What Size of Battery Charger Do I Need?

What Size of Battery Charger Do I Need? If you are screening for a new battery charger, you will come across multiple options, which makes it difficult to choose the right battery charger for your needs.

However, the choice can be made easier if you understand the size of the battery charger you need. Therefore, this article discusses everything you need to know about the battery charger’s size.

What is “Battery Charger Size?”

The rating of battery chargers is in Amps (A); further, “the battery charger size” is the maximum output current of the charger.

By supplying the optimal charging current, your battery charger will;

  • Prolong the life cycle of the battery
  • Minimize charging time
  • Efficiently charge your battery
  • Improve battery performance
  • Avoid overcharge, overcurrent, and overvoltage

Will an incorrect battery charger size charge your battery? The answer is “YES.” However, you will experience a huge decrease in service life and performance of your battery and charger over time.

What Size Of  Battery Charger Do I Need?

As a guideline, the battery charger must be 10-20% of your battery’s rating (Ampere hour). 

Therefore, to understand the size of the battery charger you need, you should first understand your battery’s rating (Ah), which is closely related to the type of battery.

First, determine the battery types.

There are several battery types. However, the most common battery types used in recreation and automotive environments are;

The battery type is usually indicated on the battery’s label on the side or top of your battery.

Most automobile cranking batteries can be either maintenance-free Calcium or wet (flooded) batteries.

 However, you should always check and confirm the battery type of your automobile, as some present-day cars have a Stop-Start AGM-style cranking battery.

We also recommend checking your battery charger to ensure it is compatible with your type of battery and choosing the correct chemistry while charging.

Moreover, the battery rating can be unclear as different batteries use different ratings. For example,

  • GEL, AGM, and Lithium batteries are generally rated using a “C” rating.
  • Calcium or wet (flooded batteries are generally rated in Reverse Capacity (RC) and Cold Cranking Ampere (CCA).
Battery charger

A new car battery

Second, convert the C/RC/CCA rating to the Ampere hours (Ah) rating

When charging your battery, you should determine the battery’s Ampere hour (Ah) rating, a universal number that will assist you in understanding the electrical capacity and size of your battery.

On Calcium or wet (flooded) batteries, like the start batteries, the Ampere hours (Ah) are usually not displayed. Besides, converting CCA (Cold Cranking Ampere) to Ampere hours (Ah) is a complex task.

 For instance, although there is a traditional guide where you can divide 7.25 into the Cold Cranking Ampere (CCA) to get the battery’s Ah rating, the conversion is not always accurate.

 Therefore, we highly recommend downloading the manufacturer’s datasheet to check and confirm the right Ah rating of your battery.

Besides, understanding your battery’s “C” rating allows you to determine and understand more information about the battery.

 For instance, for a C20-rated 100 Ampere hour battery, you need to divide the 100 Ah by 20 to get 5A, which means your battery is going to take 20 hours to discharge 5A until it reaches a universal test benchmark for a dead flat battery (which is usually 10.5 volts).

Furthermore, if your battery does not have an Ah rating label, you can multiply 0.6 by the (RC) Reserve Capacity rating to get an approximate Ah rating.

For example, 0.6 x 90 RC = 54 Ampere hours.

NOTE: You can find all this information on the battery label on your battery’s sides or top. 

However, we recommend contacting the battery manufacturer for assistance and clarification if you can’t see this information.

Now, calculate the size of the charger.

The Ampere hour (Ah) rating of a battery charger must be between 10% and 20% of that of your battery.

 For example, a 100 Ampere hours (Ah) battery will need at least a 10 Ampere charger.

We recommend maintaining the charger size below 30% of the battery’s total charging capacity to prevent overcharging.

 Therefore, if you have a 100 Ampere hours (Ah) battery, you will require an almost 30 amp charger.

However, if you have a Lithium battery, you should refer to the manufacturer’s specified maximum charging current, as it can go up to 100% of the Ampere hours (Ah) capacity.

NOTE: Most battery chargers are designed with selectable current output, which allows you to use a larger charger on a small battery by reducing the output current to a safer rating.

Battery charger size chart

The different battery charger size charts that you can use include the following;

Charging current

The charging current chart for LiFePo4 batteries

LiFePO4 Battery Capacity1C Charging Rate (about 1-1.5 hours to charge)0.5C Charging Rate (about 2-2.5 hours to charge)0.3C Charging rate (about 3.5-4 hours to charge)
20 Ah20 Amps10 Amps6 Amps
50 Ah50 Amps25 Amps15 Amps
80 Ah80 Amps40 Amps24 Amps
100 Ah100 Amps50 Amps30 Amps
120 Ah120 Amps60 Amps36 Amps
150 Ah150 Amps75 Amps45 Amps
200 Ah200 Amps100 Amps60 Amps
250 Ah250 Amps125 Amps75 Amps
300 Ah300 Amps150 Amps90 Amps

The charging current for SLA ( GEL and AGM) batteries

SLA Battery Capacity0.3C Charging Rate (approximately 6-8 hours to charge; 0.3 is the maximum recommended)0.25C Charging Rate (approximately 8-10 hours to charge)0.1C Charging Rate ( approximately 10-12 hours to charge)
20 Ah6 Amps5 Amps2 Amps
50 Ah15 Amps12.5 Amps5 Amps
80 Ah24 Amps20 Amps8 Amps
100 Ah30 Amps25 Amps10 Amps
120 Ah36 Amps30 Amps12 Amps
150 Ah45 Amps37.5 Amps15 Amps
200 Ah60 Amps50 Amps20 Amps
250 Ah75 Amps62.5 Amps25 Amps
300 Ah90 Amps75 Amps30 Amps

Recommended Charging C-rate and Average Charging Time

Battery ChemistryRecommended Charging C-rateAverage Charging TimeCharging Stages
Lead-Acid batteries (GEL and AGM)Between 0.1-0.25C (should not exceed 0.3C)Approximately 10 hoursAbsorption, Bulk, and Float Constant Current (CC)
LiFePO4 batteriesBetween 0.3-0.5C (1C can also be accepted but you should not apply it regularly)Approximately 4 hoursConstant Voltage (CV)

Charging Voltage

System VoltageCharging Voltage Range (at 77ºF or 25ºC)
GEL 24 volts27.6-28.3 volts (absorption)

26.4-27 volts (float)
GEL 12 volts14.1-14.4 volts (absorption)

13.1-13.4 volts (float)
AGM 24 volts28.4-29.2 volts (absorption)

27.0-27.6 volts (float)
AGM 12 volts14.6-14.8 volts (absorption)

13.4-13.8 volts (float)
LiFePO4 24 volts28-28.6 volts
LiFePO4 12 volts14-14.6 volts

NOTE: For SLA batteries (GEL and AGM): The charging voltage must be slightly higher at low temperatures. Most SLA battery chargers provide temperature compensation.

Can a battery charger be too big?

A battery charger can not be quite big in terms of current. Why? If your battery charger is working properly, it must be able to “communicate” with the battery to understand what charging current to supply.

Moreover, the ampere rating of the battery charger expresses the maximum current your charger can supply; it doesn’t mean that it can only supply this amount of current.

 The charging current decreases (varies) throughout the battery charging process, which shows that a battery charger can supply a lower current than it is rated for.

You have a 12-volts battery charger rated at 50 Amps and want to charge a 12-volt 100Ah AGM battery. 

Given the recommended charging current range for an Absorption Glass Matt (AGM) battery of 0.1-0.25C, the suitable charging current for this battery should be between 10-25 Amps.

Therefore, the 50 Amps charger would only supply a charging current of 10-25 Amps to your battery because that is what the charging microprocessor would “communicate” to the charger.

The charger will supply the current that the battery draws, and the battery will only draw the required current. Therefore, a “large” charger will not damage the charger or battery. 

Even though a battery charger can not be “too big”  because it will significantly impact your wallet. Further, the price increases with an increase in the amperage rating.

So, you do not need to select a larger battery charger if you will never utilize its maximum current. 

Rather, we recommend selecting a battery charger whose amperage rating matches your battery’s recommended charging current range.

Battery charger

Battery charger


Using the right charger size is essential in maintaining your battery’s health, offering high performance, maximizing cycle life, and improving charging efficiency.

Conversely, an incorrect battery charger size can undercharge or overcharge your battery.

Therefore, if you need help choosing the right size battery charger, contact Cloom.