BenzWorld Junior Member
Date registered: Feb 2012
Vehicle: 2006 CLS 500
Location: West of Boston, MA
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If you changed the battery and still just hear the relay clicking, then either the current is not making it to the starter or else the starter is toast.
Seeing as how it was running ok, then I would concentrate on the connections first. The easiest way to find what's going on is with an inductance ammeter. Cheap and simple to use. Like $10-$15 or so.
You don't have to make any connections, just hold it over the positive cable while someone turns the key. Lots of amp draw, like 400-600 and the starter is probably no good.
Very little draw, like 50-100 amps means a connection is poor: ground to body, ground to battery, positive cable to starter, solenoid connection or cable to starter brushes.
The interesting thing you said about battery charging to 50%, if you didn't have the charger connected directly to the battery posts and to somewhere else, like battery positive and negative to ground on the vehicle, then it's the ground cable/clamp or like that.