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Choosing AGM Battery for Car

Choosing an AGM-Batterie 40Ah? In this article, we’ve selected the best 40Ah car batteries.

40 Ah batteries are used for small cars. AGM batteries can work even in the most difficult conditions and last much longer than conventional lead-acid batteries.

There are two types of batteries: lead-acid and glass mat (AGM). Lead-acid batteries are outdated technology – you no longer need to refill them with distilled water – while AGM batteries are modern and suitable for cars with more advanced electrical systems.

BCI Group 51 and 51R batteries are very popular automotive batteries and are used in most passenger cars. They are most commonly used as starter batteries and dual-purpose batteries. Group 51 batteries are often used as universal/auxiliary batteries in a variety of applications requiring stable and reliable power sources.

Although Group 51 and 51R batteries are of a certain size, they can vary slightly in size. Some manufacturers have optimized their batteries and made them slightly smaller (for easier installation in cars and vehicles). Usually, these are 40 Ah batteries.

Sometimes they can be larger, for better performance. So before buying a Group 51 Battery, check the actual size of the battery and the size of the battery compartment.

Group 51 batteries are usually made as vibration-resistant lead-acid batteries (SLA) with an absorbent glass mat (AGM) that can easily fit into most car battery compartments designed for these batteries.

To simplify installation, some batteries are manufactured as Group 51 (positive left pole) and as Group 51R (positive right pole).

The best brands of 51 batteries

  • Optima REDTOP Batteries
  • XS Power AGM
  • Bosch S6 AGM
  • Delphi MaxStart AGM
  • ACDelco Professional AGM
  • Optima YELLOWTOP Batteries
  • Odyssey Extreme Series
  • Powertex Automotive Battery
  • NorthStar Ultra High Performance AGM

Best AGM Batteries 43Ah

XS Power D975 XS Series 12V AGM Battery

XS Power D975 XS AGM Battery 12


  • Rating: 4,7 ∗∗∗∗∗
  • 12-Volt Deep Cycle AGM Power Cell
  • Perfect for 1000W – 2000W Car Audio Stereo Systems
  • 100% Sealed and Maintenance Free

XS Power S975 12V AGM Battery

XS Power S975 S Series 12V


  • Rating: 4,4 ∗∗∗∗∗
  • 12V AGM Starting Battery
  • Max Amps 2100 CA: 604A, Ah: 43

Best AGM Battery 44Ah

Optima Batteries OPT8022-091 8022-091 75/25 Starting Battery

Optima Batteries OPT8022-091 8022-091 75/25


  • Rating: 4,4 ∗∗∗∗∗
  • 12-Volt, 720 Cold Cranking Amps.
  • Reserve capacity of 90 minutes for constant performance.

Best AGM Batteries 45Ah

ACDelco Gold B24R AGM Battery

ACDelco Gold B24R AGM Battery 51 Group


  • Rating: 4,4 ∗∗∗∗∗
  • Offering 325 Cold Cranking Amps (CCA).
  • 3-times the cycle life of conventional auto batteries.
  • AGM technology.

Bosch S6535B S6 Flat Plate AGM Battery

Bosch S6535B S6 Flat Plate AGM Battery


  • Rating: 4,0 ∗∗∗∗∗
  • 440 CCA.
  • The perfect choice for all vehicles including Start-Stop applications.

Best AGM Battery 46Ah

Delphi BU9051P MaxStart AGM Battery

Delphi BU9051P MaxStart AGM Battery


  • Rating: 4,0 ∗∗∗∗∗
  • 325 CCA (cold cranking amps) and 60 R.C. (reverse capacity).
  • 20x the vibration resistance of conventional batteries.

Best AGM Batteries 48Ah

Powertex Batteries Lithium-Ion LiFePO4 Automotive Battery

Powertex Batteries Lithium-Ion LiFePO4 Automotive Battery


  • Rating: 4,9 ∗∗∗∗∗
  • Super high rate Lithium Iron Phosphate battery technology.
  • With service life three times longer than that of a Lead Acid battery.

Optima Batteries OPT8040-218 D35 Battery

Optima Batteries OPT8040-218 D35 Battery


  • Rating: 4,3 ∗∗∗∗∗
  • 12-Volt, 650 Cold Cranking Amps.
  • 98 minutes reserve capacity for constant performance.

OPTIMA RedTop AGM Spiralcell Automotive Battery

OPTIMA RedTop AGM Spiralcell Automotive Battery


  • Rating: 4,3 ∗∗∗∗∗
  • Voltage: 12 Volts.
  • Product Category: Auto/Light Truck.

What is an AGM battery?

The AGM battery is especially suitable for vehicles with an auto-start and stop function.

The AGM battery is a modern type of car battery that is mainly used as a starter battery in cars with a start-stop function. The AGM battery is often used as a power source for boats or mobile homes. The AGM battery is also used in the solar sector.

The importance of the AGM battery can be attributed to fiberglass construction. “AGM” stands for Absorbent Glass Mat. As a result, the special car battery has a long life and is largely sealed. In addition, there is no need to fill the AGM battery because there is no need to refill water. An AGM battery can be recognized by the corresponding label on the case.

In contrast to conventional lead-acid batteries, the so-called AGM-VRLA (valve-regulated lead-acid battery), AGM automotive batteries are very typically used in vehicles equipped with a so-called start-stop system. The engine automatically shuts off when the vehicle is stopped (e.g. at a traffic light). If the driver finally reengages the gearbox again, or the clutch starts the car again.

Below is an overview of the most important advantages and disadvantages of an automatic start-stop system:


  • The engine automatically shuts off when the vehicle is stationary.
  • Fuel savings of up to 15% (depending on the manufacturer)
  • Protects the environment
  • Highly recommended for city traffic


  • Virtually no effect on fuel consumption on long trips
  • Increased wear and tear is possible

Types of car batteries

Car batteries come in two main types: the more traditional, maintenance-free batteries and the more advanced absorbent glass mat (AGM) batteries.

Lead-acid (conventional)

Batteries once required drivers to periodically refill water into the electrolyte solution, the liquid inside being the battery’s power source. Today’s maintenance-free batteries consume much less water than traditional batteries with flooded cells. Maintenance-free batteries retain their fluid for the life of the battery, and the covers on these models are not designed to be removed. There are still batteries that can be topped up with distilled water; with proper care, they can last longer in hot climates.

A lead-acid battery will usually cost much less than an absorbent glass mat battery. However, it will not hold a charge as long and will tolerate a deep discharge worse.

Absorbed Glass Mat (AGM)

AGMs can withstand repeated discharge and recharge cycles better than standard batteries. They are becoming standard equipment in more cars because modern features such as fuel-efficient stop-and-start systems, electronic safety and convenience features, and outlets for mobile electronics all increase energy demand.

But AGMs can cost 40 to 100 percent more than conventional, highly rated batteries. Consider buying one if you sometimes don’t use your car for long periods of time and the battery runs out. An AGM battery is better able to withstand a deep discharge and is more likely to fully recover if it accidentally runs out of power.


  • Has low self-discharge
  • long life
  • insensitive to cold


  • often expensive to buy
  • prone to high temperatures

What is the capacity of a 40 Ah battery pack?

Battery capacity is one of the most important technical characteristics of a battery pack. 40 Ah is the amount of time that an autonomous power source is able to power the electrical consumers connected to it. The capacity of a 40 Ah battery is the maximum amount of electricity a battery can store in a full charge cycle.

The unit of measure for capacity is Ah (ampere-hour), for smaller batteries is mAh (milliampere-hour).

AGM 40 Ah Automobile Batteries

AGM 40 Ah automotive batteries are recommended for all new and modern vehicles. These automotive batteries are mainly used in vehicles with Start-Stop technology and vehicles with regenerative braking energy.

Which 40Ah battery should I buy for my car?

The main parameters of the battery are:

  • Date of manufacture of the battery
  • Starting current (cold start current);
  • Battery capacity (nominal and reserve);
  • The need for maintenance;
  • Design.

Date of manufacture of a 40 Ah battery

The date is important due to the battery’s property of self-discharging. The battery loses its capacity during use. Therefore, you should buy a battery with the most recent date.

Starting current of a 40 Ah battery

Cold start current characterizes the capacity of the battery in amperes. The physical meaning of this value, marked on the battery by a three-digit number is the energy the battery gives in 10 seconds when starting at 18 degrees below zero. The higher this value is, the better. But for a particular car, overpaying for too much current may be unnecessary. Diesel engines are the most sensitive to this parameter.

Battery capacity 40 Ah

A higher ampere-hour capacity is not always appropriate. The capacity should not be less than that of the base battery, perhaps a little more. Low capacity shortens the life of a lead-acid battery, leading to a deep discharge. In addition, as the charge drops, so does the terminal voltage, and thus the amperage as well. It is more difficult to start on a dead battery, especially in the cold. Reserve capacity in practice is measured in the minutes that the car can drive powered by the battery, without the alternator.

Selecting the battery by size

The battery must stand in its original place. The battery compartment differs from vehicle to vehicle. While European and domestic models allow for a little variation, in most Japanese cars these dimensions are severely limited.

When selecting an AGM-Batterie 40 Ah in the car, you need to consider:

  • The length, width, and height of the battery;
  • Compatibility with the vehicle’s mounting mechanism;
  • The polarity of the battery (forward or reverse terminal arrangement).

Design and mounting of the 40 Ah battery

The battery must fit the dimensions of the compartment in which it will be installed. While some cars have a backlash, others have minimal clearance. Once placed under the hood, the battery must be secured.

There are three types of fasteners – top clamping bar, two- or four-way clamping from the bottom. Well, and polarity: buy a battery for the car the correct polarity – means to save yourself the manipulation of the wires. Straight polarity: “+” and “-” from left to right, reverse – vice versa.

optima battery

By design, a distinction is made between low-maintenance liquid electrolyte batteries, maintenance-free AGM batteries, and gel batteries.

Labeling of 40 Ah Batteries

The labeling of batteries for vehicles produced by different companies varies significantly, as do the batteries themselves.

Different vehicles have very different batteries, both in terms of their electrical characteristics and dimensions. Different countries have different requirements for technical information on batteries. Mandatory battery labeling requirements are described in several international standards.

Battery manufacturers themselves strive to put all the necessary information on the battery case:

  • The name and trademark of the manufacturer’s plant;
  • The marking of the nominal voltage of the battery;
  • Battery capacity value marking;
  • type of battery according to one or more international standards;
  • number of battery cells;
  • polarity marks of the battery terminals;
  • date of manufacture of the battery;
  • Warning marks for operation and transportation.

Battery sizes

Batteries come in a variety of sizes. It is important to choose the right one to ensure a secure fit and sufficient power. If the terminals are in the wrong location, your vehicle’s cables may not reach, or they will not fit securely. Refer to the store manual or setup guide. Many retailers will install the battery for free.

Size 24 / 24F (top terminal): fits many Acura, Honda, Infiniti, Lexus, Nissan and Toyota vehicles.

Size 34/78 (double terminal): fits many large Chrysler cars and many 1996-2000 GM pickups, SUVs, and midsize and large sedans.

Size 35 (upper terminal): fits most Japanese nameplates, including many recent Honda vehicles and most Nissan, Subaru and Toyota vehicles.

Size 47 (H5) (upper terminal): fits many Buick, Chevrolet, Fiat and Volkswagen models.

Size 48 (H6) (upper terminal): fits many European as well as American cars from Audi, BMW, Buick, Cadillac, Chevrolet, GMC, Jeep, Mercedes-Benz, Mini, Volkswagen and Volvo.

Size 49 (H8) (upper terminal): fits many European and Asian Audi, BMW, Hyundai and Mercedes-Benz vehicles.

Size 51R (upper terminal): fits many Japanese Honda, Mazda, and Nissan cars.

Size 65 (top terminal): suitable for large cars, trucks, and SUVs from Ford or Mercury.

Size 75 (side outlet): suitable for some General Motors midsize and compact cars and some Chrysler vehicles.

The best brands of AGM Batteries

There are many no-name brands available on the market that have no quality. The brand of the battery has a big impact on the overall quality of the battery. Make sure that you choose brand name models when buying an AGM battery. There are several well-known brands that guarantee quality. You will be able to use their product decisively.

These popular brands have earned a reputation for producing high-quality batteries that pair well with cars, SUVs, travel trailers and other types of vehicles. If you focus on reputable brands, you won’t have a problem choosing the best AGM battery because they usually use high-quality materials when manufacturing their batteries.

Proven battery manufacturers are: XS Power, Optima, ACDelco, Bosch, ODYSSEY.

AGM Battery Charging and Maintenance Instructions

Procedure for charging an AGM battery:

The battery can be charged at an optimum rate at the proper temperature. Consequently, room temperature is required to charge the battery quickly.

However, if it is too hot or too cold, it will take many hours to charge the AGM battery.

The optimal charging rate is 2.4 to 2.465 volts per cell. And the temperature should be around 25°C / 77°F.

The recharge rate also depends on the depth of discharge of the battery. You can’t just plug it in to charge when you want to. Each AGM battery has a different DoD number.

So, for example, if your battery has a DoD number of 60%, you need to use at least 60% or more before charging it.

Also, partially charging a battery is not good for this. You may lose the ability to recharge it to 100% at all.

So, you need to let your AGM battery reduce the required DoD rate and then fully charge it to keep it working perfectly.

AGM battery maintenance and care:

Now an AGM battery needs no maintenance, which is not wrong. But you do need to do certain things to extend its life.

For example, if you are not using the battery, store it in a cool place. Otherwise, its performance will decrease. And at least charge it every 4 months. Even if you don’t use it, the battery slowly drains energy. So, it needs to be refilled.

Battery sulfation is common with AGM batteries. To avoid this, you need to fully charge the battery before you put it away for storage. Follow these steps and your battery will last for years to come.

How do you know if you need a new car battery?

There may be signs that your car battery is running low. Excessive corrosion around the rack and cable connectors probably means acid leakage. Some cars will have a flashing “check engine light” if the battery is low, although in most cases you won’t know exactly what the light means without access to an OBD II scanner. An obvious crack, bulge, or war page on the battery case is a good indication that you should look for a new one.

However, the above conditions are relatively rare, and their absence does not necessarily mean that the battery is serviceable. Ten or 20 years ago, battery deterioration gradually manifested itself in one obvious way: increasingly slow and difficult engine starting or starting, especially during cold weather. Some recently manufactured batteries may still behave this way, but most modern batteries will continue to start normally until they no longer crank.

Modern batteries generally continue to turn momentarily until they no longer turn. A battery inspection is necessary. If your battery is 3 years old or older, have it checked annually. Many retailers will do this for free on the spot. It’s easy, and it’s the only way to be sure.

A car battery needs two things: cranking power and reserve capacity. Cranking capacity is essentially the large electrical discharge needed to start the engine. It is measured as the cold start current, or CCA, or the number of nearly instantaneous amps the battery can deliver at 0 degrees Fahrenheit. Reserve capacity accounts for the smaller and slower flow of electricity to power devices such as lights or radios when the car is off and the battery is not charging. This is measured in ampere-hours or Ah of total stored energy, and sometimes in minutes – how long the battery will provide power at a certain rate of discharge. Manufacturers evaluate their own batteries, and lab tests have shown that in real-world conditions, batteries sometimes don’t meet the manufacturer’s claim.

Nevertheless, the two requirements for a 12-volt car battery can work in different directions. A battery with enough starting power to start a cruise liner will not necessarily keep the headlights on for 20 or 30 minutes. All automotive batteries are a compromise in this balance of cold start booster power and reserve capacity, and much of the technological development of lead-acid batteries has been aimed at improving this balance.


AGM batteries have a long life and last a long time. Just make sure that the battery is not discharged below 60%.
If you plan to store your car for the winter, be sure to purchase a battery charger or battery tender to keep the capacity of the AGM battery in the storage.
Indeed, an important aspect of choosing the best AGM battery is not only the brand but also how you take care of the battery. Be sure to replace the battery if you notice cracks or signs of wear on the outer casing.

AGM 40 Ah Batteries Price:

AGM Batteries 40Ah Under $200

  • XS Power D975 XS Series 12V AGM Battery

AGM Batteries 40Ah Under $250

  • XS Power S975 12V AGM Battery
  • Optima Batteries OPT8022-091 8022-091 75/25 Starting Battery
  • ACDelco Gold B24R
  • Delphi BU9051P MaxStart AGM Battery

AGM Batteries 40Ah Under $300

  • Optima Batteries OPT8040-218 D35 Battery
  • OPTIMA RedTop AGM Spiralcell Automotive Battery
  • Bosch S6535B S6 Flat Plate AGM Battery

AGM Batteries 40Ah Under $500

  • Powertex Batteries Lithium-Ion LiFePO4 Automotive Battery

How to Pick a Car Battery – Video

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Max Welder (rvneeded)

Max Welder (rvneeded)

Max Welder is a mechanic enthusiast. Currently, working at a small company. Experienced in electronics, car tools, and security systems. In his spare time, he writes articles for various blogs.

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