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Exploring Automotive Batteries

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Jay Kopycinski | Writer
Posted July 1, 2011

Chemical Energy

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.

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.

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.

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.

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