Dead SLA lead acid

[CJay]

13.6V is too low.
Standby (float) is typically 13.8 but that's for continuous connection to a voltage source.

Cyclic usage is more typical of portable battery use and charge voltage is higher at 14.4-15V depending on brand.

Depends on the battery, for float charging the 1.2AH batteries I have used as an example here Yuasa quote 2.275V per cell +/- .005V, which works out to somewhere between 13.62V and 13.68V.

I figure 20mV below the lower recommended voltage isn't that big a deal and adjusting my 10A bench supply that accurately isn't that easy so it's a 'nominal' 13.6V.

13.8V would also work just fine I think, again, 120mV over the upper recommended limit isn't a big deal in this situation either so 13.6 or 13.8, it's not going to make a huge difference, it'll just take a marginally more or less time to reach full charge.

For Cyclical use they quote a higher voltage, I think it's 14.5V as you say but my prime aim when trying to recover a battery is to keep it from heating up if at all possible so low and slow seems to work better in that respect, I can charge at higher rates once I've worked out if the battery is likely to work in a useful fashion and it's been through a couple of charge/discharge cycles.

[IanMacdonald]

Frankly if it's this far gone it isn't worth bothering with. If they are left discharged for a long time the plates pick up a  thick layer of lead sulphate, which is an insulator.

[floobydust]

In flooded lead-acid batteries, all I've seen work is pumping very high charge current through them; basically a bubbling acid, hydrogen-generating festival to dislodge or dissolve the sulfate crystals.

Yep, it's called a gassing charge or sometimes equalisation charge, already mentioned above.

They hit a (flooded) car battery with >25A charging current, regardless of cell voltage, to blast off the sulfate out of the plate's pores.
I saw it a battery recycling company. I guess nothing else worked for them.

I've never seen eons of trickle charging do anything to help a sulfated battery.