It still won't work.

We already need lots of hydrogen for various industrial use cases. We currently get it from methane. First thing we should do with green hydrogen is make it replace the fossil based hydrogen.

Once that's done, we might have the abilities to expand the facilities to create more hydrogen. Those are expensive, so they won't just run if the electricity is almost free. You don't buy such a machine and have it idle most of the time - up hydrogen won't ever be free, for the cost of the electrolysers alone. The tech overhead needs to be paid for. Same goes for transport.

You know what can be done with surplus electricity more easily and with the existing infrastructure? It can be put in batteries.

But as I write, it's all probably moot, because the conditions will arrive too late, so batteries will probably have taken over everything.

It won't be because hydrogen's late but because batteries are - in cars - the less complex, more reliable and cheaper solution.

But maybe in planes because of the better weight energy ratio, and maybe also in trucks to be able to have higher load capacity. And as I write instead of fuel cells, the hydrogen can be used directly in jet engines, but also in an only slightly modified ICE car.

It makes sense for planes, I never argued against that. Especially because the weight is reduced while you use the fuel you're carrying. I don't see it for trucks simply because the infrastructure for battery electric trucks is already everywhere, but yeah, charging several hundred kWh takes time and such large batteries have a price tag attached to them - but that's not what we were talking about, no?

Disregarding the problems you describe there are actually hydrogen fuel cell cars on the roads, that have been sold commercially and been available since 2021.
For instance the Toyota Mirai:
https://en.wikipedia.org/wiki/Toyota_Mirai

So kind of weird to claim a product that is actually available now, isn't possibly feasible in a future where it can have basically free fuel!
It probably boils down to which form is cheapest to create AND store, hydrogen in containers or electricity in batteries. Batteries will always be more expensive than a container, but hydrogen has way greater loss. So it's not an obvious calculation with one solution that fits all cases.

I'm well aware of hydrogen cars existing. Hydrogen based cars have been around for so long that most of them got discontinued quite some time ago, such as the Mercedes GLC F-Cell. It's not that I say that you can't build them, I just say that there's no economically viable use case for them. There won't be free fuel for them because hydrogen will remain rare as manufacturing and transporting it will remain difficult. Fuel cells and tanks that withstand 400 to 800 bar pressure aren't free either - and neither is the whole infrastructure to distribute it, which goes way beyond the transport as a fuel cell gas station is a much larger hassle to set up than hooking up a charging station to an existing energy grid. The point remains that even if electricity gets to be free at times, it's much more efficient to just store it in batteries than to translate it to hydrogen, ship that around the country or to another continent, store that in a cryo tank for days or weeks and then translate it back to electricity to then be stored in the battery your fuel cell based car still needs to operate (albeit smaller than those in BEVs, granted). It just doesn't make sense to assume that hydrogen, with the given overhead attached, will ever be free. Even electricity often isn't when it seems to be (e.g. I might see a price of -2 cents at night but the bill then includes a 12 cents "network fee" per kWh), so it surely won't be the case for hydrogen either. And then, with all the overhead attached, you still need several times the energy to move the car the same distance as a battery electric car which makes it even less of an economical use case, assuming there's always a price attached to the energy you need (which there is - even for my own photovoltaic setup, I usually calculate about 6 cents per kWh to account for the depreciation of the modules).

So yes, it's an obvious calculation. Batteries aren't free, I get that, but neither are fuel cells, containers and smaller batteries. Just as with ICEs, the running costs will be the defining aspect of the TCO of a car and there's just no way for hydrogen to ever meet the price of putting electricity in a battery because a fuel cell car does that, too, plus all the conversions. Its operation is literally a superset of a BEV, so the running costs will be higher, unless you use fossil hydrogen, which i hope nobody ever seriously suggests as a large scale solution to de-carbonify traffic.