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I recently left my Harley Davidson 883 Superlow in ignition on for 8 hours, headlight drained all the battery. This was my first time jumpstart experiment. Had following few questions, happy to be corrected or redirected to previous questions. Thanks.

  1. Voltage dropped to 4.5 V I am currently charging the battery with battery tender, is it impossible for this battery to come alive? I read many places saying once battery goes below 10.5 its dead, should I just tow my bike or continue charging? ( After 24 hours charging now voltage is around 6.8v, I've got battery tender junior .75 amp charger)

  2. When I tried to jump it using my car battery, (afer multiple failed attempts) all the lights came up, engine did the booting sound, but when I pressed start button engine is trying to start and fails. I checked the voltage (at the bike end) its 12v only thing is cables are 12ft, and my theory is because of this length voltage is maintained but not current flow (amperes) is that right?

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  • The answers here are good but I would add that draining a battery that low does have a chance of permanently damaging it. (below 10v is usually bad). If that's the case, typically you can charge the battery up, but it will not be able to hold a charge and you will have to constantly jump/charge it. If this happens just replace the battery.
    – Christopher Hunter
    Commented Jul 5, 2019 at 19:01

2 Answers 2

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Basic answer - no the length will not matter. For all practical purposes the resistance of the jumper leads can be assumed to be zero in this situation. Jump leads will be designed to cope with very high current draw (hence the thickness of the wire, and the insulation around it)

From your comment that you had the car engine off, it is extremely likely that is your only real problem here.

Run the car engine while trying to start the motorbike. Sometimes when trying to start a car with a discharged battery you might need to rev the engine (on the car which is running) but for a small engine like a motorbike engine this should not be necessary.

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It is generally thickness that matters on jump leads more than length, however a motorbike shouldn't require a huge amount of current to get it going, so cables capable of starting a small car ought to be fine for most bikes.

You're doing the right thing by trying to trickle-charge the battery. If it is a relatively new battery it ought to be OK, but an older battery (more than 3-4 years) will be affected by being fully discharged like that, and chances are will struggle to hold a charge for long. If you've only got up to 6.8v on a 12v battery then I'd suggest that is the case unfortunately, so you may well need a new battery.

If you're trying to jump it from a car battery, make sure the car is running at the time so you can use the car's alternator as a power source - you should get around 13.8v if the alternator is in good condition.

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  • Thanks Nick. I read multiple places that the car should be off, I was trying to jump it just with battery, without car alternator.
    – Sanjeevakumar Hiremath
    Commented Sep 19, 2011 at 17:28
  • I agree with what Nick says. Also, you may be able to "fix" your battery (for a while) by adding water to it or replacing the acid. Some maintenance free batteries have caps you can pry off for that. (This is probably not recommended by the manufacturer, so it may be dangerous.)
    – xpda
    Commented Sep 19, 2011 at 19:27
  • @xpda Do motorcycle batteries have liquid acid electrolyte?
    – juhist
    Commented Feb 11, 2017 at 12:52
  • @xpda totally depends on the type of battery. some are traditional lead/acid and some are sealed.
    – Christopher Hunter
    Commented Jul 5, 2019 at 18:58

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