Originally Posted by dereckbc
OK, this may help or put you to sleep. I am a telecom engineer who designs a lot of battery plants so batteries are up my alley so to speak. There are basically four types of batteries used for carts and EVís: Gel, AGM, Flooded Lead Acid (and its cousin Valve Regulated Lead Acid), and Lithium-Ion.
Gel cells have an acid that has been gelled with silica. The advantage gel cells is that it is almost impossible to spill acid even if they are broken. However, there are several disadvantages. They must be charged at a slow rate (C/20) to prevent excess gas from damaging the cells by drying out and cracking the electrolyte. They cannot be fast charged.
Some other disadvantages of gel cells is that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 1 year to cause premature battery death.
So if you were to use gel cells you would need a special charger which in all likelihood would have to be custom built or ordered. Not too mention that gel cells are more expensive like 2 to 3 times flooded lead acid.
Is a newer type of sealed battery uses "Absorbed Glass Mats" between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse.
AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:
Since the electrolyte is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.
Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use 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.
The charging voltages are the same as for any lead acid battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents.
AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries.
The plates in AGM's are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery. AGM's will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don't have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice. AGM batteries main advantages are no maintenance, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes. Not everyone needs these features.
Flooded Lead Acid & VRLA
Since both are like technology I will treat them as the same.
Sealed batteries are made with vents that (usually) cannot be removed. The so-called Maintenance Free batteries are also sealed, but are not usually leak proof. Sealed batteries are not totally sealed, as they must allow gas to vent during charging. If overcharged too many times, some of these batteries can lose enough water that they will die before their time. Most smaller deep cycle batteries (including AGM) use Lead-Calcium plates for increased life, while most industrial and forklift batteries use Lead-Antimony for greater plate strength.
A few industrial batteries have special caps that convert the Hydrogen and Oxygen back into water, reducing water loss by up to 95. Lead-Antimony batteries have a much higher self-discharge rate (2-10% per week) than Lead or Lead-Calcium (1-5% per month), but the Antimony improves the mechanical strength of the plates, which is an important factor in electric vehicles. They are generally used where they are under constant or very frequent charge/discharge cycles, such as fork lifts, golf carts, and floor sweepers. The Antimony increases plate life at the expense of higher self discharge. If left for long periods unused, these should be trickle charged to avoid damage from sulfation - but this applies to ANY battery.
As in all things, there are trade offs. The Lead-Antimony types have a very long lifespan, but higher self discharge rates. The lead acid battery is the most economical choice used for golf carts for most applications, and no one makes a battery better than Trojan for golf carts.
I am not going into this too deeply because Li is not really a good choice right now for several reasons. Most importantly is they loose on average 10% capacity per year starting from the date of manufacture, and is accelerated when kept at 100% charged up.