Trucking batteries around to replace drained ones with charged ones is the only thing sillier than using electrolytic hydrogen to make "renewable" methane for NGVs.  In any first-world country, it's much more sensible to have a "pole pig" on a skid that fits in a truck and run cables to a charging station.

The only real use for this battery is as a buffer to permit high power charging when the power feed to the site is limited.  Everything else, such as buffering unreliable power sources like wind and PV, is better done with existing resources by having the PEV fleet plugged in when parked and operated as demand-side managed loads.  It only takes a relatively small number of PEVs (a few tens or hundreds of thousands) to provide all the regulation services a regional grid requires, and many areas have that many already.  Level 1 connections will usually suffice for this, and the typical PHEV charging time on a Level 1 connection is a decent match for the length of the work day.

Putting these buffered high-power chargers at highway rest stops, where the installed power feed was specified for just one building and can't be upgraded easily to the 100's of kW level, is one of the few real applications today.

people prefer having their car battery fully charged whenever the want to start driving.

In other words, by the end of the work day is perfect.

EV-batteries are too expensive for this application.

The only expense is in the chargers.  The EVs are already there.

I doubt many people will be willing to damage the batteries of their cars to stabelize the grid.

There is no such damage.  Operating the chargers as a variable load only (no backfeed to the grid) is sufficient to provide regulation and soak up quite a bit of variability in supply.  The AC Propulsion V2G test actually improved the capacity of the used lead-acid vehicle battery.  (full archive of ACP white papers here.)

Mass-produced dedicated battery packs (which are cheap because often second-hand cells) would be preferable I foresee.

Dedicated battery packs are net energy and money sinks.  The goal should be to integrate PEVs as the preferred method of grid regulation and build the fleet up as quickly as possible.  The dual-use technology adds benefits without adding significant costs.  Profits generated thereby can be used to help roll out more chargers and subsidize PEVs.

Assuming that most of the future extended range BEVs will be equipped with an average of 100KW batteries

Waste of money in the short run.  Until most of the LDV fleet is at least PHEV, long-range BEVs use far too much battery to replace far too little petroleum.

a high percentage (50% or so) of the accumulated-stored energy could be made available:

(1) to regulate the grid.
(2) to supply fill in energy when REs are not producing enough to meet demands.

Harvey, is THIS your "miracle" energy storage which shoves hypedrogen out of its starring role?  Vehicle batteries, which hold hours of net energy demand when the "renewable" system requires months?  Ye gods, my estimates of your intelligence were WAY too high.

(3) to supply energy for electrolysers to produce H2 for local industries and FCEVs.

Yup, I was right.  You want to replace hypedrogen reservoirs at perhaps 1-5$/kWh with batteries which are $100/kWh and up.  You are insane.

The question is how to make better use of the costly battery packs installed in current and future extended range BEVs.

That is misdirection, disinformation.  The question is how to reduce petroleum consumption specifically, and fossil fuel consumption in general, the fastest for the least money.  Extended-range BEVs are so ineffective at this, they should not be any kind of priority for at least another 10 years.  Turning almost every vehicle sold into something between a start-stop hybrid and a PHEV should be the top priority for the transportation system; cogeneration should probably be the top priority for heating plants.  We have enough battery manufacturing capacity in the pipeline to make several million PHEVs per year, so making everything else mild or full hybrids should be no problem.  Things like electric power steering and electric A/C are major fuel savers by themselves, and electric A/C gets rid of refrigerant leaks from flex hoses to engine-driven compressors.  This is a no-brainer, unless of course you have no brain.

Using some of the 100 KW batteries accumulated energy to help meet peak demand periods and help to make intermittent REs = 24/7 is not impossible

For electric power plants, high-temperature nuclear with molten salt energy storage is vastly preferable to the unreliability of "renewables".  S-PRISM based plants with 934 MW(e) steam turbine generators and 1.78 GWh(t) of salt storage are sufficient to achieve 99.9975% emissions-free electric generation based on Bonneville Power Administration load curves and 90% reactor capacity factor.  "Renewable" schemes require gigawatt-YEARS of storage per unit.

If Harvey was (a) numerate, (b) sane and (c) not a shill for ruinables über alles, you'd think he'd favor the cheap and achievable nuclear replacement for fossils over the 4000x as difficult "renewable" scheme.  He has shown no hint of taking the route of the possible over the impossible.  Innumerate, insane, shill:  pick one or more.

PHEVs are the most economic measure for replacing petroleum with electricity, until almost all vehicles are PHEVs.  That will be a while.  The USA has some 250mm LDVs; at 10 kWh apiece to convert to PHEVs, the total battery capacity is 2.5 TWh.  This is 50 years of pack production from a single Gigafactory.  It would take only 3-4 Gigafactories to put a 10 kWh pack in every new LDV in the USA, so the HEV/PHEV route is the fast track to replacing 2/3 or more of motor fuel consumption with electricity.  The USA purchases about 17mm LDVs per year; putting 10 kWh apiece into 250mm vehicles at 170 GWh/yr (3.4 Gigafactories) would be accomplished in just under 15 years.  The aggregate storage of 2.5 TWh would constitute about 6 hours of average US electric consumption.  This is adequate to replace at least 2/3 of petroleum demand, but grossly insufficient for Harvey's delusions of buffers for his precious unreliables.  Harvey isn't smart enough to cook this scheme up; it's his handlers/sources who are working to deceive everyone else, and (not coincidentally) maintain the dominance of fossil fuels for the foreseeable future.

If Harvey gave a damn about the planet, he'd shut up now and never post again.

(a) numerate, (b) sane and (c) not a shill for ruinables über alles
You need mental help.