Can You Jump Start A Motorcycle With A Car Battery

Having a discharged battery can leave the rider stranded on the side of the road, forcing one to try to crank the motorcycle's engine in various ways, including push/bump starting, using a motorcycle jump starter, car jump starter, and similar.

However, people often wonder if they can jump-start a motorcycle with a car battery. After all, in most situations, both motorcycles and cars use 12V electrical systems, right? Well...

Updated: May 17, 2024.

Discharged Motorcycle Battery

When trying to start a motorcycle or a car, the last thing that a rider/driver wants to hear is a dreaded "click" (or "clicks"). Or maybe the battery is not completely dead, and one can hear the engine starter making a few slow, lazy cranks...

When that happens to a motorcycle rider, one can try several ways to start the engine, including:

• push/bump start,
• using a jump starter,
• using jumper cables and another motorcycle,
• using jumper cables and a car.

While the first three methods are probably tried and tested by most riders, the last one, jump-starting a motorcycle using a car and a car battery, tends to confuse many riders leaving them wondering if such a method will damage their bike.

Why Motorcycle Battery Gets Discharged

There are many reasons why motorcycle batteries can get discharged, including high parasitic drain, lights left turned On, issues with the alternator/stator generator, etc.

However, as the battery ages, its ability to store the charge and release it when needed decreases. But, this ability gradually decreases - as long as the battery is able to crank the engine, it is still good enough to, in combination with a working alternator/stator generator, power the onboard electronics.

Thus, if the battery is not completely dead, after jumpstarting, the motorcycle's engine (and onboard electronics) will continue to operate as long as the alternator/stator generator is working.

On the other hand, if the alternator/stator generator is not working properly and the battery is discharged, the motorcycle's engine can be jumpstarted. Still, the very moment the external power source is removed, the engine will most probably shut Off.

For short, jump-starting a motorcycle engine works when the battery is almost dead, but the alternator/stator generator is working more or less properly.

Jump Starting A Motorcycle With A Car Battery

When all other options are unavailable, can a motorcycle be jump-started using a car battery?

Yes, it can.

Is it a safe way of starting a motorcycle?

No, it is not.

And here is why...

Both motorcycle and car electrical systems operate at 12V, at least in theory.

In real life, voltage depends on the battery condition, remaining battery charge, if the engine is working or not, and similar.

But, the voltages of both electrical systems vary between ~12V (voltage of almost discharged lead-acid battery) and 13.7-14.7V (charging voltage of the alternator, when the battery is completely charged).

So, what is the problem?

When You connect a 10Ah or 70Ah 12V battery to a motorcycle starter, the starter will draw the current (given in Amps) required to start the engine, and this current will be practically the same for both 10Ah and 70Ah batteries.

This is because the internal resistances of 10Ah and 100Ah batteries are much lower than the internal impedance of the motorcycle starter.

However, if You connect the fully charged car battery (let's say 70Ah) to a discharged motorcycle battery (let's say 10Ah), the initial current from the car battery to the motorcycle battery can be rather large, much larger than the recommended lead-acid battery charging current (which is usually around 0.1C to 0.15C when the battery is charged using dedicated battery chargers).

And if the car's engine is running and the car's alternator is keeping the voltage even higher, this initial charging current may fry the motorcycle battery permanently.

Are there any dangers to motorcycle electronics? When the car battery is connected to the motorcycle battery, generally, there are no dangers for motorcycle electronics. Still, if the engine is running, high voltage and possible voltage spikes may damage the motorcycle electronics.

Since modern cars use voltage charge controllers that, in combination with the car battery itself, smooth out the voltage well, the danger of spikes is limited, but why risk it ...

How To Crank The Motorcycle With A Car Battery

Again, this should be done only if other methods are unavailable.

Take jumper cables and connect the red (positive) terminal of the car battery with the red terminal of the motorcycle battery.

Now, carefully connect the black (negative) terminal of the car battery with an exposed metal part of the motorcycle - not connecting the batteries directly gives additional protection to the motorcycle's battery.

When the car and motorcycle are connected, let the car's battery recharge the motorcycle battery for 30-60 seconds - this surface charge increases the voltage of the motorcycle battery, letting the motorcycle alternator/stator generator charge it.

Note: if you are worried that the battery is fully discharged and that this charging current may damage the battery, try to crank the engine right away and remove the cables ASAP.

Crank the motorcycle engine and disconnect the cables in the opposite directions of connecting them.

If the engine doesn't start or the engine stops the very moment the black cable is disconnected, let the car battery recharge the motorcycle battery for 30-60 seconds more.

Try to crank the engine the second time.

Now, regardless if the cranking was successful or not, remove the cables, let the car's battery recover for a minute or two, crank the car and let it recharge the car's battery for a few minutes, just in case.

Note: This can be done even with the car's engine idling, just in case the car's battery is, for example, older and you don't want to risk both vehicles getting stranded.

If the cranking was successful, feel free to ride the bike for at least 30-60 minutes, after which check the battery's voltage and check if the battery is strong enough to crank the engine - of course, turn off the engine only when You get home or to a workshop.

If the cranking is unsuccessful and especially if the engine shuts off the very moment the black jumper cable is removed, take the motorcycle to the workshop and let them check the battery, alternator/stator generator, charging electronics, and the rest of the bike's electrical system.

Long Story Short: It is possible to crank a motorcycle using a car engine, but some risks are involved.

Most motorcycle manufacturers, and especially the manufacturers of motorcycle batteries, will tell You not to do it. In fact, it is easily possible to lose the warranty on the battery. If there are issues with motorcycle electronics after cranking with a car battery, the motorcycle warranty may also come to question.

Note: The author of this article has cranked motorcycles several times using car batteries, even with car engines on, with no issues. Knock, knock on wood...  But that was me and my responsibility.

But, whatever You do, it is your responsibility. Again, whatever you do, stay safe.