A battery is an electrochemical device that stores and delivers energy. Energy is chemically stored in the battery and when the terminals of the battery are connected through a resistive load, electrical energy passes through the circuit.
Automotive lead-acid batteries are commonly referred to as starting batteries, which are generally 12 volts (12V) and are rechargeable. These batteries supply power to the starter and ignition system to start the engine and to internal clocks and computers that require constant power. Lead-acid batteries also supply extra power needed when the vehicle’s electrical load exceeds the supply from the charging system (alternator) and acts as a voltage stabilizer in the electrical system, evening out voltage spikes and preventing them from damaging other components in the electrical system.
Anatomy of a Lead-acid Battery
A 12V lead-acid battery contains six separate cells at two volts each. The cells are connected in series by welding connections through the cell partitions. Each cell contains an element or book that consists of stacked positive and negative plates. The battery is then filled with liquid electrolyte (a diluted solution of sulfuric acid and water) or as it is commonly called, battery acid.
Batteries are made of five basic components:
- Positive plates
- Negative plates
- Separators – usually a micro-porous polyethylene synthetic material
- Polypropylene container
- Liquid electrolyte
Plates are comprised of two parts – the grid and the paste, both made of lead. The paste is soft and is the active material that chemically stores the energy. The grid provides a skeletal framework for paste adherence. Grids are made of lead alloys that resist corrosion and conduct electricity to and from the plate.
Positive and negative plates are stacked in an alternating fashion, with separators between the plates to prevent electrical shorting and to enable battery acid to flow back and forth. Both the positive and negative plates are connected at the top by a cast-on-strap that is welded to the plates. When a battery goes from a charged state to a discharged state, it is called battery cycling. During discharge, energy is released from the battery as is demanded by the electrical system. The battery is recharged with energy generated by the alternator.
Battery Council Industry (BCI) specifies standard battery sizes and configurations by group size. This includes standard overall size – length, width and height of the battery – and standard terminal type and position.
Cold Cranking Amps (CCA)
This industry rating measures the power a battery has available to start a vehicle’s engine at zero degrees Fahrenheit. For a 12V SLI battery, BCI defines CCA as the amount of current (number of amperes) a lead-acid battery at zero degrees Fahrenheit can deliver for 30 seconds while maintaining at least 7.2 volts.
Reserve Capacity (RC)
A battery’s RC represents the length of time a battery can maintain a vehicle’s electrical needs in the event of a charging system failure (alternator failure). BCI defines RC for a 12V SLI battery as the amount of time (in minutes) that a battery can deliver 25 amps at 80 degrees Fahrenheit while maintaining terminal voltage of at least 10.5 volts.
What are the major types of lead acid batteries?
Batteries can easily be divided in two ways, by application and construction. The major applications are starting, dual purpose, and deep-cycle. The major construction types are flooded (wet), gelled, and AGM (Absorbed Glass Mat).
Sealed Maintenance Free Battery
Sealed batteries are commonly known as maintenance free batteries. They are made with vents that (usually) cannot be removed. A standard auto or marine maintenance free battery is sealed, but not fully leak proof. Sealed batteries are not totally sealed since all batteries must allow gas to vent during charging. There are sealed lead acid (SLA) batteries that are non-spillable.
AGM or Absorbed Glass Mat Battery
The newest type of sealed non-spillable maintenance free valve regulated battery uses "Absorbed Glass Mats", or AGM separators between the plates. This allows them to take much more abuse. Because of the way they are constructed, an AGM Battery will not leak acid if broken.
What are the advantages of the AGM battery?
The primary advantages an AGM Battery has over a conventional battery are: zero maintenance, vibration resistance, quicker charge recovery, no outgassing of fumes, non-spilling (even if they are broken), and can survive most freezes.
AGM batteries are "recombinant" – which means the Oxygen and Hydrogen recombine inside the battery. These use the gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost..
AGM batteries have a very low self-discharge rate (from 1% to 3% per month). So they can sit in storage for much longer periods without charging.
The plates in AGM's are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.
What is a Gel Cell Battery?
A gel battery design is typically a modification of the standard lead acid automotive or marine battery. A gelling agent is added to the electrolyte to reduce movement inside the battery case. Many gel batteries also use one way valves in place of open vents, this helps the normal internal gasses to recombine back into water in the battery, reducing gassing. "Gel Cell" batteries are non-spillable even if they are broken. Gel cells must be charged at a lower voltage (C/20) than flooded or AGM to prevent excess gas from damaging the cells. Fast charging them on a conventional automotive charger may permanently damage a Gel Battery.
Wet Maintenance Free
Wet Maintenance Free batteries have a Lead with Calcium alloy in the positive and Lead with Calcium alloy in the negative plate chemistry or formulation. They are available in non-sealed (with removable filler caps) and sealed (with non-removable filler caps) versions. The non-sealed versions are recommended for use in hot climates, so lost water can be replaced.
- Less preventive maintenance due to less water loss
- More forgiving when accidentally overcharged
- Reduced terminal corrosion and ventilation
- Smaller self-discharge rate
- Less risk to consumers because there is less to service
Wet Marine/Recreational Vehicle (RV)
Wet marine/RV batteries are available in three different versions--starting, dual purpose, and deep cycle.
- The starting marine/RV battery is basically a wet car/starting battery with carrying handles and is designed for high current and shallow discharges (up to 5% Depth-of-Discharge).
- The wet dual purpose marine/RV battery is a compromise between a starting and deep cycle battery that is specifically designed for high vibration in marine/RV applications.
- The deep cycle marine/RV battery is designed for deep discharge applications such as a trolling motor.
- A marine/RV deep cycle or dual purpose battery will work as a starting battery if it can produce enough current to start the engine.
Good ventilation is required for all wet (or "flooded") batteries to dissipate the gasses produced during charging. For saltwater applications use only a sealed AGM or Gel Cell battery to prevent the formation of deadly chlorine gas that can occur if battery electrolyte is mixed with saltwater.
AGM (Absorbed Glass Mat)
Sealed Absorbed Glass Mat have a very fine glass mat between their plates. They have all of the advantages of the "Maintenance Free” batteries plus:
- Much safer then wet batteries (due the hydrogen gas recombination during charging)
- Does not require water
- Shorter recharge time
- Lower self-discharge rate (typically 1%-2% per month)
- Longer service life (typically up to twice as long)
- Better vibration endurance
- Can be used in saltwater applications
- Spill proof and can be mounted in virtually any position (because they are sealed)
- Can be used inside a semi-enclosed area, like the passenger compartment or trunk
- No sulfation from electrolyte stratification or water loss
Gel batteries use a thickening agent to immobilize the electrolyte within a battery. Gel Cell batteries have a lot of the same advantages and disadvantages of AGM.
If a battery is to be used in a starting application the cold cranking amp (CCA) rating is a very important consideration. The battery's CCA performance rating should meet (or exceed) the vehicle manufacturer's recommendation.