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A few years back I picked up a 1989 Harley Davidson Sportster. I t was originally an 883, then the previous owner put in a 1200 cc motor. He threw a rod, so he said and had to have the cylinders bored out to 80 cubic inches. I was having trouble with the battery and finally replaced it with a marine battery. I had to store the bike for almost 3 years. I replaced the battery with a Duracell Ultra. At first start it was alright, but then noticed the headlight dimming. I replaced the voltage regulator and put a new battery, Another Duracell in. It lasted about 3 days and then wouldn't turn the starter over. Should I put another marine battery in it and see if that solves the problem? Any help would be greatly appreciated.

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There are several possibilities:

  • The bike's battery is bad. This is by far the least likely cause, as the previous battery behaved exactly the same way. The chances of having two batteries fail sequentially in the same manner is very low.
  • The battery is not being charged when the motor's running. If there's something awry with the charging circuit or its components or wiring or connections, sufficient electricity won't be delivered to the battery, which will become weaker and weaker and then unable to start the machine.
  • The electrical system has a parasitic drain: even with the ignition off, electricity is being "used" by some component on the bike, and is slowly flattening the battery's charge. This can occur because of a wiring or component fault.

The second and third possibilities are the most likely. You will need to test the charging system for correct output with the motor running, and test the bike's electrical system at rest for a parasitic load. Then you'll have to rectify whatever faults you might discover. If you can't do this work yourself, you'll need a competent motorcycle mechanic to do so.

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    Agree. I would charge the existing battery off the bike then refit it. At least you will know If the battery is capable of being charged.
    – HandyHowie
    Sep 10, 2019 at 5:30
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    be aware that if the charging system is the fault, then likely the battery is also bad at this point. especially given that it's been replaced twice now, and failed twice. batteries that have been drained severely will be permanently damaged.
    – Christopher Hunter
    Sep 12, 2019 at 22:36
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This answer assumes there's a fault in the electrical system somewhere.

With the bike off, remove the cable from one terminal of your battery and put a multimeter (IN CURRENT MODE!) in series with the battery terminal and cable.

You should see zero amps flowing through the meter. If there is current flow, even in the milliamps range, you've got some kind high-resistance fault in electrical system somewhere.

A dead short would allow massive current to flow and would likely damage the battery and the metal around where the short occurred, but a high-resistance fault could be leaking small currents from the battery to the chassis/frame through wear spots in the cabling, and rust, oil, grime, etc., is providing the resistance that prevents this from becoming a dead short. It will likely not be obvious.

If you see current flow, you can switch your meter back to volt mode and do continuity or resistance tests along the cabling, disconnecting behind you if possible as you go, to isolate the fault point.

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