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Discussion Starter · #1 ·
This has nothing to do with our cars, but I wonder if the experts here can explain this.

My Honda motorcycle owner's manual says
Jump starting using an automobile battery is not recommended, as this can damage your motorcycle's electrical system.
I don't understand this. I thought electrical devices draw needed current up to what is available. I can plug 1 amp devices into an outlet that can provide 15 amps and nothing explodes. Apple officially approves using an iPad charger to charge an iPhone.

How might the bike's electrical system be damaged by using an automobile battery? The specified fully charged voltage is 13.0 - 13.2 V, which sounds common.
 

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the starter motor in a motorcycle will try to draw as many amps as it can. A motorcycle battery has an amp limit due to it's size. This limit is more than enough to spin a motorcycle engine and the associated wiring can handle the load.
Using a car battery can potentially allow more amperage than the wiring can handle. The wiring can fry before the starter motor will.
This is how i've understood it from way back when owning my honda's (70's). ....I currently own an original, unrestored, pristine 550F.......hope this helps.
i should add, if the engine fires right up, you may get away with jumping a bike with a car. If you have to crank and crank........you might smell smoke...
 

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the starter motor in a motorcycle will try to draw as many amps as it can.
No.

Amp draw by the starter is dependent on load. A starter will not just draw unlimited amps. That's not how electricity works.

As for why you shouldn't jump a motorcycle using a car, the answer, at least for my Harleys, has to do with how the voltage regulator works.

On modern car alternators the amount of energy produced is variable and determined by how much current is supplied to the electromagnets in the armature. This allows variable output determined by the voltage regulator even when rpm is constant.

On some motorcycles the alternator users permanent magnets, which means that the alternator always puts out maximum power for a given RPM. Voltage through the rest of the electrical system is controlled by a shunt regulator which, instead of "dialing down" the alternator output, shunts the excess current to ground. This is simpler, but generally only suited to relatively low amounts of current. Motorcycles don't usually have high output alternators, so this is fine.

The problem comes when you hook up a running car engine to a motorcycle system and the two voltage regulators have different set points. Specifically when the car's system is attempting to regulate to a higher voltage than the bike's (say, the car is seeking 13.5 volts but the bike is trying to regulate to 13.2 volts). When this happens, the motorcycle's regulator attempts to shunt current to ground until the voltage drops to 13.2, but whenever the car's regulator sees the voltage drop below 13.5 it increases current from the car's alternator. This results in a cycle that ends with the bike's regulator trying to shunt far more current than it's designed to handle and quickly burns itself out.

Strictly speaking, using a car battery that is not mounted in a car will be perfectly fine. The problem is using a running car engine. However, this distinction is a bit too nuanced for an owner's manual and so, out of an abundance of caution they simply say not to use any automobile battery.
 

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Discussion Starter · #6 ·
Strictly speaking, using a car battery that is not mounted in a car will be perfectly fine. The problem is using a running car engine.
Can I infer that it's OK to use a battery mounted in a car with the engine off, or is its voltage regulator always "live?"
 

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Can I infer that it's OK to use a battery mounted in a car with the engine off, or is its voltage regulator always "live?"
Voltage regulator controls alternator output, so as long as the car is off, you should be able to jump from a car battery just fine. Just be sure that the car is off - engine not running, key not in the ignition, lights not on, completely off. (Technically, as long as the engine's not running it *should* be OK, but better safe than sorry so turn it all off.)

Also, take care with the jumper cables. Automotive jumper cables usually have quite large clamps and it can be very easy to accidentally short an automotive jumper cable clamp against the motorcycle's frame as you attempt to get a connection to the much smaller motorcycle battery terminals.

Going forward, I highly recommend some form of battery tender for your motorcycle batteries if you aren't already doing it. The batteries in my motorcycles went from lasting about 2-3 years to lasting 6+ years when I got a battery tender and started hooking up to it religiously. It paid for itself in battery savings very quickly.
 

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Thanks for an interesting (non Volvo) question and a great answer. In the good old days we avoided this issue by using the kickstarter.
 
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