Merc. 350 SL. 2nd battery goes flat in storage. If I connect a 'trickle charger' to the main battery, will this charge the 2nd battery too ?

If both batteries are connected in the vehicle the trickle charger should keep them both charged.

Hi so the issue is about a drain on the battery as car are meant to be driven not left standing. if a battery has drained below a certain point it is a waste of time trickle charging as it will not return to its full charge state.

it depends. Is there a battery isolator between them? A battery isolator only allows current to flow one way, BUT the charging system on the car has to be operating. Otherwise each battery would need it's own trickle charger. If they are paralleled (+ to + and - to -) then the 2nd battery would drag the first battery down with it. It sounds as if there is an isolator or something that controls the 2nd batterys charge. I'd put a trickle charger on both in storage. if the 2nd battery is going dead yet the main battery is fine, your 2nd battery is shot and you need another. in that case I'd recommend replacing BOTH batteries at the same time and then use a good quality trickle charger on each battery to keep them in good shape while in storage