Has anyone here tried a battery desulphator?

Waste of money. Put it towards a new battery

I had to get a new battery a few months ago. I was thinking more of 'next time'. A long time away, I hope, but I was a bit curious about these things.

If the charging system on your car's healthy, and you use the car regularly, then you won't need it. If the car's not used regularly, then disconnect the battery if it's going to be unused for long periods, or use a smart charger, which'll do the exact same job if and when it thinks the battery needs it - but it won't, because the smart charger will stop it from getting discharged and sulphating in the first place.

Basically, a load of partial bollocks.

You don't check a battery by measuring the off load voltage. You do so by measuring the maximum current it can deliver. Modern sophisticated testers do this - and also tell you the actual capacity in amp.hr. The important bit is whether it can deliver enough current to start the car. And has enough in reserve for more than one attempt.

You can recover (to some extent) a battery which has been left flat by applying a simple high voltage charge. About 30 volts is fine - sometimes for a week or more. No need to pulse it. But it won't be as good as new ever again.

I already have a bench top power supply where I can set maximum voltage and current, so doesn't cost me. But it's not going to be worth paying good money for a device which only does this.

Depending on the current these gimmicks could simply boil the sulphate off the plates. Pretty brutal really.

One of my 'Ring' smart chargers has a desulphating program, I believe it puts short sharp shocks in, rather than just blast continually, but really why would you bother?, sulphation generally only occurs when the battery is very old or has been allowed to go dead flat, in which case it is probably knackered or very below par anyway. If it is a vaguely modern car the battery is a fairly critical component that is not worth scrimping on.

When a battery is sulphated, you can't push enough current through it to boil anything - at least with the sort of power supply you could sell legally. At 30 volts or so it will only draw a fraction of an amp, due to the internal resistance being sky high. Which is why it takes so long to work - if indeed it does.

As you say the int. R is very high. I tried to "solve" this once with a full wave bridge (isolated) mains and via a 8uF capacitor (using the reactance to give some R(buffer). :-) It kind of worked - it worked so well it actually warped the internal cell links in the battery. :-)