so high that there is no choice but to use it, which is common with some of the CGG units.  He gets about 2-3 years per unit, with heavy use.   

FYI- these do NOT regulate to RMS voltage so won't work well with a nasty PF load like a cheap high current , switch mode batttery charger.  Output voltage will be too low for a very bad PF load.

One of the issues regarding longevity is that even in the ST-3 with a normal harmonic, the voltage peak is over 400V, while RMS voltage is much lower. 

The standard harmonic system of the ST3 sorta regulates output voltage by sensing load current via the harmonic winding, so for a big load spike, like starting a motor, it really works well.  I have a dual mode setup, my own design AVR for normal running, but I can switch to harmonic only for running a small switch mode type MIG welder.  I have an adjustable resistor to limit harmonic current to get the desired voltage. 

For cleaning up the output waveform several methods have been reported depending on what the problem is.  The cheapest digital o-scope or usb scope will let you see what needs to be done.  The common methods are- adding motor run capacitors on the line output (for some ratcheting and spike distortions),  running an AVR on the line voltage instead of the harmonic (if you have "harmonic hump" distortion.  Some found adding capacitance after the bridge diodes helped with harmonic hump distortion.  If the ratcheting and spikes are really bad, I'd add two 200-400 uH chokes plus capacttance on the line output.  A surplus line reactor plus motor run capacitor will also help but is pretty spendy. Putting massive amounts of capacitance (> 60uF) of motor run capacitors starts to affect your peak motor starting capability and starts to show a little in fuel consumption. 

For the high frequency EMI on the AC line,  I found that adding 0.1 uF snubber capacitors parallel to each diode in the bridge (4 total) made a world  of difference.  With these small caps (240VAC rated) the EMI went from detectable across much of the AM band from 8 feet away from AC wiring to 8 inches in just a few spots.  A standard dual common mode choke filter will also be helpful but I found the snubbers got it good enough for me and only cost me a few bucks.

That's my pitch on ST heads-  I envy the guys with Stamford clones who just belt them and forget about it!  The variation  in different ST heads is huge, in terms of waveform quality.  At one point I had 3 different ST-3 rotors and stators on my bench- and none were physicially identical to the others.  I"ve seen lots of waveform screen shots from ST-5's also, and they also vary dramatically.  The best thing I can say about ST heads is they are cheap, and simple. If you get copper windings and put in good bearings, and a modern bridge rectifier, they are reliable in creating a high THD waveform for thousands of hours.  My custom low EMI inverter with a 5 step sine has lower measured THD (measured THD of 12%) than my ST-3 (measured THD of 15%), even when running on mains excitation instead of harmonic.  Either runs everything I need just fine, but the timer motor on my washer makes some noise on the generator.

olNick,
Bravo on that repair. 
I expect a 90's design with IGBT's would not be power factor correcting, so it would need a much higher rating on the generator head to cope.

Inverter-welders have always been at the cutting edge of power designs.  I can't imagine a more difficult power design application; high current arcing and shorts are normal.