Batteries are designed for many types of duty. The common car starter battery needs to produce very high currents for a short time and is not cycled deeply. It has many thin plates with lots of electrolyte contact to give very low resistance. The thin plates shed their active material easily and fall to bits after a few deep cycles.
Old type stationary batteries has a few very thick plates and could stand 20 years of deep discharge cycles. They were heavy, costly and needed lots of maintenance but given that they were excellent.
Most batteries now are of the car type with thin plates or a compromise with thicker plates and longer cycle life. Marine type batteries are not much more than better car batteries and will usually work well enough as starter batteries and stand a few more discharge cycles.
The next step up is semi traction types with thicker plates, these wont work well as starter batteries but suit golf carts ans small electric vehicles. they are good for RE.
All these are lead acid and the difference between flooded, gel and AGM is mainly in the type of electrolyte. There are also differences in plate grid hardening materials.
Wet ( flooded) tend to have Antimony as the alloy. these have many good properties but are prone to self discharge. You can get calcium flooded cells and these are usually classed as low maintenance as they tend to use less water.
For conditions where they need to be true low maintenance the electrolyte is restricted and either contained in a gel or it is absorbed in glass mat (AGM).
As these forms can't be topped up they are invariably Calcium based so that they can operate with less need for equalisation.
AGM and gel are clean to deal with and can be used in equipment such as UPS and are a perfect choice for unattended use.
Where you can give some attention Flooded have some advantages, particularly in completing the final charge quickly and they are less fussy about over charging.
Gel and VRLA ( AGM) have better properties with regard to self discharge.
In general the flooded cell will be considerably cheaper but sealed versions have a long life if treated correctly, They cost more but you get some of your money back.
I would suggest you use sealed varieties if you are away from them for any long time and you want them more as back up than cyclic use and be prepared to spend money on clever controllers.
If you want them to work hard, can top them up regularly, equalise them occasionally and don't want to spend a fortune on controllers the flooded cells are your best bet.
For small solar installations the low self discharge of sealed cells gives them some merit but more panel may cost less when used with a flooded battery.
All have their place and all have their problems.
Flux
