How Long Does it Take to Charge a Car Battery with a 10 Amp Charger

How Long Does it Take to Charge a Car Battery with a 10 Amp Charger

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How Long Does it Take to Charge a Car Battery with a 10 Amp Charger


Frequent use of your car for long tours and daily to and fro highlights the importance of keeping your battery fully charged and in peak condition.

This is especially important during the winter months when most of the car utilities like heaters, highlights, wipers, etc. are constantly in use.

The battery won’t be able to consistently crank the engine the motor if excessive current is drained from it and not replaced.

In recent years, technology has improved considerably, with upgraded energy density and faster charging periods.

One of the major difficulties with car batteries has traditionally been their frequent use combined with slow battery charging.

However, as battery technology advances, these problems are being solved. Modern batteries can now be considered an important source of backup power.

Let’s look at the factors that influence battery charging speeds.

Battery Condition

When thinking about your battery’s condition there is a number of variables to consider.

State of Charge (SOC%)

An expression that describes the present capacity of the battery in percentage form, SOC can be ascertained by current integration that detects the change in capacity of the battery for a specific period of time.

Depth of Discharge (DOD%)

The percentage value of battery capacity that has been discharged for a long duration. 80% of DOD refers to the stage of deep discharge.

Terminal Voltage (V)

The difference of voltage in battery terminals with the load applied. Usually, the terminal voltage changes with the SOC and discharging current.

Internal Resistance

The resistance present within the battery that generally differs with respect to the charging and discharging which is directly dependent on SOC. The higher the resistance, the lower the efficiency.

Importance of State of Charge

A significant factor affecting charging is the maximum rate of charge a car battery can sustain for a specific period of time.

Thus it becomes important to ascertain the State of Charge of the battery. Determining the SOC will make it possible to predict the average duration of charging.

State of Charge describes the state of the battery i.e. charge capacity. Therefore, when the battery is completely charged, SOC will be 100%.

Note: information like the State of Charge and other important metrics can generally be found on the battery or in its instruction manual.

If this information is not available, you can determine the State of Charge through the process of modification.

It’s a process where you can connect the charging unit directly into the battery.

Receiving an abrupt load of current, the charging unit will discharge the battery and measure the SOC.

Detecting the Battery Charge

Car batteries are usually lead-acid batteries which are composed of cells. Each cell contains approximately 2 Volts; thus a 12 Volt car battery comprises of 6 individual cells.

A completely charged 12 Volt battery provides 12.9 Volts, whereas a fully discharged battery will emit 11.4 Volts.

Therefore the difference between a completely charged and a discharged battery is only 0.25 Volts.

Let’s figure out a formula to calculate the residual charge in the battery.

Residual charge in a battery

The following formula applies to 12 Volt batteries:
Percent charge = (Measured battery voltage- 11.4Volts)/1.5Volts x 100.

In order to work with this formula, your battery should be in the resting state, which means the battery must not be supplying power to any load.

According to experts, the battery should be in an idle state for at least 24 hours in order to record an accurate measurement.

The above formula is an accurate method of calculation as long as the load current is always less than 1% of the battery’s capacity in amp-hours.

Duration of Battery Charging

The time it takes to completely charge your battery depends on the present condition of your car battery and most importantly on the charger and its ampere-hours (amps).

The lower the charge in the battery, the longer the duration of charging. If the battery is severely discharged it will take several hours to charge the battery fully.

Step 1: Before starting the charging process, one must determine the reserve capacity of the battery. The rule of thumb to calculate the number of amp-hours in a battery is to multiply the reserve capacity by 0.6.

Example: If your car battery has 100 minutes of reserve capacity, multiply those 100 minutes by 0.6 and that would give approximately 60 amp-hours at a 20-hour rate.

Step 2: Check the open circuit voltage of your battery with the help of a voltmeter. This is the voltage with no load or connected circuits.

Do not charge the battery before testing the open circuit voltage. If the voltmeter reads a voltage of 12.2 Volts, then it has 50% of charge.

Example: If your battery has 50% charge, there are approximately 30 amp-hours left in your battery. Thus it becomes necessary to add more amp hours to balance the internal resistance within the battery and provide complete charge to the battery (30 amps x 0.2 = 36 amp-hours)

Step 3: Charge your car battery with a 10 amp charger in order to add that accurate amount of amp hours into the battery.

To determine the charging duration, you simply have to divide the amp-hours of the battery by 10 (with a 10 amp charger).

Charging Duration= amp-hours left in the battery / 10 amps of the charger

Example: If your battery is left with 50 amp hours then the approximate duration of charging will be the amp hours left divided by 10 amp hours of the charge: i.e. it will take 5 hours to completely charge your car battery (50 amp hours / 10 = 5 hours).

10 amp chargers are ideal for 12 Volt car batteries because they automatically control the amperage and voltage during the charge cycle.

After charging your battery, let it cool at room temperature and use a voltmeter to determine that the voltage is at equilibrium.

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Steve Brown


Steve is a gadget enthusiast who's always been intrigued by batteries. The founder and editor of Battery Chargers Info, he's assembled a group of like-minded experts to cover every facet of portable power His aim is to help you learn more about your favorite gadgets and their batteries so you can maximize both their performance and their life.

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