DC Electrics - Batteries — Page 66, Lesson 61

I want to walk you through the next part of DC Electrics — Batteries. We're going to look at how we check a battery's condition, how it gets charged in flight, and then a summary of the two main battery types. Let's start with a simple check. If a battery isn't supplying any load — nothing is drawing current from it — it will likely show its nominal voltage. That's the off-load voltage, the voltage you'd expect to see stamped on the battery, like 12 or 24 volts. But that reading alone doesn't tell you much about its health. Now, if you load the battery up — by switching on things like the pitot heater, landing lights, or blower motors — and the voltage stays steady, then the battery is in a good state of charge. It's holding its voltage under demand. But if the voltage falls below a stated value within a time limit that's specified in the aircraft manual, then the battery is in a low state of charge and should be replaced. So the key test is: load it, watch the voltage, and compare against the manual's limits. Now, let's move to battery charging. Most lead-acid batteries use a Constant Voltage Charging system. The idea is to keep the battery fully charged during flight. With this system, the generator's output voltage is held constant — 14 volts for a 12-volt battery, and 28 volts for a 24-volt battery. So the generator voltage exceeds the battery voltage by 2 volts for every 12 volts of battery potential. That 2-volt difference is what pushes the charging current into the battery. Alkaline batteries are different. They're susceptible to thermal runaway — a condition where charging causes heating, which increases current, which causes more heating, and so on. So for alkaline batteries, a constant current charging system may be used, controlled by a dedicated battery charger that monitors both battery temperature and voltage. Some charging systems use a method called pulse charging. Once the battery reaches 85% capacity, the charger delivers short pulses of charging current instead of a steady flow. Here's an important note: after you start an engine using the aircraft's battery — whether it's lead-acid or alkaline — the generator, once it's on line, recharges that battery. You can see this happening on the ammeter. If you have a generator load ammeter, you'll see a high initial reading. If you have a centre-zero battery ammeter, you'll also see a high reading on the charge side. That reading should quickly reduce as the battery gets recharged. But if the charge rate increases, or stays high, that could indicate a faulty battery. A high charge rate that doesn't come down could result in the battery overheating and subsequent damage. Finally, let's look at the summary table for secondary batteries — that's rechargeable batteries. I'll walk you through the key columns. For a lead-acid battery: when it's charged, the positive plate is lead peroxide, the negative plate is spongy lead, and the electrolyte is sulphuric acid. When it's discharged, both plates become lead sulphate, and the electrolyte becomes weak sulphuric acid. The specific gravity — SG — of the electrolyte changes: charged it's around 1.270 to 1.300, discharged it's about 1.170 to 1.240. And if there's spillage, you neutralise it with sodium bicarbonate and water. For an alkaline battery: when charged, the positive plate is nickel oxide, the negative plate is cadmium, and the electrolyte is potassium hydroxide and distilled water. When discharged, the positive becomes nickel hydroxide, the negative becomes cadmium hydroxide, and the electrolyte stays as potassium hydroxide and distilled water. For spillage, you use boric acid. So that's the summary: lead-acid uses sulphuric acid and its specific gravity tells you the state of charge; alkaline uses potassium hydroxide and is more susceptible to thermal runaway, so it often needs constant current or pulse charging.