Batteries: energy packaged under control

What to know about our boat batteries: main battery types, how they work and charging status.

Often forgotten in the corner of a box, we usually agree on them only when we produce a problem. From the engine start to the most forgotten detail of our boat you need a power supply, an energy that, in the sea, we can not obtain so easily. Batteries make up for this lack of supply and offer, preserved, an essential of which it never hurts to know every detail element.

When we run out of battery in the car, the problem, though annoying, no longer a mere incident relatively simple solution. Start the car downhill or seek help from a neighbor with cables, we can easily take the trouble. But at sea we can not start downhill and, without power, most of the electronics on board, if they can not run on batteries, will not work.



A battery stores electric energy through a chemical reaction. Conventional lead-acid batteries consist of plates in which molten lead is inserted the active material, these plates are immersed in a liquid compound of dilute sulfuric acid. The plates are called electrodes and electrolyte liquid. Connecting plates alternately the two poles, positive and negative are achieved.

As the batteries are discharged, part of the acid separating from the electrolyte to form a chemical compound with the active material of the positive plates, making lead sulfate to be deposited on the negative plates, producing water dilutes electrolyte.

When the batteries are charged, sulfated acid material negative plates, returns to the electrolyte. At the end of the load, the excess current decomposes water into hydrogen and oxygen, which are released as gases.

This conventional type of battery has two drawbacks, the first is that lose charge when at rest; every day can lose 1% of their capacity at normal temperature, but under conditions of high temperatures can lose up to 3% per day of capacity, which in three months of inactivity will lose all its load. The second drawback is that the gasification process consumes water and therefore have to add water periodically. This is why the conventional batteries have plugs filled with vents for exhausting the gases. Use in nautical these batteries has several drawbacks, by continuous movement to which they are subjected, the electrolyte can reach out through vents plugs and worse, the lead plates can get to work without being the all covered by the acid, which results in a loss of performance and premature aging.

Maintenance free batteries

The R & D manufacturers soon discovered that what caused these difficulties was used antimony alloy lead to stiffen the grids. Thus, the antimony content decreased or even, in some cases entirely eliminated by substituting other alloys. This other type of battery does not lose its capacity as quickly and as gasification is drastically reduced, may contain a greater amount of electrolyte on the grids, which means they do not need to be completed for several years. In most cases the caps have been removed and are called “maintenance-free batteries.”


Another development, has been to solidify as a gel, the battery electrolyte. Thus, the batteries can work in any position, even inverted yield can reach one hundred percent and have no danger of leaks, even in case of breakage of the outer box. Another great advantage of these batteries is that they are able to maintain several deep discharges, such as those generated when starting the engine, and self-discharge time is less. Ultimately, gel batteries, despite being more expensive, are safer and more profitable long.


We have seen that a battery is a very peculiar and delicate type of deposit. You can not give all the stored electricity back to be filled later. A battery has discharged more than 80% capacity can be considered lost. If the download long hard, the negative plates are coated by excess lead sulfate and can not restart the charging process. It is therefore important to always know what the status of our battery charging, to proceed to load as soon as possible.


Until recently, the most accurate way to check the state of charge of a battery system was the hydrometer. This apparatus consists of a glass syringe with a rubber bulb in its end and a float graduated inside. This simple contrivance serves to measure the specific gravity of the electrolyte (mixture density of acid and water). The more the battery is discharged, the acid is diluted more and lighter is the electrolyte. To know how to interpret the readings of the hydrometer, we must consider that a fully charged battery will have a specific gravity of 1.280 at a temperature of 27 ° C, half load will give a reading of 1.200 which tells us that can still be used, but must be loaded as soon as possible. A discharged battery will mark 1,150 and must be charged immediately. Most float hydrometer have three colors for easier reading and show whether the state of charge is good, average or bad.

The hydrometer also warn us of a possible battery fault giving the internal state of each of the vessels. If one of the vessels is in poor condition and gives us a reading below 0.050 than the average of other vessels we can not expect the battery to continue to work successfully for much longer.


But we know that is not usual practice a hydrometer on board pleasure craft and on the other hand the recondite location of the batteries would not allow us, in many cases, these measurements. Usually we will have on board voltmeters only as a reference for the status of the batteries.


At full load, an electrolytic battery has an open circuit voltage of 2.1 volts per jar, 12.6 volts in total (for 12 volt batteries) rated voltage. Upon discharge of all the battery, the same voltage drops to 11.8 volts; intermediate values ​​of load and strain are related almost linearly

Following the parameters of this table, it would seem easy to know at all times the state of charge just by skimming the voltmeter, however, this measure would only be valid if it had been a minimum of 24 hours without loading or unloading (48 hours for dry batteries) time required for ions to stabilize the plates. Otherwise, the voltmeter reading would say little state of charge and discharge can be misleading. For example, when starting the engine the voltage drops rapidly because the chemical processes are relatively slow, limiting the rate at which a battery can release the stored energy (chemical reactions are not as fast as to maintain the voltage required for the supply current demanded by the engine).

However, the battery does not take long to recover because the chemical reaction is reversed, recovering the tension imposed by the regulator, normally between 14.0 and 14.4 in 12-volt batteries during recharge. When disconnecting charger, the diffusion process immediately reduce this value to the stable voltage between the plates.

OPTIMIZATION board batteries

Installed on a ship batteries must be able not only to energize all electrical equipment, but also to ensure a minimum autonomy. To calculate the dimensioning of the batteries from our boat, we will add the consumption of all electrical and electronic instrumentation to obtain total consumption on board. For example, the average consumption of a boat with fridge and normal equipment of electronics, hovers around 80A / day, of course this value will increase as we introduce new devices, we mount refrigerators increased capacity or simply sail a lot at night, with consequent increase in electricity consumption caused by navigation lights and interior.

Anyway, on board modern ships, this consumption has been significantly reduced thanks to LED technology bulbs. Continuing the example, if instalásemos batteries that should give the 80A / day required, we would be doing them to work at full capacity, being more than likely the risk of shock. To avoid this problem, we mount batteries capable of doubling or tripling the required consumption better. In this case, we would install batteries capable of withstanding currents of 240A / day, thus ensuring the functioning of the plates without being in continuous discharge. When installing equipment on a boat, it is important to know their consumption, especially if they are electromechanical; for example a refrigerator consumes about 50A, and a 35-watt radar. Clearly, if we install several hit these elements, the current expenditure is triggered and may require a new dimensioning of the electrical system.