Not sure what your point is?
In addition to charging from a current source, a battery-powered EMU would be able to recover some energy from regenerative braking.
I looked up the timetable for the Windermere branch which was mentioned earlier in this thread and found that, under the current timetable, the largest number of Oxenholme-Windermere-Oxenholme round trips operated by a unit before it goes elsewhere is five, which happens three times a day.
The time it takes to run those five round trips is around five hours, with the time spent waiting at each end of the shuttle service during that time being 106, 102 and 77 minutes respectively. These times are based on the public timetable, the actual layover time at the termini might be even more if the public timetable differs from the working timetable on the 'under-promise, over-deliver' principle.
So to operate this branch with battery power, you would need the following amount of energy to be enough to get through five round trips:
- The unit arriving from somewhere else carrying a full charge, courtesy of running on electrified lines.
- Energy stored from braking for between 32-38 station stops.
- 77 minutes worth of time spent charging from the wires while waiting between shuttle runs at Windermere and Oxenholme.
That actually seems like a fairly decent basis for a specification - running five shuttle trips on a 10 mile branch line after starting with a full charge and having it partially replenished during the shift by some charging from overhead lines.
As already mentioned, there are questions about how appropriate it would be for real world usage. There are some pretty exotic minerals used in making batteries which will increase in price if the most easily extracted sources get depleted by increased building of battery-powered vehicles, while the classic materials used in stringing up the overhead lines are relatively abundant.
Could this technology not be used on the the FGW electrification? Surely it would be much cheaper and less disruptive to use batteries through the Severn tunnel?
No.
Getting a small to medium-sized fleet of battery EMUs to run lower speed services on some short lines which branch off electrified routes
might be cheaper than proper electrification, fitting batteries to every single train which uses a fairly important mainline is most definitely not.
All that is needed to do the Severn Tunnel with minimal disruption is good phasing of the works - or serious consideration of when it will become advantageous to stop spending money on it and build a modern twin-tube replacement which would incorporate the 140+ years of expertise gained since then, and be equipped with future compatibility for 250+ km/h high speed rail and larger loading gauges.