Constant drain of 1 of 2 batteries

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crazy charlie

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1988 Regal 280 twin 5.0LX with alpha drives.Replaced battery that I assumed was bad. It was not. The replacement is constantly getting drained to zero. Every switch and light is turned off.Fridge is on shore power. I'm puzzled??? Where to look?? Nothing is on. Charlie
 
alldodge

alldodge

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Alternator, Bilge pump, Mercathode
Is it a start Bat only or combined with house Bat?
 
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Chris1956

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Mar 25, 2004
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28,937
Usually, alternators bilge pumps and Mer Cathodes bypass the battery switches.
Alternators have diodes which can short to ground. The test is to check resistance from battery pos cable to ground, with the battery switch off. You should have infinite resistance on the Rx1 scale. If you do not have infinite resistance on that scale, you will need to trace it to the pump, cathode or alternator.
 
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cwella

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Jul 10, 2026
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crazy charlie said:
1988 Regal 280 twin 5.0LX with alpha drives.Replaced battery that I assumed was bad. It was not. The replacement is constantly getting drained to zero. Every switch and light is turned off.Fridge is on shore power. I'm puzzled??? Where to look?? Nothing is on. Charlie
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A new battery going flat with everything off points to a parasitic drain. Check with a multimeter between the negative terminal and cable to see if there is current draw.

Common culprits on boats are the bilge pump float switch, stereo memory, alarms, engine electronics, and a faulty alternator diode feeding power back. Finding the draw is the quickest way to locate the problem.
 
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