Being of a curious nature and having bought deep cycle batteries for a boat many years back, I was curious to find out if AGM batteries were not available as deep cycle types by reason of their internal make-up. I googled AGM deep cycle batteries and it seems there are many advertised as such...?
I`m no battery expert however as best I understand it deep cycle batteries are designed to hold their charge longer than standard lead acid batteries during periods of inactivity thus perhaps more designed for Boat, caravan and other similar usage situations..?
These days with some vehicles being left unused for weeks if not months at a time, I suspect there may be just a bit more chance of firing it up when needed after lay-up if it has a deep cycle battery fitted...?
With the ability to attach an automatic float charger during periods of lay-up, there seems little requirement to pay extra for a battery with deep cycle functionality..? Or have I got that all wrong..?
Apologies for correction George but just wanted to explain the substantial difference between a true deep cycle battery with the other extreme, a cars starter battery.
A deep cycle battery has a lesser number of thick plates for any given size allowing the electrical to chemical energy conversion to be stored deep into them. This makes the battery very robust in terms of longevity as it allows discharge to low levels without undue damage, hence the name. It does mean that charge and discharge currents are lower so take place over a longer period of time. The main reason is the much reduced electrolyte to plate surface area since it's at the surface of the plates that the electrical/chemical and vice versa conversion takes place. It also takes a finite amount of time for the chemical change representing the electrical charge to sink deep into the plates composition and of course vice versa when it's asked to discharge back into electrical energy. This makes them ideal for boat or home overnight storage where they would typically be part of a battery bank dissipating the charge/discharge current to a modest level between batteries.
A starter battery by comparison is a single battery required to give a very large current for a short period of time, typically seconds. This is achieved by a large number of thinner plates giving a much greater surface area of electrolyte to plate interface and so accomodating the high current demanded. Although seemingly more fragile it achieves reasonable longevity by light use, typically discharging to only 10% or less of fully charged capacity before the cars alternator tops it back up to 100% again. A true deep cycle battery would therefore be compromised if used as a cars start battery.
There is a halfway house known as the leisure battery popular on caravans and campers although favouring start battery requirements.
The above relates to AGM or normal lead/acid construction although deep cycle requirements favour the latter.
