Clattery when cold
Join Date: Feb 2012
Location: United Kingdom
County: Greater London
The plate is the small flat box of connections, fuses and straps that sits on top of the +ve battery terminal.
But it sounds as if you do have a genuine battery drain happening whilst the car is parked, and then things go nuts because of the low volts.
So you need to establish whether there is a drain and what is causing it. This is alas, a fairly common fault, though there are lots of possible causes ranging from boot or glovebox lights not going out, to aftermarket radios taking current when they are switched off, to a defective ignition switch, alarm or alternator diodes. Finding out what is a PITA, and needs a meter capable of reading amps, not volts. That is connected in series with the battery +ve terminal and the circuit that is draining is isolated by removing and replacing fuses. If the drain disappears you've localised the circuit that is stealing your electricity.
If you search the forum for 'battery drain' you'll find a lot of information. Unless you are obsessive, determined and not easily frustrated, this might be the time to take it to a specialist auto-electrician, who should be able to pin it down quickly.