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sailing and diving around the world
What is a battery? In this case we're talking about the batteries that power a boat and, in this case, a sailboat. There are different kinds of batteries taht are used on boats, but, based on my experience, from what I've seen on the boats I've crewed on, and from talking to countless other yacht owners and Captains, the Trojan T-105, deep cycle, industrial golf cart battery, seems to be the way to go, for a sailboat. The Trojan, 6v, seems to be very popular. These batteries are specifically designed to be discharged down as much as 80%, which is why they are called 'deep cycle.' Sailboats need long-term power, as opposed to the quick, powerful bursts of energy, say, required to start a car. 'Cycle' means the process of using a fully charged battery, down to a particular state of discharge.
A volt is a unit of measurement of electrical potential, or 'pressure.' On a 6 volt battery, a single cell is 2 volts. A watt is a term used to measure total power. A watt is amps, multiplied by volts. A battery that can supply 240 amp hours (AH), at 12 volts, is equal to 2880 watts.
So, what is a battery? A battery is a device that, rather than making electricity, as many people think, simply stores the electricity, in chemical form. Batteries are basically the only method to store direct current (DC) power, produced from sources like solar panels, wind generators, or generators. Think of your batteries like a bucket of energy, where the voltage is equal to pressure, and amperage equates to flow rate. Imagine that you are slowly pouring water into a bucket that has a small hole in the bottom. As you pour the water into the bucket, its slow leak will mean that you'll probably use 12 gallons of water to fill a 10 gallon bucket, by the time it is full. In the same way, it takes more energy to charge a battery than it will actually store. The size of the bucket is analogous to the amp hour capacity of the battery bank. Amp hour is the unit of measurement used to express the storage capacity of deep cycle batteries. The amp hour rating will tell you how much amperage is available when discharged evenly over a 20-hour period. 20 hours has been the standard time length for rating batteries, although shorter or longer time variables may be used, depending on the application.
When you have a deep cycle battery sitting on a table in front of you, you're basically looking at a square, plastic box, with two posts, called terminals, sticking out of the top, one positive +, one negative -, to which, cables are attached. These posts are also called electrodes and are also called the 'anode' and the 'cathode,' and they change from being the anode or the cathode, depending on whether the current is flowing into the terminal or flowing out of it- anode when current is flowing in and cathode when current is flowing out. So, when the battery is being used (discharging), the positive terminal is the cathode and the negative terminal is the anode. The negative terminal becomes the anode when the battery is being used, because the flow into the battery happens in the negative terminal. The terminal that was the cathode during discharge, becomes the anode when the battery is being charged. In other words, the anode is the terminal where current flows into the battery from the outside, and the cathode is the terminal where current flows out of the battery.
Inside the 'plastic box' are lead plates, stacked side by side. At the manufacturing plant, some of these lead plates are coated with a paste that contains lead oxide and acid, and some of the lead plates are left untreated. The plates are made to be either positive, or negative. The plates that are coated with the lead oxide are positive, and the untreated plates are negatively charged. With the paste on them, they now have stored energy. The plates are dropped into sulphuric acid to charge. After 48 to 72 hours, the plates are washed to remove excess acid, which, if left on, would corrode. Then the plates are wrapped in a fibreglass material, which serves to ensure the plates don't short-circuit. Then the plates are stacked, alternating between positive plates and negative plates. Each stack will become one cell. A 6 volt battery has 3, 2v cells, each cell providing 2v of electricity. The terminals, or electrodes, are attached with molten lead, one to each side of the battery. Then the plates are fitted into their polypropylene casings, a plastic cover put on top, and now the assembly looks like a battery.
The medium that allows the energy, or electrons, to flow from terminal to terminal, is called electrolyte (the water in the battery), and it is composed of 35% sulphuric acid and 65% water. The chemical reaction between the positively charged lead plate and the negatively charged acid allows the battery to store and give 'electricity.' What happens, at its most basic level, is when you run a wire from one terminal, into, say, a light bulb (the load), and run another wire from the light bulb back to the other terminal, you a create a circuit and allow electrons to flow, which give you the power to light the bulb.
More specifically, when you turn on the light in your cabin, and electrochemical reaction occurs inside the battery, and the stored electricity (electrons), which is in chemical form, flows through the acid/water mixture (electrolyte), and gives power to the bulb. The reaction in the anode creates electrons, and the reaction in the cathode absorbs them. The result is electricity. The battery will continue to produce electricity until one, or both, of the electrodes run out of the substance necessary for the reactions to occur. In other words, when you do something to the battery, discharge it or charge it, chemicals in the battery change and electric energy is either released or stored. The current of a battery can on be DC, direct current, and an inverter is used to convert DC into AC, alternating current, which the current we need for our appliances.
Let's talk about sulphation, for a moment. This is a hard lead sulfate coating that builds up on the lead plates, and it can reduce the battery capacity, dramatically. Even though lead sulfate is created in the materials of the plates during normal discharging, this term is used to describe the creation of a different form of lead sulfate, which will not readily convert back to normal material when the battery is charged. Sulfation mostly occurs when a battery is stored too long in a discharged condition, if it is never fully charged, or if the electrolyte has become abnormally low, due to excessive water loss from overcharging and /or evaporation. Often, sulfation can be corrected by charging very slowly (at low current) at a higher than normal voltage, usually about 2.4 to 2.5 volts per cell, at 2 to 8 amps. During the discharge, acid is consumed and water is produced and when the battery is charging, water is consumed and acid is produced.
Here are some things you want to avoid:
-connecting batteries of different age (shelf life is age too)
-connecting different capacity batteries
-connecting batteries with different nominal voltage
-connecting batteries which, at the moment of connection, have different charge status.
The above mistakes are typical, however it can be deceiving, as most of the consequences of these mistakes will not manifest themselves immediately. Inevitably, however, the batteries' capacities will become reduced substantially. For example, if you connect a full battery with an empty one, in parallel, the full one will attempt to charge the empty one- a large current will be formed instantly, causing temperature increase in both batteries, sparks, and possible insulation break down. You could instantly end up, in the worst-case scenario, with two damaged batteries. If a newer battery is connected with an older one, the fresh battery will degrade faster because it will be constantly 'supporting' the older battery, whose capacity surely has dropped over time.
Adding water to batteries:
This must be done at the right time and in the right amount or the battery's performance and longevity will be reduced. Never use tap water. Always use 'battery water,' which can be purchased in most marine hardware stores. Water should always be added after fully charging the battery. Prior to charging, there should be enough water to cover the plates. If the battery has been discharged (partially or fully), the water level should also be above the plates. Keeping the water at the correct level, after a full charge, will prevent having to worry about the water level at a different state of charge.
Depending on the local climate, charging methods, application, etc., Trojan recommends that batteries be checked once a month until you get a feel for how often your batteries need watering.
Important things to remember:
-Don't let the plates get exposed to air. This will corrode them.
-Don't fill the water level all the way to the cap, just til the water covers the plates. Any fuller and the battery will probably spill over, losing capacity and the acid will damage most things it touches.
-Use distilled or deionized water only, never water with a high mineral content.
Equalization and why many feel it's important:
All batteries are built of of individual cells, and there is variation between the capacities of each cell in the battery. As the battery ages, this variation increases. Since the battery is a chain of cells, that is only as strong as the weakest link, some procedure is required to ensure that all cells stay at peak capacity.
The procedure, called 'equalizing,' is commonly performed. Equalizing temporarily elevates the charging voltage of the entire battery bank to above the normal voltage. The elevated charging voltage allows all cells, including the weak ones, to accept more current from the charger than they would at float voltage. The purpose for the greater equalize voltage is that all cells in the battery become overcharged. This is acceptable for short periods provided the battery has sufficient electrolyte. Overcharging greatly increases the rate at which the water in the battery electrolyte is electrolyzed into oxygen and hydrogen gas. Since low electrolyte levels will permanently damage the battery, it is important to limit when, and for how long, the battery is 'over-charged' during equalization.
Equalization extends the life of the battery. Batteries are nearly always used in series combinations of multiple cells. When a series string of cells is charged as a group, a single current is imposed on all the cells. However, if the voltages begin to differ, the result is a charge imbalance that can lead ultimately to battery failure. In any useful series battery charge process, some type of charge balancing or equalization must take place to restore balance or at least prevent it from growing. In most conventional battery charging practice, equalization is addressed, either by driving the charge to a sufficient potential, to assure some degree of overcharge for all cells, or with a separate, higher-voltage charging step, intended to reach the weakest cells.