New car, flat battery: causes?

I certainly wouldn't put any faith in it and new ones are so cheap that it's not worth the hassle.

I've had this with railway batteries, the 2.2 volt type which stand about a foot tall (3 or 4 per tray, 48 odd to make 110volts and a couple of thousand amps cranking power :o) ). The cells were recovered from Mk3 Restaurant cars at a breakers as I needed a set for a shunter engine whose original set were life expired at 18 years old. The cells were initially found drained of juice and were first reconditioned by each cell spending a week being trickle charged at 5 amps to recondition the cell plates. Once I had enough to make up 110 volts they were all stringed together and put on the proper charger unit to bring them all up to the same charge level. Fitted, five or six deep discharges, all OK and still running.

Personally, as a battery for a car is cheap by comparision, I'd fit a new pack but also check the start/charge circuit at the same time.

PDH

Im tempted to agree here, they are really cheap these days. They can be semi-fixed by a strange form of pulsed charging and draining at high current which destroys the sulphate bridges (IIRC) that cause the internal drain but its only something like 50% effective and will cost you a tenner in parts to build assuming you have the knowledge to make up your own PCB.

An ancient charger with no form of regulation - the type that has merely a resistor in series with the DC supply will often give quite high off load volts - which it will do into a sulphated battery which has gone high impedance. This might well sort it out enough to be useful again, but expect it to take over a week...