This might sound like a stupid question but I have several 14Ah lead acid batteries that i accidently left energising a relay and checked them recently. They all read between 0.5v and 1.5v which I know is very bad for this type of battery, and thought I can still charge them but they don't draw any current from the charger now. Why (chemically?) don't they charge anymore? And is there any way to bring them back from this?

There are some circuits out there on the web which are designed to"desulfanate" a lead acid battery, but as yet I have not tried to build one. I use a lot of the Gel Cells as emergency battery packs for in the field battery systems for our radios.
Try a search for "Desulfanater" or some other spelling.

Thanks. One of the batteries was recoverable when i put three power supplies in series to get like 75v. Although I don't know how long it will last. I've read somewhere that you can recover Ni-Cads in the same way.

I might get a charge regulator which can manage the charging / discharging of the battery pack.

Another thought is to get some bigger (car batteries), would those be better than 40 14Ah ones?

I have fix some lead acid battery car type with a charger I made it could put 20volts at 50amps in them but it didn't work on all of them found a chart some where that showed how much it took to

desulfanate" a lead acid battery

you may be able
to find it on the web.

I've asked my dad about this, and he says the acid gets stronger when it's discharged. Thats why the plates are disolved quickly. Does this sound like sulfanating?

I just thought, how does he know that the acid got stronger, he doesn't have anything to measure that - he must have tasted it! lol!

I built the circuit found here battery gizmo but I do not have a beat-up battery to test it on. Hope to have one soon back from a friend.

On a good battery I can get the audible RF noise so I believe my circuit works, though putting a multimeter's leads across the battery terminals does not show a voltage "spike" at all. Either that way of testing does not work or I screwed up somewhere w/ the 555 I guess.

You can use a battery hydrometer to measure the specific-gravity of the electrolyte.
As you can see from the diagram, the "float" rises in the "battery acid" when the battery gets charged "more-and-more." This means the acid gets "heavier" not lighter, when the battery is fully charged - as the denser fluid pushes the float higher out of the water, just like a ship floating in salt water compared to fresh water (plimsoll line - or load line) .

I've done the hydrometer test too many damn times in the Navy lol.

My friend has a "beat" dead battery that won't come back to life in his old Toyota, so hopefully I can be testing the circuit soon.

Lead sulphate is like the previous posts mentioned your problem .
Desulphation is in itself (partly) possible .
Over your plates you have a layer of lead sulphates which isolates .
The internal resistance of your battery is very high and it prevents the interaction between the plates and the now dilluted acid .
Lead sulfates resonate if i remember correct at 3.3 Mhz .
What you want in your desulfator is that it has powerfull pulses which contain high frequency's to have resonance effect take the sulfates of your plates .
I used switchmode controllers with (intentional) ringing from the mosfet to create that 3.3 Mhz needed to desolve the sulphatelayer .
It did (measurable) improve the state of health of the battery but it takes time and the battery doesnt come out as new .
You can follow progress by measuring the internal resistance of the battery from time to time given you don't use a method that draws large currents .

The VX-6 (?) battery rejuvenator fluid was based on baking soda (or baking powder, I cannot remember which).

On this website they give a relative good explanation of the processes at work .
Personaly i don't know the VX6 fluid ,but i can ask one of our customers that runs a battery rejuvenation service .
At the moment i am doing trials with the chargers he uses (in combination with additives)
(those chargers do pulse but at just 60 Hertz and relative low powerlevels)
I assume that when that VX6 is based on backing soda the intention is the have the alkalic solution dissolve (neutralize) the sulphates .
To me it seems it would dillute the acid mixture ,taking a few % of the efficiency of the chemical processes but with cleaner plates the battery would function again just not at 100 % of its rating .
Its like cleaning the waterboiler with vinegar .
Personaly i would try to inform yourself on the working of desulphation pulsers .
I don't like meddling with the acid mixture especialy not when it might be possible to recover it from the sulphation on the plates .