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Hi, my name is***** there is only one battery on the ML. If your battery light remains on I would suspect either a defective replacement alternator or a fault with the wiring to the alternator. I've attached the wiring diagram for the alternator. G2 is the alternator. The blue wire is your charge light. When the alternator charges it puts 12V to the blue wire to cancel the 12V coming from the light bulb and extinguish the light. Either the alternator is not putting 12V to the blue wire or the blue wire is shorted to ground between the alternator and the light.
Let me know if you have any questions.
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