Charging A Car Battery: 2 Amp Or 10 Amp?

how long to cahrge a car battery at amps

There are many ways of charging a battery, plus a myriad of chargers to choose from. Yet one of the most common questions is that, what charger should one use when charging a car battery, 2 amp or 10 amp?

This is mostly a question that focuses on charge time, because determining how long to charge a battery is important so that you can schedule tasks in line with when the task finishes.

Another reason why you should know what charger to use is so that you can accurately measure how long you have to charge your battery without it overcharging, or worse, overheating. (Learn how to use a car battery charger).

Determining whether to use a 2 amp or 10 amp charger will help you accurately predict the charge times of a battery, but it is not the only factor. The battery capacity and its type also plays a big role in calculating charge time.

As for car batteries, it is a rule of thumb not to fast charge them using anything that exceeds 20 amps. Slow charging using 10 amp chargers or less is generally accepted to be better. This is because repeatedly fast-charging a car battery may lead to overcharging and will severely reduce its performance.

The short answer: 10 amp chargers are generally preferable than 2 amp chargers. But to really dig into the reasons why that is so, we need to learn about how charge times are determined on different kinds of batteries.

Significance Of Amp Hours

The question of what charger to use is anchored on the amp hour— a measurement of energy output used in batteries. Automobile batteries commonly have a capacity of 48 amp hours. This means that it can deliver 1 amp per 48 hours, or 2 amps in 24 hours, 8 amps in 6 hours, and so on.

So a 2 amp charger will deliver 2 amp per hour to this “flat” battery, and if it requires 48 amps that in means it needs 24 hours to fully charge. A 10 amp charger, in comparison will take a little under five hours to fully charge the 48 amp battery.

There are some that prefer a slow charge because fast charging can lead to overcharging and buckling of the battery plates. Also, these charging rates are just the ideal conditions, circumstances when all goes perfect.

However, calculating actual charge times is much more complicated than crunching simple numbers. It actually depends on the battery material, the reliability of the charger, and the current charge rate on the battery.

Accurately Determining Charge Time

Regardless of whether you will use a 2 amp or 10 amp chargers for your car battery, there are standard steps on how to determine the charge time. Now, if you are curious about the 2 amp or 10 amp preferences to determine which one is faster, or which one is just more efficient in general, an understanding of charge time is useful knowledge.

Calculating the charge time is easy, and the only special equipment you will need is a voltmeter.

However, this is important: you should always wear protective gear when charging or handling batteries. Safety goggles, protective sleeves, and latex gloves are a must to protect your eyes and skin from accidental electrocution and corrosive materials that may leak from the battery.

1. Find Out Your Battery’s Amount Of Amp Hours Via Its Reserve Capacity

The first step in determining the actual charge time is checking out the amp hours on your battery. This is done by first figuring out its reserve capacity, which may be found in the manufacturer’s guide or user manual of the battery.

You can derive the battery’s amp hour ratings by multiplying the reserve capacity by 0.6. For example, if the reserve capacity is 100 minutes, then 100 multiplied by 0,6 is 60. This means that your battery contains 60 amp hours.

2. Find The Battery’s Open Circuit Voltage Using A Voltmeter

Open circuit voltage means the voltage rating of a battery when it on “zero load” or when it is not connected to any circuit. This can easily be measured by a voltmeter. However, one must make sure not to charge the battery before measuring.

Once you get the voltage rating, refer to the list below to determine the charge state of your battery, depending on type. The voltage rating on the left corresponds to a charge state on the right column.

For Flooded Lead Acid batteries:

12.60 to 12.70 – 100%

12.40 – 75%

12.20 -50%

12.00 -25%

11.80 -0%

For Gel batteries:

12.85 to 12.95 – 100%

12.65 – 75%

12.35 -50%

12.00 -25%

11.80 -0%

For AGM batteries:

12.80 to 12.90 – 100%

12.60 – 75%

12.30 -50%

12.00 -25%

11.80 -0%

So for example, you measured your flooded lead acid battery and found out that its voltage rating is at 12.20. Then that means that the battery is currently at 50 percent charge.

Now, considering our example earlier that the battery has 60 amp hours of capacity, it then follows that the battery in question currently has 30 amp hours— 50 percent of 60 amp hours is 30 amp hours, after all.

This also means that you need at least 30 amp hours more to have the battery fully-charged. But that is not all. You also need for the battery’s internal resistance, so you have to put about 20 percent of the needed charge rate as additional amp hours.

30 amp hours multiplied by 0.2 (20 percent) is 6 amp hours. Add that value to the original needed charge rate and you get 36 amp hours. This is the amount of amp hours you actually need to charge the battery in our example.

3. Determine Charge Time By Rate Of Charge And Amp Hours Needed

Now that you know how much amp hours you need, you just need to find out the type of charger.

In our example, if we use a 10 amp charger to replenish the needed 36 amp hours, we could simply divide 36 by 10, which yield a result of 3.6. This means that when charging the example battery with a 10 amp charger, it will take 3.6 hours to fully charge.

Likewise, a 2 amp hour charger will need 18 hours to fully charge the same amount. 36 divided by 2 is 18, after all.

A word of advice: a 3-stage automatic charger that is rated at 12 volts / 10 amp is the best choice for charging automotive batteries.

However, keep in mind some chargers rated at 10 amp do not actually constantly output that the whole 10 amps steadily during the whole charging cycle. This is because chargers like these usually limit the amperage on the charge cycle.

Most likely, it will run on half its actual rating, so you have to give more leeway to your calculated charge time. Doubling your calculated result is a good bet— for example, charging calculated in 3 hours amy take approximately 6 hours to actually finish.

In addition, the reduced voltage may also add up to the time required for the battery to reach 100%.

4. Check The Battery Temperature And Confirm Charge Level With Voltmeter

While charging, it should go without saying that you have to make sure your battery is at safe temperature levels. Excessive heat may indicate a problem with the circuit, and so if you are experiencing overheating, you might want to check out for faulty wirings and the like.

A little heat is natural, though, and it is expected that your battery will heat up a little during the whole charging process. But after your calculated charging time, you should make sure that your battery cools up to room temperature, at least.

After this, you can now use the voltmeter and consult the tables listed above to confirm that your battery is fully charged.

Additional Points: Maintenance

Before actually setting up to charge the battery, check its electrolyte levels. If the battery needs topping up of electrolytes, then you need to do so before you go ahead with the charging process.

Also, the terminals and battery posts need cleaning regularly. If you see that your battery needs this, it may be best to do it before you charge.

It is safe to leave the battery in the car while charging when it has a power point and about 3 to 4 amps in charge.

However, if there is an alternator installed, the battery should be disconnected. Some alternator models may be damaged by the charging process.


You now have a fair idea of answer to the question, “What charger should one use when charging a car battery, 2 amp or 10 amp?” The pros and cons of each type of charger have been presented, as well as how they actually contribute to the charging process.

2 amp chargers are good for those who like slow charging, 10 amp chargers are for those who like it a bit faster but with reduced risk of overcharging. The judgment on what type of charger to use, according to your needs, is not entirely up to you to decide.