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Automotive Batteries Explained-What's Best For You?

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Ensuring You Use the Right Battery for Your Application
AGM batteries, like this Odyssey Group 78, are growing in popularity.

For many, an automotive battery is just another wear item like your tires. When it comes time to replace the battery in their vehicle, they use the specs in their owner’s manual or perhaps the group size information listed on the current battery in the car. Just like their tires, they simply go with the current size on their vehicle for a replacement and move on with their day.

This may work for the daily driver in your garage, but when it comes to building a hot rod, a race car, or updating your classic muscle car or truck, you really need to place more consideration on your battery needs. Just like these specialty rides roll on non-stock tire sizes, so too will your battery need most likely be far from stock as well. To that end, understanding battery sizing, capacity, and internal cell composition, along with your ride’s electrical system demands, engine size, and even climate and storage situation and how they all affect your vehicle’s battery is the best way to help choose the right SLI (starting, light, and ignition) battery for your vehicle’s electrical system storage needs.

What Battery Group Size Do I Need?

While there are many aspects of an automotive battery to consider, first and foremost is the battery’s dimensions. The [Battery Council International] ( (BCI) provides a standardized battery group size in numeric or alphanumeric configuration for a specific set of battery dimensions, which include the length, width, and height of the battery and even the terminal location and type of terminal. These dimensions are provided to fit a specific battery tray size and battery location in an OEM application. For our use though these numbers are more helpful to determine just what battery may fit a custom location such as a frame mounted battery tray in a hot rod, or even underneath a seat or mounted in the trunk of your muscle car. See our car battery size chart below for dimensions on popular group sizes.

Dual terminal batteries, like the popular Optima Yellow Top Group 34/78, provide a straightforward way to connect electrical accessories or provide for external battery charging terminal connections.

Measure your battery mounting tray/box and using the chart below you can determine what battery BCI group will work for your custom application. Keep in mind what terminals your battery cables are configured for or what terminals you would like to use for the best cable routing and clearance. Just because your GM muscle car may have come with a side terminal battery doesn’t mean you can’t use a top terminal battery when you move it to the trunk of your car. Typically, you will find top post terminals on group 24, 34, 35, 41, 42, 48, 51, and 65 batteries. Side posts are more prevalent on group 75, 78, 79, and 100 batteries and these use a standard 3/8-16 internal thread. If you are looking for a dual post configuration to run accessory wiring or remote charging terminals these are usually found on group 34/78 batteries.

This car battery size chart shows the common 12v car battery BCI group dimensions (in inches). We also offer this expanded list of additional BCI group sizes.

What Is the Difference Between Cold Cranking Amps and Cranking Amps?
A battery’s label will list CCA rating and often CA rating as well.

Cranking amps and cold cranking amps, often shortened to CA or CCA on a battery label, are both amperage ratings used to determine the amount of starting power a battery can deliver at full charge. Simply put, the higher the number, the more power available to the engine’s starter to crank the engine over at sufficient rpm to start the vehicle.

Where these two ratings differ is in their temperature ratings. Cranking amps is the amount of amperage that the battery can supply at 32 degrees Fahrenheit. Whereas cold cranking amps is the amount of amperage that the battery can supply a 0 (zero) degrees Fahrenheit. An automotive battery, no matter the internal construction design, will lose starting power as temperatures decrease, as such the CCA rating will always be lower than the CA rating. Therefore, CCA ratings are much more popular on battery identification labels, though these ratings can vary from brand to brand even within the same group size.

How Many Cold Cranking Amps Do I Need?

Because CCA is more prevalent on battery labels it is easier to use CCA for cranking needs. A general rule of thumb is that you need one CCA for every cubic inch of engine. So, a 302 Ford would need a minimum of 302 CCAs. It is common to double the CCA for diesel applications (which is why many diesels have two batteries wired in parallel) but because performance applications usually see higher compression and other performance upgrades, we prefer the cushion of doubling the CCAs as well. In our 302ci Ford example we would then recommend a battery with at least 604 CCAs. Keep in mind things like stroker engine displacements too where your 350 Chevy is now a 383 and is 11:1 compression. For such an engine having double the CCA (766 or simply round up to 800) would be preferred to ensure you have enough amperage to turn over that stroked high compression engine when it is hot.

Is Reserve Capacity Important?
A battery’s reserve capacity is important for engine off load needs or to provide that extra bit of power to get you home safely in the result of charging system failure.

Reserve capacity is a measurement, usually displayed in minutes, which states how long a battery can provide 25 amps of power until the battery is considered discharged (which is 10.5 volts for most 12-volt automotive batteries). Most applications where the battery is used for starting purposes only, the CCA is a much more critical measurement than reserve capacity. However, in applications where there will be engine-off electrical system needs, such as car audio or lighting displays, race cars without an alternator, or if you simply want to have an extra “cushion” in case of charging system failure to allow you to drive home with lights, ignition, and other critical electrical sub-systems working strictly off the battery’s reserve capacity to allow you get to a safe location, then reserve capacity is something to be cognizant of when battery shopping.

What Is A 16 Volt Battery Used For?
A 16 volt battery and charging system is commonly found in racing applications.

While our primary focus for this buyer’s guide is the typical 12 volt automotive starting or SLI batteries, we would be remiss in not mentioning the popularity of 16 volt charging systems and the 16v battery used in racing applications. So why 16 volts? A standard 12 volt car battery has six cells at 2.1 volts each. A fully charged battery will show 12.6 volts on a voltmeter. Adding two additional cells brings the fully charged battery’s capability up to 16.8 volts. Under heavy electrical loads, even with a high amp alternator, your charging system can “dip into” the battery’s reserve power. Just think of the typical race car with high voltage ignition system, electric fuel and water pumps, electric cooling fan, and more. When the charging system’s ability to run all these loads drops the battery must take over. Having those two extra volts in a 16 volt system allows an extra bit of breathing room to maintain those pumps, fans, and more at full speed. The downside is you must run a 16 volt compatible charging system (if you run a charging system at all) and use a 16 volt charger for maintenance.

What is The Best Type of Car Battery?

Knowing which type of battery to use for your application will be determined by several criteria, including BCI group/case size, type of terminals needed, performance level of the battery required, mounting location/angle, and of course budget. The various battery internals available provide a mix of price level, fitment, and performance for just about any application, with each one having their plusses and minuses. Below we provide details on the major battery styles used in vehicles today.

A flooded, or wet cell automotive battery.

Conventional Wet Cell/Flooded Lead Acid Battery

These are the most common 12v car battery and have been in constant use since manufacturers have been using in-vehicle electrical systems, charging systems, and starting systems at the onset of the 20th century. Their construction features a housing with lead plates and non-conductive separators that is flooded with an electrolyte solution typically made up of sulfuric acid and water which surrounds the lead plates inside the battery. This is where the “wet” or “flooded” terminology is derived from. Early flooded batteries used cell caps and required routine maintenance of water level checks. Today’s flooded lead acid batteries are generally found to be maintenance free and are of a sealed design. However, that does not mean that they cannot leak. Damage from road debris or vibration can cause cracks in the battery case which will allow the battery’s electrolyte to escape. Some reproduction batteries are not of the sealed type as well and can only be mounted upright and require routine maintenance of the battery fluid level. Generally, any flooded 12v car battery should be mounted upright regardless of if it is a sealed design or not due to the vent valves. The flooded battery is also the lowest cost option with the largest BCI group offerings; thus, it is still a quite common battery to use in most non-performance applications today.

One of the most recognizable AGM batteries is the Optima Red Top series

AGM (Absorbent Glass Mat) Battery

Just what is an AGM battery? The AGM battery may be new to some, but it has been around for quite some time. While the AGM still uses a liquid electrolyte solution like the flooded lead acid battery, where it differs is in how that solution is stored. The AGM design utilizes fiberglass mat material sandwiched between the lead plates of the battery that are saturated with the electrolyte solution. This absorbed fiberglass material means there is no electrolyte solution sloshing around as the vehicle moves and is a non-spillable design, allowing the battery to be mounted in just about any position. This can help with battery placement in the vehicle, such as a sideways mounting in the trunk or under the car for easier terminal access. Due to the non-spillable design and the inherent vibration and shock resistance that the AGM configuration provides, these batteries are quite popular in motorsports activities where high g-loads and vibration will easily kill a standard flooded battery. AGM batteries are often 1-1/2 to two times the cost of a flooded battery, but their longer life and better low-temperature use make them a definite possibility for most any application. The one caveat of AGM batteries is that when it comes to AGM vs standard battery charging, they do require a compatible battery charger since the AGM battery can be discharged further than a standard flooded lead acid battery. The good news is that most battery chargers today, like the Optima battery charger, have an AGM setting, so it is not a major concern unless you’re using an obsolete charger.

Gel Cell Battery

The gel battery is in some ways like the AGM in that it is not a “wet” or “flooded” battery, but it accomplishes this in a different manner. By using a silica gel mixed with the electrolyte solution the battery’s solution is now in a paste-like gel state that provides many of the same advantages as an AGM design. However, the gel cell battery enjoys lower freezing and higher boiling points, which makes them suitable for high load/vibration installations with the same leak-free design and mounting benefits as the AGM. While there are gel cell applications that might be used for a particular automotive fitment, most gel cell batteries are used as a traction battery (think power chairs, scooters, etc.)

Lithium-ion batteries are known for their diminutive size and weight, making them perfect for racing applications where mounting space is limited and weight is a crucial factor.

Lithium Ion Battery

While the standard flooded lead acid and AGM-style batteries may be much more common, we cannot ignore the benefits of a Lithium-ion battery, even though they are a bigger hit to the budget costing three to five times as much (though they are known to easily last 3 to 5 times longer than a traditional battery design). Lithium-ion cells are becoming more commonplace with the broader use of EV vehicles, however the construction of a Li-ion battery is different than traditional lead acid designs, as it uses a lithium oxide cathode, a graphite anode, and an electrolyte solution to store energy. These batteries are much smaller and lighter than traditional flooded or AGM battery designs, often weighing just a few pounds while having similar power capabilities. Li-ion batteries are also often used in motorsports where there is no alternator/charging system and the whole ignition and other electrical aspects must rely on a clean voltage signal directly from the battery. Besides the higher price tag, the Li-ion battery does require a specific Li-ion compatible charger as well.

While common in marine use, deep cycle batteries are employed by some in automotive applications for their noted reserve time capabilities.

Deep Cycle Battery

Deep cycle batteries, by design, are meant to be deeply discharged (as low as 20 percent remaining power) and be recharged without hurting the battery. This low charge state allows these batteries to be used for extreme “key on, engine off” load times. Deep cycle batteries are widely used in the marine environment for such things as trolling motors, bilge and bait pumps, and so on to keep the SLI battery isolated to allow the boat to start and return to land. Many have used deep cycle batteries in automotive applications for the same reasons—powering audio, lighting, and other items when the engine is not running, and thus the alternator is not charging. Since a standard SLI battery is not designed to be discharged in this manner without damage (anything below 90 percent is considered discharged) a deep cycle battery is becoming a more popular option since discharging a SLI battery significantly will reduce the life of the battery, especially when this is done frequently. Deep cycle batteries are known for their higher reserve capacities and is how they are usually shopped or compared.

As you can see, there are many factors that go into sourcing the proper battery for your application. It isn’t just about size, but also the intended application, what the vehicle is expected to have electrically, and more. Answer these questions and you will come to the right battery solution for your ride.

Updated by Mark Houlahan

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