DC Electrics - Batteries — Page 60, Lesson 59

Let’s talk about battery checks — how we measure a battery’s health and what the pilot needs to know before flight. First, the capacity of a battery. Capacity is defined as the product of the load in amperes that the manufacturer states the battery will deliver, multiplied by the time in hours that the battery is capable of supplying that load. So it’s current times time, and the unit is ampere hours, abbreviated Ah. For example, a 40 Ah battery, when discharged at the 1-hour rate, should supply 40 amps for exactly 1 hour. That 40-amp figure is called the rated load. Alternatively, that same battery could supply 4 amps for 10 hours at the 10-hour rate. So the same total capacity, but at a lower current draw, it lasts proportionally longer. Now, that’s the rated capacity — what the manufacturer says it should do. But in service, batteries deteriorate. That gives us actual capacity. Actual capacity is determined by the battery’s deterioration in service. Suppose you have a 60 Ah battery, and when you discharge it at the 1-hour rate, it only lasts 0.7 hour — that’s 42 minutes. Then the actual capacity is 70% of its rated capacity. In other words, the battery is only 70% efficient. To find out the actual capacity, we perform a Capacity Test. This is a test to determine the actual capacity of aircraft batteries. It is carried out every 3 months, and the efficiency must be 80% or more for the battery to remain in service. So if a battery drops below 80% of its rated capacity, it’s removed from service. Why 80%? Because that capacity will ensure that essential loads can be supplied for a period of 30 minutes following a generator failure. So the battery is your backup power source after an alternator or generator failure, and it needs to keep critical systems running for at least half an hour. What are those essential loads? They include: attitude information — that’s your artificial horizon and attitude indicator; essential communication equipment — your radios; lighting; pitot heat; plus any other services necessary for continued safe flight, or loads which cannot easily be switched off — that’s what we call load shedding. So the battery must be able to power all of that for 30 minutes. Now, what about spare batteries? Spare batteries are held ready for use in the electrical workshop. Lead acid batteries are stored in a charged state to prevent deterioration of the battery by sulphation. Sulphation is a chemical process that damages the plates if a lead acid battery is left discharged. NiCad batteries — nickel-cadmium — can be stored in a discharged state with no detrimental effect, and therefore they have a longer storage life or shelf life. Next, the On-load Check. This is carried out by applying the rated load to the battery circuit for a short period of time. During that time, the battery voltmeter reading must remain constant and not fall below a stated value. On modern aircraft, the time is as low as 10 to 20 seconds with the rated load selected. So you put a heavy load on the battery briefly, and watch the voltage — if it holds steady above a minimum, the battery passes. Finally, the pilot’s preflight check of a battery may include comparing the on-load voltage with the off-load voltage to give an indication of the state of charge of the battery. Off-load voltage is the voltage with no load applied; on-load voltage is under the rated load. The difference between them tells you roughly how much charge is left. So to summarise: capacity in ampere hours, rated load, actual capacity, the 80% efficiency threshold for the 3-monthly capacity test, the 30-minute essential-load requirement, storage differences between lead acid and NiCad, the on-load check with a voltmeter, and the pilot’s preflight voltage comparison. That’s the complete picture of battery checks.