The Oxford English Dictionary defines a battery as:
“A device, consisting of one or more cells, in which chemical energy is converted into electricity.”
There are two main types of battery, primary batteries and secondary batteries. Primary batteries are those designed to be discharged and discarded (disposable). Secondary batteries are those which can be re-charged and used multiple times. Our canal boats use secondary batteries for both engine starting and for the domestic or leisure circuits.
Going back to the OED definition, chemical energy is released as a result of a chemical reaction. That tells us that within the battery itself there must be a process going on between at least two different substances to provoke the reaction.
The simplest battery would be one cell with a lead anode plate, a lead cathode plate and a separator between them. In reality, plates can be flat sheets or tubular and cells contain multiple positive and negative plates depending upon how much current, amperes, we need the battery to provide. One plate is of a spongy lead, the other lead dioxide.
These plates are stacked and welded together and then suspended in the case. The case is then filled with an electrolyte, a mixture of sulphuric acid and water, which activates the cell.
The electrolyte of sulphuric acid and distilled water is highly reactive and distributes itself evenly throughout the battery. This ensures an even reaction between the positive and negative plates. The electrical output of a lead acid cell is around 2.1 volts. Therefore, a conventional 12v marine lead acid battery will contain 6 cells of 2.1 volts.
What Happens Inside?(This is the only really complicated bit)!
Very simply, a battery cell consists of a negative electrode (the anode) that holds charged ions and a positive electrode (the cathode) which holds discharged ions. An electrolyte allows the ions to move from the anode to the cathode whilst the battery is under load and to move from the cathode to the anode whilst it is recharged.
The chemical reaction between the electrolyte and the spongy lead of the anode and the lead dioxide of the cathode coats the surface of both plates with lead sulphate. As this process occurs the hydrogen in the acid combines with oxgen in the lead dioxide to form water. As ions move between the plates, an electrical potential difference is created. This becomes our output voltage at the case terminals.
The terminals on the outside of the battery allow current to flow out of the battery when we require it to do work.
That’s as detailed as we are going to get but if you can imagine these ions flowing through the acid in opposite directions from one terminal to the other during a discharge/recharge cycle then that’s really good enough!
Simply put, when all the ions have moved from the anode to the cathode the battery has used all it’s stored power, so we have to move them back across by recharging the battery.
If you want more detailed and accurate information look up redox reactions! If we can visualise what is happening inside the battery we can be aware that if we are putting the battery under load and causing the reaction to occur in one direction, then we must reverse that reaction if we want to use the battery again. Hence the need to recharge.
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