Choosing A Battery
One of the leaders in AGM technology is Optima. Their Spiralcell construction gives their batteries an unmistakable "six pack" look, but also provides more plate surface area, closer plate spacing, and the ability to use high-purity lead. The result is extremely low internal resistance, which results in more power in a smaller package, the ability to recharge much faster, and higher and cleaner voltage characteristics during discharge.
Optima offers three different categories of batteries, each is identifiable by the color of the top and case.
These are designed for engine starting where an alternator immediately monitors the state of charge and provides energy to the battery whenever it is needed. For a vehicle with average electrical demands, this is the battery to use.
Yellow Top: Considered a deep-cycle battery, this is the battery to use when electrical loads are higher than average, or when the discharge cycle is more than typical engine starting, such as vehicles without alternators. This also includes racing vehicles without a charging system, cars with audio/video applications with large electrical demands.
Blue Top: This dual-purpose battery can be used for both starting and deep cycling; it is a true deep-cycle battery with extremely high cranking power. The difference between Blue Top and Yellow Top deep-cycle batteries is that Blue Top batteries have both automotive (SAE) posts and threaded posts, while Yellow Tops (other than D31T) only have SAE posts.
Here are two examples of parallel...
Here are two examples of parallel connections of a discharged Optima and a charged battery.
To keep things simple, when it comes to Optima batteries if it has a dark gray case then it is a starting battery; if it has a light gray case then it is a deep-cycle (dual purpose) battery.
Optima Tech Tips
Charging Your AGM Battery
Low and slow is best. A low-amp charger (1 to 10 amps) is always the best choice for charging any lead acid battery. It's quicker to charge at higher amperage, but it also generates a lot of heat, which reduces the life of a battery, just like the raging heat of summer.
Many newer battery chargers, or Smart Chargers, have microprocessors that collect information from the battery and adjust the current and voltage accordingly. Some have different settings for charging wet cell (flooded), gel, and AGM batteries.
All lead acid batteries can experience sulfation-the formation of lead sulfate crystals upon discharge. Look for a charger with a de-sulfation mode to help condition your battery and keep it performing at its best.
Alternators are NOT chargers. Don't rely on your alternator to do the work of a charger. An alternator is meant to maintain a battery, not charge it.
Batteries eventually die. Batteries are a consumable product. No battery will last forever. The goal is to consistently maintain your battery to get the most life out of it.
Resuscitating a Deeply Over-Discharged AGM Battery
Don't Throw Away Your "Dead" AGM Battery. In time, AGM batteries, including Optima batteries, may fail. Failures are typically caused when a starting battery is used in a cycling application, in which a deep-cycle battery is the better choice. However, in many cases, Optima batteries that are assumed to be bad may actually be perfectly fine, just deeply over-discharged.
AGM batteries, including Optimas, have incredibly low internal resistance. This allows very high amperage output and for the battery to accept a charge very quickly. However, due to that very factor an AGM battery doesn't react like a traditional flooded lead acid battery when recharging is required.