All of the Hitachi AT300 trains, like all other EMUs on the UK railway that offer charging power to passengers, AFAIK interrupt non-essential services for a few seconds when the train is not connected to the primary power sourece, i.e. 25kVac or 750VDC. The trains are supplied against a procurement specification that the customer provided (DfT I assume). The manufacturer hasn't supplied a train that "switches off every few minutes uncontrollably", as switching off is very much controlled, every time the track magnets tell the train that a primary power supply interruption is coming, and the specification would have included that ancillary supply in the specification because the customer was offereing it as a charging facility. Had Hitachi put that supply in the permanently on services group, the batteries would need to be heavier, have a shorter off-line capacity, and certainly increased the capital and running cost of the fleet.
As far as giving every passenger a specification of the charging supply, that's nonsense. The railway doesn't give out an air conditioning specification, stating the maximum deviation from setting, nor the maximum ambient temperature that it works in, simijlarly, there isn't a maximum brake retardation spec., so that passengers no how much their drinks might slop. Battery charging power is not a contractual part of the deal, and as edwin_m says, there's no claim for it not working at all, let alone any interruptions that individual passengers might not have prepared their portable devices for.
* the IC225 is irrelevant, - it isn't an EMU, - it's a rake of coaches pulled by a locomotive, thus it needs to have some battery power for lights etc., when it is without a loco connected to it.