Hooking up to any convenient wire that carries 12v when the ignition is off
and the key is out of the car (i.e. the brown on an MGB) and ground will be
fine. Hopefully the charger will be fused and so any reverse connection
will blow that rather than causing any damage to either charger or car. If
you have a circuit that is 'always on and fused' (like the purple circuit in
the MGB) then you will have 'belt and braces'.
However you shouldn't leave a conventional trickle charger connected for
long periods or it will boil the water out of the electrolyte. There are
'conditioning' chargers which are intended for long-term connection which
modulate the amount of charge according to the state of the battery.
But having said that an MGA should be able to hold its charge for a couple
of months at least. If it doesn't either there is a drain or the battery is
bad anyway. A drain could be caused by any aftermarket electronics that
might have been added such as alarm, or audio systems. But if you have none
of those and the electrics are standard and the battery good there must be a
fault. You can track this down by removing the battery ground strap and
connecting an analogue voltmeter in its place switched to its 12v scale. I
say 'analogue' as I cannot be sure what results a digital instrument may
give connected like this. If you see anything registered on the meter there
is a drain. If you see 12v registered it is a significant drain. Note that
if the car has been fitted with an alternator you may well get a few volts
registered, this will be from the normal reverse leakage of the alternator
diodes and can be ignored. Unplugging the alternator should drop the
reading to zero. Then it is matter of disconnecting things that are
'always live like (in an MGB) the interior light and horns fuse, then
disconnecting the live wires from the ignition switch, lighting switch, and
any other components that have 12v connected to them all the time until the
reading drops to zero. When it does, the problem lies in the last circuit
to be disconnected. Note I have no experience of MGAs so I don't know how
relevant the above is.
But at the end of the day a battery disconnect switch is a good idea anyway,
if it has a removable handle as something of an immobiliser, and for safety
if one of the many (in an MGB at least) unfused 'always live' wires should
happen to short out.
PaulH.
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