What is the Best Boat Battery?

Choosing the Best Boat Battery For Your Vessel

Having researched the properties of Flooded Lead Acid, AGM and Gel batteries, boat owners could be forgiven for not being able to see the wood for the trees. So what is the best boat battery technology to choose?

Theoretically, Flooded Lead Acid Batteries should have the longest lifetime and be the most cost efficient. In practice, they will hardly ever achieve the cycle life that they are capable of, because it is conditional on optimal charging, discharging and maintenance.

Such a charging and discharging regime is usually at odds with the operational requirements for the boat, i.e. the battery will be discharged as needed and recharged when possible or convenient for the owner. Many boat owners are also not technically equipped to do the routine checks and maintenance, or they simply don’t have the time to do so regularly. We see many owners who bring in FLA batteries that have failed after two years.

FLA batteries have a low charge acceptance rate, so it takes longer for them to be fully charged. If engines have to be run longer to charge them, this could mean that the lifetime cost of these batteries is significantly more due to the incremental cost of fuel, servicing, and engine wear and tear.   Add to those factors the disadvantages such as high maintenance requirement, the risk of acid spills and in severe emergencies, the risk of chlorine gas poisoning when acid is mixed with sea water, the venting of acid gasses during charging and the problems and inconvenience caused by their high self discharge rate.

The bottom line for FLA batteries is thus that their only real advantage is their slightly lower initial cost. Their lifetime cost is usually higher than for other battery types, and they are a lot less pleasant to live with.

Absorbed Glass Mat and Gel batteries have many advantages over FLA batteries. To start off they are spill proof and don’t gas, so they are a lot safer and more pleasant to have on board.
They have a high charge acceptance and discharge rate. That means that they can be recharged quickly, provided that there is sufficiently high charging ability. That can mean that engines have to be run a lot less, or it may not be necessary to run engines solely for battery charging at all. The high discharge capacity of particularly AGM batteries can mean that a smaller cranking or winch battery can be fitted, saving cost and weight. Their low self discharge rate means that they can be left for long periods without charging.

The one proviso that must be taken into account when considering AGM or Gel batteries is that they do require more sophisticated charge controlling, which can mean a greater once off cost for adequate battery chargers, alternator regulators and/or solar and wind charge controllers.

In our opinion, the slightly higher initial cost of AGM or Gel batteries is well worth the advantages down the track. The prices of these batteries have come down in recent years, and good quality AGM batteries are now available that cost only 10% to 20% more than comparable quality Flooded Lead Acid batteries.

Our usual advice is therefore to buy AGM batteries for use on occasionally used boats, i.e. boats that are used only two or three weekends a month and for the odd week or so during holidays.
For live aboard situations, i.e. where the batteries will be deep cycled on a continuous basis on cruising boats or permanently occupied house boats, no battery type will outlast correctly managed Gel batteries. They will probably last twice as long as AGM batteries, so their lifetime costs will be lower despite the higher initial outlay.

Gel batteries have a relatively low cranking ability, but if there is a large enough gel deep cycle battery on board, that should be adequate to start any medium-sized engine.  Cruisers that want the security of a separate, dedicated starting battery may consider using an AGM battery for that purpose. In that case it is recommended to select an AGM battery that can be adequately charged at the settings required for charging the gel batteries, or the AGM battery must be charged by a series charger such as the interVOLT SPCi that is capable of raising the charging voltage.  So for most boat owners the best boat battery is an AGM Battery but you should give Reliable Batteries a call to confirm your choice.