Most general aviation aircraft use a lead-acid battery, and it works just like the battery in your car, motorcycle, or boat. It converts chemical energy into electrical energy and visa versa. Inside the battery, porous plastic separators lie suspended inside a hard rubber case between a series of lead plates surrounded by fluid (acid) electrolyte. The case has vented caps to fill and refill the cells with electrolyte.
The positive plates are connected in series and have an external electrical connection for the battery cable. The negative plates are set up the same way. The positive plates are made of lead peroxide (Pb02) and the negative plates are made of spongy lead (Pb). The electrolyte is sulfuric acid (H2SO4) and water (H2O).
When you charge a lead-acid battery, you apply a direct current (DC) in the opposite direction of battery discharge. (The current must have a voltage higher than the battery's capacity.) This converts water into sulfuric acid and releases hydrogen and oxygen gases. Because hydrogen and oxygen is an explosive combination (remember the Hindenburg?), you should charge any lead-acid battery in a well-ventilated area.
As the battery charges, it creates a sediment on the plates, which falls to the bottom of the case. It gradually builds up in the space between the bottom of the plates and the battery case. If this space fills with sediment, it shorts out the plates, rendering the battery useless. High-quality batteries have more space at the bottom to extend the life of the battery. When the battery discharges electrical power, it converts sulfuric acid back to water.
Batteries are rated in two ways - voltage and amp/hour capacity. The voltage depends on the number of cells in the battery. In a lead-acid battery, each cell delivers 2.2 volts DC at full charge, but 2.0 volts is a good approximation for field use. A 12-volt battery has six cells and a 24-volt battery has 12 cells.
Simple math reveals that a 12-volt battery is actually a 13.2-volt battery and a 24-volt battery is actually a 26.4-volt battery. Aircraft electrical systems are designed for a specific voltage and the battery must match it. A 12-volt battery won't provide enough power to energize anything with a 24-volt system, and a 24-volt battery attached to a 12-volt aircraft system will quickly burn out virtually everything electrical on the aircraft.
A battery's amp/hour rating is a function of total plate area, amount of active material in the plates, and the amount of electrolyte surrounding the plates. In other words, the size of the battery and the number of plates it can hold determine the maximum rating. Generally, a 20-amp/hour battery will deliver 20 amps for one hour or one amp for 20 hours. This rating indicates the battery's ability to continue cranking the engine in cold weather or to operate electrical equipment, such as the radios, following an engine or generator/alternator failure. How long battery power lasts - assuming the battery is fully charged - depends on how many amps your electrical devices draw.
The electrolyte's specific gravity (density compared to distilled water, and measured with a hydrometer) determines how "charged" the battery is. In a fully charged battery, the electrolyte is approximately 70 percent sulfuric acid and 30 percent water by volume, and the specific gravity is between 1.275 to 1.300. A low battery charge reads between 1.200 to 1.240. The hydrometer gives an accurate reading when the electrolyte is between 70?F and 90?F. You must apply a correction factor if you take a reading when the electrolyte temperature is outside of this range.
Keeping a lead-acid battery charged protects it against freezing in cold temperatures, and a fully charged battery will freeze at around -95?F. A low battery can freeze at -16?F, a temperature often seen at many northern airports. Even if the battery doesn't actually freeze, temperatures close to its freezing point may cause irreparable damage.
In the aircraft, the battery is vented to the air, usually by a length of clear plastic tubing. Some battery cases are fully enclosed and some are designed to be housed in a vented battery box. Some twin-engine aircraft, especially turbine-powered aircraft, and some single-engine aircraft used in cold weather, have dual batteries that can be connected in parallel or in series. Connecting two batteries in parallel doesn't increase the voltage, but it does increase the amp/hour capacity.
The gel-cell is another type of lead-acid battery that you see occasionally in general aviation, and it's especially popular with the aerobatic crowd. The gel cell operates just like any other lead-acid battery, but its electrolyte is a gel that prevents leakage in unusual attitudes and under negative G loads.
The nickel cadmium battery is also used in aviation. A "ni-cad" is similar to a lead-acid battery in that it has positive and negative plates suspended in an electrolyte, but that's where the similarity ends. In a ni-cad battery each cell is a self-contained unit. It's like a box of saltine crackers with individually wrapped "cells" housed inside a common outer case.
The ni-cad's positive plates are made of nickel hydroxide, the negative plates are made of cadmium hydroxide, and the electrolyte is 30 percent potassium hydroxide (KOH) and distilled water. The battery's fully charged voltage is a bit lower than that of a lead-acid battery, around 1.5 to 1.8 volts per cell. This is why ni-cad batteries are larger and heavier than lead-acid batteries for a given voltage and amp/hour rating.
Unlike a lead-acid battery, which gradually loses power as it discharges, a ni-cad maintains a nearly constant power output until it's almost totally discharged. This important difference makes the ni-cad an ideal battery for turbine engines, which draw high starting currents for a longer time than piston engines do. The ni-cad also recharges quicker than a lead-acid battery, which means it quickly regains much of its power between engine starts in multiengine turbine aircraft.
Another quirk of the ni-cad is that it loses internal resistance as it recharges, unlike a lead-acid battery that increases internal resistance as it recharges. The loss of internal resistance means you must monitor the charging process closely. Less internal resistance means more charging current goes into the battery. This causes heat, which further reduces the battery's internal resistance. A ni-cad can become so hot it triggers a chemical reaction that will not stop even if you remove the charging current. This reaction is a "thermal runaway," which can be disastrous if it happens in flight. Ni-cad battery systems have a temperature warning system and cockpit indicator that allows the crew to monitor the battery temperature and stop the charging process if the temperature exceeds a certain level.
The specific gravity of the ni-cad's electrolyte doesn't change significantly as the battery discharges so it's of no value in determining the state of charge. A ni-cad battery's charge must be assessed by a load test. Also, low temperature is less likely to affect a ni-cad battery, but it will freeze at approximately - 75?F.
Usually, you can replace a lead acid battery with a ni-cad but this is rarely done. The ni-cad battery is considerably more expensive, larger, heavier, and you need special servicing equipment to maintain it.
You can do several things to preserve a battery's performance and extend its useful life. You should make sure the electrolyte level is always up and that the battery is always at least close to fully-charged.
If you must store an aircraft for a long period, consider putting a trickle charger on the battery. Make sure the battery connections are tight and that the vent hose is connected, undamaged, clear, and runs to a point outside the aircraft. If you must store the airplane during the winter or can't put a charger on it, remove the battery and store it in a warm location.
Aircraft batteries are rather simple devices that provide essential power for flight. They will give reliable service provided you operate and maintain them properly.
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