Clattery when cold
Join Date: Feb 2012
Location: United Kingdom
County: Greater London
It seems your battery is either not getting charged, or is incapable
of holding one, and/or incapable of delivering one. Here are a few things to check, probably in this order.
What is the battery voltage? It should be at least 12.5v across the terminals with nothing turned on. If less, you'll need to stick the battery on a charger. If you don't have a multimeter, a 12v 21w (-ish) indicator bulb, sticky tape and wire should give bright, white light. Yellow-orange = flat.
Since you've disturbed the alternator, is it fully plugged in to the loom?
Possibly a poor battery or earth connection that is going high resistance once the starter load is applied. Remove battery connections, use fine glasspaperr or a wire brush to clean the battery posts so you have bright metal - not gray oxide, which is a non-conductor. Do the same to the connectors then do them up tight.
If still no good, don't assume a brand new battery isn't defective, they quite often fail in the first few weeks from a bad internal connection between cells, and/or a defective cell or two. The battery appears fine with no load, then you get about 8v as soon it's asked to provide a few amps. Cheap batteries are very fond of this sort of nonsense in my experience. Get it load tested where you bought it, but you'll need to charge it up first. Should be a no-argument warranty exchange if cattled.