DC Electrics - Batteries — Page 60, Lesson 56

I want to walk you through the lead-acid battery, which is one of the most common secondary cell types you'll encounter in aircraft. Let's start with a key measurement: the specific gravity of the electrolyte. The specific gravity, or SG, of the electrolyte is an indication of the battery's state of charge or serviceability. In plain terms, the specific gravity tells you how charged the battery is and whether it's in good working order. We check the SG value using a hydrometer — that's a simple device that floats in the electrolyte and gives you a reading. Now, the electrolyte level must be maintained just above the top of the plates inside the battery. You top it up using distilled water — never tap water, because impurities can damage the battery. The reason you lose water in the first place is gassing at the plates, which happens when the battery is fully charged. That gassing is the electrolysis of water into hydrogen and oxygen, so the water level drops over time. Let's talk voltages. The on-load voltage, also called the nominal voltage, of each cell of a lead-acid battery is 2 volts. That's the voltage you see when the battery is supplying current to a load. The off-load voltage, when the battery is sitting with no load connected, is 2.2 volts per cell. So a fully charged 12-volt battery, for example, actually sits at about 13.2 volts off-load. Here's a critical safety point: electrolytes are highly corrosive. If spilled inside an aircraft, they can cause extensive damage to the structure and wiring. The neutralizing agent for an acid electrolyte — and lead-acid batteries use sulphuric acid electrolyte — is a sodium bicarbonate solution. That's ordinary baking soda dissolved in water. If you ever have a spill, that's what you use to neutralise it. Temperature has a major effect on battery performance. In low temperatures, the rate of discharge decreases because the internal resistance of the battery goes up. In warm temperatures, the rate of discharge increases. So generally, the battery performs better in warm temperatures — just like a car battery, as the book points out. As a lead-acid battery discharges, the specific gravity of the electrolyte reduces. That's because the sulphuric acid is being consumed in the chemical reaction, turning into water and lead sulphate. This is important in winter operations: in freezing temperatures, if the battery is discharged, there is a risk of the electrolyte freezing. A fully charged battery has a much lower freezing point than a discharged one. So it is important to maintain the battery in a fully charged state during winter operations. Let me show you the construction. Figure 4.5 shows a lead-acid secondary cell. You can see the plates and the electrolyte. Figure 4.6 shows a free liquid type of lead-acid battery, where the electrolyte is in liquid form — that's the traditional flooded cell design. Figure 4.7 shows an absorbed liquid type, where the electrolyte is absorbed into the active materials in the plates. That makes it less prone to spillage, which is a significant advantage in an aircraft environment where you want to minimise the risk of corrosive liquid escaping. So to summarise: specific gravity tells you state of charge, checked with a hydrometer; nominal voltage is 2 volts per cell, off-load is 2.2 volts; electrolyte is corrosive and neutralised with sodium bicarbonate; cold reduces discharge rate, warm improves it; and a discharged battery risks freezing in winter, so keep it fully charged.