It's a valid concern (I suspect cost is the fly in the ointment too), though it's worth noting that crude oil prices are notoriously volatile. Of interest, was Walmsley referring to lead/acid, LiCad batteries or both? Do you happen to know when his appraisal was made? Manufacturing costs have reduced somewhat over the last few years, at the same time as battery performance has improved (rather too slightly). There are several significant costing differences between the two technologies. The Drumm cells on the GSR managed just over a decade (the diagrammed use from 1933-43 involved around 17,500 miles per unit per month, before timetable cuts during "the emergency"). The lead/acid of the Derby BEMU (conversion) gave rather less. (The Drumm units gave 15-20 years further service as hauled stock, the Derby BEMU awaits new batteries at the Royal Deeside Railway). Another variable is the cost of recharging. The Irish installation was government driven, largely to use some of the spare generating capacity of the Shannon Hydro-electic scheme and pretty much hit the buffers when government support for the whole Drumm project was pulled, just as replacement batteries loomed. The last unit ran in service in 1949. Another downside of battery units compared with an EMU is weight, with a lot of energy consumed just lugging about the power cells themselves, though probably no worse than a loco in working order. The Drumm units had an axle load of around 17t 5cwt. I can't find the figure for the BEMU, though the batteries themselves weighed 8tons (neccessitating chassis strengthening).