Service Lead batteries, how to choose the right one:

This post is the English translation of a post published by Roberto Minoia on his blog; you can find the original Italian version here  ; a huge thank to Roberto for allowing me to traduce and share this article

The service batteries are one of the important parts of the electric system, they are objects on which we mostly rely for our comfort while cruising.

In spite of this they’re often misunderstood by the vast majority of sailors , they’re so deeply misunderstood that often when it’s time to purchase new service batteries, sailors simply lacks the competence to do the right choice among the thousands of models available on the market.

I took the decision to write this essay after a chatter on a Facebook group, in which became to me totally clear how much confusion lies on the subject of lead-acid service batteries.

I’ll start with few basic concepts on batteries, although they will be totally clear for a lot of readers, then I’ll go deeper into technical details around service batteries.

What’s a battery ???

A battery is often also called an Electric Charge Accumulator, and basically it’s a electric energy tank; yes, exactly a tank, like the freshwater or the fuel ones, the only difference is that batteries is filled with electric energy instead of gasoline or water; as the water tank has a capacity measured in liters or gallons, a battery capacity is measured in A h (Ampere x hour); we can fill a water tank with water coming form the marina water pipe or coming from an on-board desalinizing system, in the same way we can fill a battery with shore power line or with energy coming from on board electricity generators (engine’s alternator, solar panels, windmills).

We drain water from water tank opening the shower tap, in the same way we drain energy form the batteries switching on the lights or the oven.

As you can there a lot of similarities , with these examples we have just introduced an important concept: Battery capacity, that is measured in Ampere x hour (not Ampere /hour) and it’s straightforward that the bigger is the capacity and the bigger will be our energy on board storage.

We can increase capacity fitting more batteries and plugging them with a parallel link; in this way the total battery pack capacity will be the sum of the batteries’ capacity. It’s highly recommended to use identical batteries (same capacity, same brand and model, same purchase date) to set up a battery pack, if we omit to do this, batteries can get damaged quite quickly.

Assembly Technology of lead batteries

the standard lead-acid batteries present on the market are mainly divided in three categories:

• free acid batteries (or liquid electrolyte batteries)

• AGM Batteries (Absorbed Glass Matt batteries)

• Gel batteries

the difference among these three categories is the kind of electrolytic solution in which the lead plates are submerged.

• free acid batteries: the liquid electrolyte is a simple acid solution of distilled water and sulfuric acid; the main drawback of these batteries is that they can’t be heeled too much, otherwise they’ll leak acid into battery box or bilges; even the so called “sealed battery” are not totally sealed ; we’ll have the same concern in case of shocks, the battery case may get broken and the water-acid solution will leak in bilge.
• AGM batteries: in this category of batteries the sulfuric acid solution is drowned in a kind of fiberglass sponge. In this case there’s no free liquid and batteries are much more safer, almost for marine use, in an environment in which vibrations, sudden accelerations, shocks and high heeling angles are the normal working environment of the battery; an AGM battery is free form risks linked with acid leaking and may work also upside down, it is also a really free service battery; another advantage is that AGM battery have a low internal resistance, so they can be charged more quickly (with higher ampere values) than free acid ones;
• Gel Batteries: they have several advantages in common with AGM batteries, given the fact that the acid solution is a gelified electrolyte instead of a fiberglass absorbed one, so also gel batteries do not leak in case of case damage; the main difference among gel and AGM batteries is that the last ones have a higher current, comparable to a traditional free acid one; that is the reason why AGM batteries can be used as engine battery, while a gel battery may lack the current needed to start the engine. Another feature of gel batteries is that they’re quite delicate in charging operations, so they need a dedicated charger suited for this kind of batteries; formation of hydrogen bubbles within the gel may occur if charging current is too intense, damaging the battery.

Batteries for Deep Drainage operations (Deep Cycle Batteries)

The common things among the three types of batteries end here.

You may have noticed that until now we didn’t take into account the kind of use (services or engine starting) for which the batteries are fitted on board. We just told that gel batteries are not suited for engine use.

The capital question is : which is the best battery for service use ?

Most of the sailors think that the type of battery (free acid, AGM or gel) is strictly linked with a specific type of usage (engine or service), but this is definitely not true.

There are other battery features that lead the game, making each model of battery better suited for a specific use, and they have nothing to do with the battery technology.

There are a lot of AGM battery perfectly suited for engine start (like those widely used on cars), as there a lot of free acid batteries perfectly suited to be used as service batteries.

In general terms an engine battery is designed to withstand very high current for very short times (few seconds until engine start) ,but it’s not designed to be deeply drained by prolonged use at low-average currents rates. In other words this kind of batteries are never drained for more than 10% of their total capacity, and if used correctly their lifespan is around 4-5 years.

On the opposite, service batteries must have totally different features: it’s not important that they’re able to give very high current rates for few seconds, but it’s mandatory that they are able to withstand a lot of deep drainage cycles without getting damaged.

All the Lead batteries have one thing in common: the number of chrage/drain cycles that hey are able to bear before they’re “dead” is inversely proportional to how deeply we drain them.

In other words if we take a generic lead battery and charge/drain it up repeatedly up to 10% of its total capacity , it will last X years, if we charge/drain the same battery up to 50% of its capacity, it will last a LOT less than X years.

This the CAPITAL concept of this article, and it’s always TRUE for lead-acid batteries.

Each battery has a technical datasheet, in which there must be a diagram like the one we attach here; in this graphic we can observe the lifespan of the battery, in terms of charge/discharge cycles vs. The DoD (Depth of Discharge) that is how much in percentage we drain the battery in each cycle.

Summing up, the only thing you have to do, and you must do it, when you choose a service battery is to verify how many deep charge cycles it will bear, assuming a Depth of Discharge (DoD) of more than 50%. In this way you’ll have an idea of how many years it will last.

An engine battery, if you discharge it deeply, may last less than one year, while a service battery, designed for deep cycle use, may last up to ten years without concerns, given the same use.

A service battery may cost two or three times more than an average engine battery, but it will definitely pay back itself in years of service on board.

If you’re going to buy a new service battery, pay attention to technical datasheet, often the same brand puts on the market different battery lines with different technical features, also with the same technology.

The following question is: ok, this is perfectly fine, but how can i properly size my service battery (or battery pack) , how can I make an electrical budget of my cruising activities ? Soon I’ll write down something about this, stay tuned.