DC Electrics - Batteries — Page 60, Lesson 55

I want to walk you through the lead acid battery — one of the most common types of secondary cell you'll encounter in aircraft DC electrical systems. Let's start with the construction. Inside a lead acid cell, we have two plates immersed in an electrolyte solution. The positive plate's active material is lead peroxide, and the negative plate is spongy lead. Both plates sit in an electrolyte made from a mixture of water and sulphuric acid. The whole thing is housed in a container made of glass or hard plastic. On top, there's a filler cap that lets you replenish distilled water — that water gets lost through evaporation during normal use. The cap also has a vent hole, and that vent allows hydrogen gas to escape. That hydrogen is produced whenever the cell is working. Now, how do we know how much charge is left in a lead acid cell? We measure the strength of the electrolyte solution. The tool for that is a hydrometer, and it measures something called specific gravity — often abbreviated as SG. When the cell is fully charged, the specific gravity reads 1.27. When it's discharged, that number drops to 1.17. Here's what happens during discharge. When the cell is connected to an external circuit and current flows, lead sulphate forms on both plates. As that happens, the acid in the electrolyte becomes weaker, so the specific gravity falls. Once the specific gravity has dropped to 1.17 and the voltage has fallen to 1.8 volts, the cell needs to be recharged. To recharge the cell, you connect it to a battery charger. That charger applies a slightly higher voltage to the cell, which forces current to flow in the reverse direction through the cell. During charging, the lead sulphate that had built up on the plates gets removed, and the specific gravity of the electrolyte rises back up to 1.27. The voltage 'on load' — meaning when the cell is under load — should return to just above 2 volts. There's an important caution here. When you're charging a lead acid battery, you must control the rate of charge. If you charge too quickly, you get what's called 'gassing' — that's the rapid production of gas — and excessive evaporation. That can lead to boiling the battery dry, which causes damage to the plates. So charge rate control is critical.