By usage
Batteries have to be designed for the application they are to be used in, this is a summary of the main types with quite a bit of simplification:
Traction batteries: For use in fork lift trucks, golf buggies, submarines etc., they need to deliver high currents most of the time and for good utilisation they need to be able to cope with deep discharge, lots of cycles and be capable of being recharged quickly. The high currents involved means that the plates have to be very robust so that they do not buckle and short out, which generally means they are thick. Because they are thick the surface area per Kg of lead is normally relatively low (this can, at a cost, be partially mitigated by "corrugating" the plate) so the power density (Watt hours for a given volume) is low and given the high cost of lead they are expensive and they are heavy. On a fork lift truck the weight is actually a benefit as the battery is the main ballast to stop them falling over, the same does not normally apply to a golf cart, car or a yacht.
However 6 volt golf cart batteries are often used on larger boats where the weight is less critical (if it is low down) they are quite expensive and some specialist batteries (TTPL, see below) may well be preferable for not much more money.
Starter Batteries: As you have in a car, they need to provide high current (expressed as Cold Cranking Amps) hopefully for a relatively short period to start the engine, again hopefully they will rarely if ever be deep cycled. They should preferably maintain their charge for extended periods and will need to support parasitic currents (In non-marine applications those currents keeping the clock running, the radio tuned, micro computers ready etc.) for extended periods, on some modern cars these currents are relatively high and voltages need to be stable so a car, e.g. most if not all those by Mercedes, may have a small secondary battery to act as a reserve power battery in addition to the larger starter battery.
Reserve Power Batteries: Are the type you might find as back up for a home security alarm or PC Uninterruptable Power Supply (UPS), larger UPS's are extensively used in telecoms - every cell phone base station has one - data centres etc. These batteries are designed to be able to deliver relatively low currents for extended periods, they typically don't need to cycle so frequently or so quickly and should not degrade when maintained on a trickle charger. Plates can be relatively thin so they have good power density. Boat batteries need to support frequent cycling and preferably quick charging so they are not generally applicable to boats,
Hybrid / Leisure batteries: are a compromise between all of the above, as usual with compromises they are not as good at any one function as the specialist battery but should be good enough at each function and better than the wrong type for a given function. They tend to be more expensive than the above but not by a lot.
On a boat it would be possible to use a starter battery for starting and a Leisure battery for services but generally it would be better to use a Leisure battery for starting for compatibility with the service battery.
By Technology.
Flooded batteries: For years the standard type of battery, the cells are open to the air so that when stressed with high charging currents the water in the diluted acid "boils off" and you have to replace it with distilled or demineralised water. Apart from the inconvenience of doing this there is some danger involved because as the water does not actually boil as water vapour but as Hydrogen and Oxygen, a highly explosive mix, fortunately Hydrogen being lighter than air does not gather in the bilge as does cooking gas etc. so providing there is good ventilation the risk of explosion is not that great. Other dangers however remain, particular acid burns from spills and reaction with salt water.
Flooded batteries are now banned whilst racing under World Sailing's special regulations and although they support quicker charging than most other types they should be avoided on boats.
VRLA (Sealed) batteries: Valve Regulated Lead Acid batteries are essentially the same as flooded except that they are partially sealed, if gas is given off it is contained within the battery and in time will get reabsorbed. However if the pressure inside were to get to high there would be a danger of explosion so each cell has a valve that releases the gas when pressure reaches a set level. So, ventilation is still required but acid spills are not going to happen.
The down side is that if gas is vented it is not possible to replace the water and eventually the battery will have to be junked. To prevent this "Sealed" batteries, including AGM, must not be charged as aggressively so it takes longer. This is important, some chargers can sense which charging profile is required or just assumes the slower charge rate, but other chargers, alternator regulators etc. do not so they must be set to the correct battery type. Normally VRLA can be cycled to 50% of rated capacity without prematurely aging the battery, some newer types are claimed to be able to go deeper.
Someone who owned Sancerre before me replaced flooded batteries with VRLA but did not change the jump links on the external alternator regulator to select VRLA, as a result the batteries were charged to quickly, became "gassed" and had to be replaced before their time. Fortunately I discovered the cause before I replaced the batteries or it would have happened again with the AGM's I installed.
AGM batteries: Absorbent Glass Mat batteries contain a special glass mat separator that wicks the electrolyte solution between the battery plates, the glass mat helps hold the battery plates apart so that resistance to vibration and general robustness is improved. Newer versions with improved technology allow 80% of the capacity to be used without prematurely aging the battery as compared to a recommended 50% for flooded and early AGM types so the effective power density is much improved usually more than making up for the increased cost over VRLA types.
TPPL "Odyssey" batteries: Thin Plate, Pure Lead batteries, are an exception to the "compromise rule" mentioned above, the manufacturing techniques and design are proprietary technology developed by Gates then acquired by BTR (later Invensys) before the product group was involved in a reverse take over by the smaller Enersys (full disclosure: I was the IT Director for the product group and moved to Enersys). The genuine batteries for marine applications are generally sold under the brand name "Odyssey". Knock offs, mainly from China, are in the market but having been in a plant producing them doing due diligence for an acquisition we did not follow up on, I would avoid them like the plague.
These batteries have thin plates of very pure lead (Enersys built them in separate facilities to avoid cross contamination) with minimal other metals and minerals and effectively no calcium that is in fairly high concentrations in other lead acid batteries. The plates are built in such a way that they don't buckle and with the use of a special acid blend they provide very high power density, high power output and support deep discharges for many cycles and will outperform all of the above types. The down side is they are expensive to manufacture and very expensive to buy.
Things to check when buying
- Usable capacity in AHr without prematurely aging the battery is a key issue so check the recommended DoD for the batteries under consideration, some modern VRLA (sometime called "Advanced VRLA") are better than older / cheaper designs and claim a safe DoD of 70%, not as good as AGM but better than older types, I have not research this type which as only come onto the market recently.
- Warranty period which gives an indication of the makers confidence in the product and its likely quality, one maker has batteries with of similar capacity with 2 and 5 year warranties.
- Battery lifespan in cycles.
- For any battery that could be used for engine start, its suitability for engine start and the Cold Cranking Amps (CCA) available.
- The recommended charging profile, if not available assume the less aggressive possibility.
- Terminal location and type(s).
- Weight & size.
- See also rules for connecting batteries in parallel.
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