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Motorcycle and car batteries work more or less the same way. Shouldn’t it follow that the charger you use to charge the car battery can be used to do the same on the smaller motorcycle battery? That is not the case. You can learn how to charge a motorcycle battery with a car charger. It is discouraged but not impossible.

 Most motorcycle owners are also car owners. When the motorcycle battery suddenly goes flat, quite often the only battery charger available is one used for car batteries. Can it be used for the motorcycle battery? After all both are 12 volt batteries.

This is a common question. The most straight forward answer is ‘don’t do it!’  It is generally not recommended because you run the risk of overcharging and damaging the smaller motorcycle battery.

The question remains, can it be done? Yes it can. It is possible to charge a motorcycle battery with a car battery by mitigating amperage or using a smart charger. Even then, it is highly discouraged. Experts say it should only be attempted as a last resort.

 Can You Use A Car Charger Battery To Charge A Motor Cycle Battery?

The short answer to this question is that you really shouldn’t. If you can avoid charging a motorcycle battery with a car battery charger, please do. The charge rate used by a car battery charger is way higher than what a motorcycle battery can handle so it will most likely fry the motorcycle battery to destruction.

Is that to say that there is absolutely nothing you can do if you have a dead or weak motorcycle battery and the only charger available is for car batteries? No. There is a way to go about it. That is where the long answer comes in.

This method takes some careful work and more than a fair level of understanding of the battery’s amp- hour rating. This figure describes the length of time the battery can last if it is discharging at a fixed rate. It is usually expressed in relation to a 10 hour period. For example a battery with a 16 amp-hour rating means it can discharge 1.6 amps for 10 hours before it goes flat.

It also calls for some mathematical calculations. Why so? The general rule to follow is ‘don’t charge a battery at more than one-tenth of its amp- hour rating.’ If a battery has a 20 amp rating for instance, it should not be charged at more than 2 amps over a period of 10 hours.

Anything more than this is considered overcharging which can lead to a myriad of complications. For one the battery could overheat. The electrolyte could boil. In rare cases, batteries have exploded.

How to Charge a Motorcycle Battery with a Car Charger

There are two ways to go about it:

  • Mitigate amperage
  • Use a smart charger

 Mitigating high amperage

You can mitigate the excessively high amperage from a car battery charger by running appliances in parallel to ensure only what the motorcycle battery can handle gets into it.  You could use any appliance but light- bulbs are a favorite option because their amperage is clearly indicated and it is easy to tell when they are not receiving electricity. They simply do not light up.

The math involved is to ensure that the amperage finding its way into the motorcycle battery is equal to or less than one-tenth its rating in amp hours. If the battery has a 16 amp rating, no more than 1.6 amps should be reaching it over a 10 hour period.

Lining up as many appliances as are needed for sufficient mitigation may be cumbersome but it is worth the effort if you don’t want to end up with a fried battery. When undertaking the process, keep a close eye on the set up to ensure all the connections remain intact. If anything disconnects, the battery will be overcharged.

That said, this method should only be used in emergency cases and never on a regular basis. This is why many experts still maintain that you cannot charge a motorcycle battery with a car battery charger.

Using a smart charger

Smart chargers deal with the risk of overcharging. These sophisticated chargers offer different charging modes depending on the battery being charged. Most smart chargers have different charging modes tailored for small batteries like those used in motorcycles and bigger ones used in cars.

Small battery modes automatically charge at a slower rate to avoid damaging the battery. All modes are fully automatic such that you can connect the charger and leave it without having to check on it every so often.

These chargers are programmed to meet the needs of the battery at each stage of the charging process using multiple automated steps. These steps often start with a de-sulphation process which attempts to deal with the effects of a deeply discharged or neglected battery.

This is followed by a phase of high current charging when most of the required re-charging is done. After this is a bulk charging step which maintains optimal voltage. Current is dropped to bring the battery close to full charge. After this is an analysis phase where the charger analyses how the battery is holding charge and determines the best way to bring the battery to full charge.

Another advantage of smart chargers is that you can actually leave it connected to the battery permanently without causing any damage. The battery remains in perfect condition. One downside to smart charges is that some brands may fail to detect a battery if it is completely discharged.

Precautions When Charging Motorcycle Batteries

There are a few things which should alert you of a problem with the charging process.

Heat: it is abnormal for a battery to get hot when charging. If it becomes more than just a little warm to touch, something is wrong. Stop charging and let it cool before resuming.

Swelling: This is another sign that something is not right. It could be that the battery is already damaged. Whatever the cause, the important thing is to stop charging immediately.

Smoking: Definitely not a good sign. This could happen when the connections have not been done correctly or if the battery is already damaged. Either way… stop.

Must The Battery Be Removed For Charging?

Removing it causes no mechanical damage but it is not necessary. Nor is it recommended. With the very elaborate electronic systems in modern motorcycles, removing the battery causes the ECU (Electronic Control Unit) to lose memory. When the battery is reinstalled, the bike may feel different for some time before the system relearns details like throttle positions and fuel maps.

It is much easier to charge the battery while it is in its compartment. Use a charger with a quick-disconnecting plug.

Final Word

Charging your motorcycle battery with a car battery charger is indeed possible. However, it requires a lot of precaution to make sure you don’t overcharge the smaller motorcycle battery and destroy it in the process.

Figuring out how to charge a motorcycle battery with a car charger takes a good understanding of amperage and the specific load the motorcycle battery can take. It can be done after applying appropriate appliances to mitigate high amperage. Smart chargers are another option. They are as their name suggests… smart enough to do the thinking for you and ensure the battery remains in good condition.