Alternators vs Battery

[USMCmech]

Battery on a small GA airplane will provide some power for a few hours, I think it depends on the ampre hour rating (?).

It depends on the size of the battery and the load placed on it.

You probably could run your nav lights for several hours on a full battery. However nav lights + strobes + landing lights + Xponder + radios can drain it in less than 30 minutes. The number on the circut breaker will tell you how many amps that device will consume over an hours time.

Also transmiting on any radio consumes about 10 times the power that just listening does.

 

[JohnnyP]

..

erj-mech,

13-15 min's isnt a lot of time when youve got nothing left and need to get down, at least to me it isnt. Not terribly reassuring.

Maybe someone else more knowledgable knows this, i dont have a copy of the transport certification req's in front of me, but youd think they would require the aircraft to have more time than that in an essential pwr. situation? Is there anything of that sort thats req'rd for transport aircraft as far as time on batt's alone? In the dash we are supposed to get 30 min's out of them.

 

[Flyerjosh]

erj-mech,

13-15 min's isnt a lot of time when youve got nothing left and need to get down, at least to me it isnt. Not terribly reassuring.

Maybe someone else more knowledgable knows this, i dont have a copy of the transport certification req's in front of me, but youd think they would require the aircraft to have more time than that in an essential pwr. situation? Is there anything of that sort thats req'rd for transport aircraft as far as time on batt's alone? In the dash we are supposed to get 30 min's out of them.

For part 25 aircraft (transport category certification), the batteries have to power emergency standby instruments and radios for 30 minutes after a total primary electrical failure.

Here's some applicable regs:

§ 25.1351(6)(d)- Operation without normal electrical power.

It must be shown by analysis, tests, or both, that the airplane can be operated safely in VFR conditions, for a period of not less than five minutes, with the normal electrical power (electrical power sources excluding the battery) inoperative, with critical type fuel (from the standpoint of flameout and restart capability), and with the airplane initially at the maximum certificated altitude.

§ 29.1303 Flight and navigation instruments.

The following are required flight and navigational instruments:

(3) Continues reliable operation for a minimum of 30 minutes after total failure of the electrical generating system;


§ 23.1353 Storage battery design and installation. (applies to NORMAL, UTILITY, ACROBATIC, and COMMUTER category airplanes)

(h) In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing at least 30 minutes of electrical power to those loads that are essential to continued safe flight and landing. The 30 minute time period includes the time needed for the pilots to recognize the loss of generated power and take appropriate load shedding action.

 

[avbug]

Shoulda, woulda, coulda...doesn't mean the battery WILL last. Never count on it, because as often as not...it won't. That battery may be long in service and due for a deep cycle...at the next inspection...tomorrow. You might not have noticed the failure right away. It may be partially gone. You may have a cold soaked battery that's not going to give your full figure...Remember that battery requirements are certification requirements...not necessarily what the aircraft will do in the real world. Just what it's been demonstrated to do, for type certification.

The duration of the battery is very much dependant on multiple factors, not the least of which is the load under which it's operating. A ten ampre-hour battery, for example, will provide one amp for ten hours, or ten amps for one hour. In theory.

Same for backup power supplies.

If you're seeing an electrical problem, you may be seeing something you don't yet know about. Loss of electrical power or fluctuation might be a short that's starting a fire...it might be a control failure that will cost you all your electrical, including battery power. It may be a number of things...don't count on being able to fall back to battery. Often with electrical things, you're seeing the tip of the ice berg, and the problem that manifests isn't the real problem. This can be especially true of a shorted item on a bus affecting other items.

Most of the time a problem can be traced to a grounding issue. That sounds simple enough, but it can be a bear to troubleshoot...something you don't have the luxury of doing (and shouldn't do) in flight, even with test equipment, wiring diagrams, and maintenance publications in a well lighted shop. Additionally, a ground can present as any number of problems...and a short is a ground. You may have bigger fish to fry than loss of a generator, a popped circuit breaker, or the duration of your battery power.