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Exploring Automotive Batteries

Posted in How To on July 1, 2011
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The electric battery was invented more than 200 years ago and is one of those creations that has hugely effected the course of mankind. Batteries provide us with portable energy that can be saved, moved, and then used when needed. In the case of a lead acid auto battery, it allows us to run a starter motor for the engine and power whatever electrical components we need.

In short, the battery converts internal chemical energy to electrical energy that can be used to spin the starter motor, converting the energy again to mechanical energy. As the alternator runs, it uses the mechanical energy from the engine to produce electrical current that replenishes the chemical energy in the battery as it is recharged.

In recent years, electronics use in vehicles has skyrocketed and, with that, the power demands from the alternator and battery have also risen. Modern auto electronics are best served with a robust and reliable battery that can maintain a near constant voltage. However, in our case, we often demand far more from a battery than your average daily driver commuting back and forth to work in a passenger car.

2.11 + 2.11 + 2.11 + 2.11 + 2.11 + 2.11 = 12.66 volts A 12-volt battery is composed of six cells connected in series and each cell creates a maximum voltage of about 2.1 to 2.2 volts. The charge level of such a battery is actually close to about 12.7 volts.

Trail runners may often move at slow speeds where alternator-charging potential is minimal. We also subject our batteries to high current loads from winch motors or high intensity lights. And for those who don’t use their 4WD rigs on a very regular basis, the battery may sit unused for weeks or more at a time before being used hard on the trail or in the dirt.

How long a battery will last depends on its construction and its usage pattern, and how well it matches its application in the vehicle. Here, we’ll explore the ins and outs of auto batteries and how to get the best performance from them.

Battery Construction
Inside the battery is a series of lead (negative) plates or lead-oxide (positive) plates and a sulfuric acid electrolyte, which make up a cell. This is where chemical energy is converted into electrical energy. Each cell with the positive and negative plates immersed in the acid creates electrons which when hooked into a completed circuit can provide current to operate an electrical device. The plates are held apart with the battery housing by separators to keep them from shorting together. Additionally, the plates and solutions in the battery must be retained in a polypropylene container of sufficient strength to withstand heat, shock, and vibration present in an automotive environment.

The electrolyte solution of sulfuric acid (1/3) and water (2/3) has an ideal mix ratio. As this ratio changes, the charge capacity and condition of the battery will start to degrade. On wet cell batteries with removable caps, the mixture can be checked using a simple tool called a hydrometer to check the specific gravity of the solution.

This table below details the relation of specific gravity to percentage of charge and the open circuit voltage (OCV) of the battery. The OCV of a battery is only applicable when the battery is in a static (not being charged or discharged) condition.

State of Charge OCV Specific Gravity
Full charge 12.84 1.300
90% 12.73 1.282
80% 12.65 1.268
70% 12.57 1.255
60% 12.49 1.242
50% 12.41 1.228
40% 12.33 1.215
30% 12.25 1.202
20% 12.17 1.188
10% 12.09 1.175
0% 12.01 1.162

Battery Types
When it comes to lead acid auto batteries, there are three primary categories that are in use today: starting batteries, dual-purpose, and deep-cycle batteries. A starting battery is what you will find in most cars and light trucks on the road. Dual-purpose batteries are those where a combination of deep cycling and starting characteristics have been combined. Deep-cycle batteries are more commonly used in RV, marine, and golf cart applications. They may also be a good choice for some 4WD uses. In general, a starter battery is capable of greater instant energy, whereas a deep cycle sacrifices some instant energy for the ability to deliver energy for a longer time period.

Batteries may also be one of several types: There are wet-cell (flooded acid) batteries, sealed batteries, Gel cell, and absorbed glass mat (AGM) types.

A flooded acid battery consists simply of liquid cells where the electrolyte is allowed to move in the cell container. These batteries require the occasional addition of distilled water as it is lost over time due to out-gassing and evaporation from the battery container.

A sealed battery is a type of flooded acid battery where the owner does not have to add water on a periodic basis. Valves in the case allow the escape of hydrogen and oxygen gases during charging, while maintaining sufficient hydration for the intended battery life. Maintenance-free batteries are built with beefier internal components to reduce water loss and plate degradation over time.

Battery Ratings & Choice
Bigger batteries often mean more capacity. We’re all familiar with 1.5V dry cell batteries and know that a fat D cell puts out more sustained energy than the smaller AA batteries. When it comes to auto batteries, there are a number of common terms used to compare ratings such as Cold Cranking Amps (CCA) and Reserve Capacity (RC), along with others listed in the sidebar at the end of this article.

Gel cell and AGM batteries are similar as they both utilize a suspended electrolyte in close proximity to the internal plates. They are substantially more expensive than wet-cell batteries, but they offer some distinct advantages. They do not degrade as readily as a wet cell, do not outgas as much as a wet cell, do not leak or spill acid, and can be mounted anywhere in a vehicle and in almost any orientation. These facts make these batteries the choice when it must be mounted inside a passenger cab or when you want to ensure electrolyte does not leak under rough riding conditions. These batteries are also well suited for applications where the vehicle is not used on a daily or regular basis, due to their superior charge retention properties.

A typical starting battery is not designed to discharge very deeply. It may do it a dozen or so times, but will also be degraded and more prone to failure. If you have an application where you more habitually run the battery into deep discharge, then you should consider using a deep-cycle battery.

Ideally, before you start your vehicle, your battery should be fully charged. Then once it’s running, the alternator should recharge the battery to replace the energy used during starting and its current operating consumption. The alternator is not meant to restore a depleted battery but to maintain a good charge on a healthy battery.

With respect to the “wet”-type batteries there are standard batteries and those termed as maintenance-free. A maintenance-free battery has a beefier internal design and the plates may contain calcium, cadmium, or strontium to reduce out-gassing of the battery, which results in loss of water and reduced capacity and longevity. These lead-calcium designs are essentially a closed design battery and need no fluid maintenance over their life.

The technical subject of battery charging is quite complex. Many batteries are damaged due to poor charging techniques that shorten their life. Whenever a battery supplies energy to the outside world and discharges, lead sulfates and water are formed from the plates and sulfuric acid within the battery. Charging reverses this condition by forcing the water and sulfates to recombine to the acid. It is this reversible process that allows us to charge and discharge a battery over many cycles.

Whenever the recharging process is occurring some of the hydrogen and oxygen gases that make up the water may be lost. This release typically occurs between about 13.8 and 14.4 volts, and one goal of an intelligent battery charger is to minimize this gas loss.

To charge a battery, a controlled power source is connected to the battery and operates at a voltage a bit higher than the no-load voltage level of the battery. Better quality chargers use a charging algorithm where several voltage/current/time settings are used to more optimally charge a battery depending on its state of discharge.

When using an AGM or Gel battery, make sure the charger has a setting for AGM or Gel. Using a regular automotive charger on these batteries can cause them to over-charge. AGM and Gel batteries have a lower internal resistance, which means they accept a charge quicker than a regular liquid lead acid battery.

Jeff Barron, a technical service specialist at Interstate Battery System of America, offered some additional advice on battery storage for extended periods of time:

It used to be said that setting an auto battery on a concrete floor would slowly cause it to leak down charge. Polypropylene battery cases today provide excellent electrical insulation and have no such issue, so this is not a concern.

A: Charge the battery 100 percent before putting it in storage. It’s a good idea to check the battery every four to six weeks while in storage, and if the voltage is below 12.66, charge it again.

B: If the battery has removable caps, invest in a battery hydrometer and check the specific gravities to ensure the battery is at a 100 percent state of charge. Make sure to test all six cells on a 12-volt battery. If you have readings that vary by more than about 0.005, this could mean a weak or sulfated cell. The battery could fail without any warning. Replace it before that happens.

C: On regular maintenance batteries, check water levels periodically to insure the plates are completely submerged with the electrolyte. Batteries will only use water while in use. Add distilled water up to 1/8-inch below the vent well or at least 1/2-inch over the plates. Never add premixed electrolyte to a battery.


Chargers that have a float or maintenance charge mode, such as this compact Battery Tender Junior from Deltran Corporation, can be connected indefinitely with no ill affects to the battery. Leaving a charger without this mode connected to a battery for an extended time can result in the battery out-gassing and the cells drying out.

There are a variety of things you can do to maintain your auto battery and extend its useful life. A wet cell battery can be of a serviceable type or maintenance-free variety. When possible, check the water level several times per year and replenish as described above.

If you live in a cold winter climate area, a battery will last longer when kept inside a garage that is warmer than the outside temperature. However, in especially hot summer climates, the life of a battery can be diminished as well and many a battery will ultimately quit in the heat of summer.

Common Failures
The most common failure mode for an auto battery is that of sulfation build-up. That is to say the battery becomes so discharged that excessive sulfur clings to the lead plates and subsequent recharging will not sufficiently release the sulfur back into the electrolyte solution. This can happen for a number of reasons, such as: sitting too long between uses, too deep a discharge for a starting battery, high heat levels, low water level, undercharging, or subjecting the battery to excessive cold.

Of course physical constraints come into play as the battery has to fit in the space you have available, but capacity upgrades are often possible in a given space. Batteries that have both top and side post terminals can be very handy when you want to add a winch connection point or any of a variety of electrical accessories we typically add to our 4WDs.

To inhibit further corrosion, the treated felt washers that are available at most auto parts stores do a good job of slowing post oxidation. A dab of grease placed over the exposed metal on battery connectors will further prevent acidic condensation from attacking the metal surfaces.

Vibration in an auto environment, especially off-road, can cause electrical connections to vibrate loose and that includes battery terminals. Ensure they remain snug to maintain a low impedance path to your electrical system.

Securing Your Battery
As you well know an auto battery packs a lot of energy. Have you ever seen a hard shorted battery? Placing a dead short, such as the metal hood of the vehicle, across an auto battery can allow it to deliver literally a few thousand amps of current given a hefty, low resistance conductor. Keep your battery properly secured to avoid such a situation.

A sturdy mount mechanism and carefully routed cables will save you many headaches in the long run. If a battery bounces loose in a vehicle and shorts itself across substantial metal, the ensuing high current flow can generate tremendous heat, and can start a fire. This is made even more possible with the nearby presence of flammable fluids, such as gasoline or diesel, auto transmission fluid, or motor oil.

Choosing a Battery for Off-Road Use
We spoke with Jeff Barron of Interstate Battery System of America. He offered specific recommendations when asked about choosing a battery for off-road use:

A: Know what the recommended Original Equipment CCA rating is for the vehicle, if applicable. Not having enough CCA will starve the electrical system on initial start up and in time can damage the starter and computer components, and could also cause premature battery failure.

B: If possible, figure out the amperage draw with all components in use. This will help determine whether you need a starting or a multiple battery system utilizing one for starting and a separate dual-purpose or deep-cycle battery. A dual-purpose battery will enable a person to run deep-cycle discharges (winches) and still have capacity for starting. Using a dual-purpose or deep-cycle battery doesn’t mean to run the battery to a complete discharge. The user needs to be cautious when deep cycling the battery not to go below the threshold voltage where permanent battery damage can result.

Quality chargers today may incorporate up to four charging modes, each designed to operate at a specific voltage and current and work at various times during the battery charging process. The final mode is often called float charge mode and is used to keep the battery maintained for long periods of time, especially between driving use.

C: If the vehicle is truly off-road only, a completely sealed AGM or Gel battery might be what you’re looking for. The sealed batteries are known as Valve Regulated Lead Acid batteries meaning they will never leak and can be mounted in almost any position. Using a liquid electrolyte maintenance battery could result in the battery losing the electrolyte while on an incline, decline, or even in worst case scenario, when rolling over.

Common Battery Terminology

Hot-Cranking Amps (HCA)
A rating similar to CCA that is used to define the current output of a storage battery at 80 degrees F (27 degrees C). These inflated ratings should not be confused with CCA.

Many of us have a basic (and probably old) battery charger such as this that can do the job, but modern chargers with sophisticated charging sequences can do a far better job of charging a battery while also ensuring any ill effects are minimized to prolong battery life. Additionally, slow charging (10 amps or less) is the preferred method to replenish the energy in a battery. Higher current rates charge the battery faster but can cause the battery to heat and shed the paste material on the plates more quickly, and can shorten the service life of the battery.

Cranking Amps (CA)
A rating that is used to define the battery’s ability to start an engine in moderate temperature conditions. BCI defines it as, the discharge load in amps that a new, fully-charged battery at 32 degrees F (0 degrees C) can continuously deliver for 30 seconds while maintaining a terminal voltage equal to or higher than 1.20 volts per cell. This artificially high rating should not be confused with CCA, which is conducted at 0 degrees F (-17.8 degrees C).

Cold-Cranking Amps (CCA)
A rating that is used to define the battery’s ability to start an engine under low-temperature conditions. BCI defines it as, the number of amps a lead-acid battery at 0 degrees F (-17.8 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt lead-acid battery). See also CA and HCA.

Deep-Cycle Battery
A battery that is designed to withstand repetitive discharges to a 20-percent depth of discharge (DOD) or more and to continue providing its rated capacity after hundreds of cycles. Deep-cycle batteries are often used in marine/RV and industrial applications.

Dual-purpose battery
A combination battery with slightly less cranking ability than a starting battery, however, it has the ability to produce more medium-duty cycles (approximately 20 to 30 percent depth of discharge). It cannot produce deep cycles like a deep-cycle battery.

Depth of Discharge (DOD)
The percent of rated capacity to which a cell or battery is discharged. It is the reciprocal of a battery’s state of charge. Example: a battery that has a depth of discharge of 45 percent has a state of charge of 55 percent.

The usefulness of a battery is only as good as its connections through which the current flows when charging or using the battery. Terminals need to be kept free of dirt, grease, and oxidation buildup. They can be cleaned with a simple solution of baking soda and water.

Reserve Capacity (RC)
Defined as the number of minutes a new, fully charged battery at 80 degrees F (27 degrees C) can be discharged at 25 amps and maintain a voltage equal to or higher than 1.75 volts per cell (i.e., 10.5 volts for a 12-volt battery). This rating represents the time the battery will continue to operate essential accessories in the event of a charging system failure.

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