DC Electrics - Batteries — Page 60, Lesson 54

I want to walk you through secondary cells now. We've already looked at primary cells, where the chemical reaction is irreversible and once the cell is flat you throw it away. Secondary cells work on exactly the same principle as primary cells — the same basic electrochemical process — but with one critical difference: the chemical energy in the cell can be restored after the cell has been discharged. You do that by passing a "charging current" through the cell in the reverse direction to that of the discharge current. So instead of the cell supplying current to a load, you force current back into the cell from an external source, and that reverses the chemical reaction. In this way the secondary cell can be discharged and recharged many times over a long period of time. During recharging, electrical energy is converted into chemical energy. That chemical energy is then retained inside the cell until the cell is discharged again, at which point the chemical energy is converted back into electrical energy. So it's a reversible energy storage system. Now let's talk about capacity. The capacity of a cell is a measure of how much current a cell can provide over a certain time. Capacity is measured in Ampere hours, abbreviated as Ah. And it's determined by the area of the plates inside the cell — the bigger the cell, the greater its capacity. So a physically larger cell with larger plate area can store more chemical energy and deliver more current for a longer period. Let me give you a concrete example. A cell with a capacity of 80 Ah should provide a current of 8 amps for 10 hours. Or alternatively, 80 amps for 1 hour. Theoretically that should be true — it's a simple multiplication: current times time equals capacity. But in practice, the capacity will reduce as the rate of discharge is increased. If you try to draw a very high current from the cell, you won't actually get the full rated capacity out of it. That's why capacity is normally measured at the 1 hour rate — that's the standard reference point. Now, a single cell battery may be used on its own, or cells may be connected together in series or in parallel, depending on the voltage and capacity you need. Let me explain both arrangements clearly. For cells connected in series, the positive terminal of one cell is connected to the negative terminal of the next cell, and so on down the line. The total voltage across the whole series string is the sum of the individual cell voltages. So if you have three 2-volt cells in series, you get 6 volts total. But the capacity of the series combination is that of one cell — it doesn't add up. So three 2-volt, 10 Ah cells in series give you 6 volts at 10 Ah. For cells connected in parallel, the positive terminals are all joined together, and the negative terminals are all joined together. The total voltage is that of one cell — it doesn't increase. But the capacity is the sum of the individual cell capacities. So three 2-volt, 10 Ah cells in parallel give you 2 volts at 30 Ah. That diagram on screen shows exactly this — on the left you see cells in series adding voltage while capacity stays the same, and on the right you see cells in parallel adding capacity while voltage stays the same. This is fundamental to how we build battery systems for aircraft: we combine cells in series to get the required voltage, and in parallel to get the required capacity.