View Full Version : APU start with a low battery

4th Apr 2010, 14:16
I'll put it simply: is it true an APU start attempt with a low battery can damage the battery?

4th Apr 2010, 16:18
It may decrease the life, not sure if it could creat a runaway scenario on a sick battery. I would not let it worry me one bit though, you are doing far more damage to your apu (longer hotter spool times).

4th Apr 2010, 18:00
How hard the battery works affects the amount of heat it generates (http://www.mpoweruk.com/thermal.htm). The thermal profile of a battery affects its cycle life (http://www.mpoweruk.com/lithium_failures.htm#lifetime).

On the 737 (-300 at least), as well, if the battery voltage falls below 18V during the APU start, several of the relays in the start sequence can fail to operate, which can have unexpected electrical effects.

4th Apr 2010, 18:55
APU start with low battery power can damage the battery, mine goes back to 747-200, start power 50 amps, at least that is what I remember from my course

4th Apr 2010, 23:54
As checkboard noted, the battery is largely inconsequential in light of the potential damage to the APU. If the battery voltage is low, slower spool up times for the start will occur, and potentially hotter starts may occur.

A low battery results in longer starter engagement times, and potentially the inability to spool the engine fast enough.

A battery which has been drained will take greater current when receiving a charge after the start, which can result in higher batter temperatures. In the case of NiCad batteries the potential for a thermal runaway exists...which definitely does damage to the battery...as well as the surrounding airframe.

Flight Detent
5th Apr 2010, 08:37
This just got me thinking...

in the 737NG, with a low main battery, trying to start the APU,

just thinking, when standby power is activated is the only time the main and standby batteries are connected electrically!

so, will activating standby power help in this situation by increasing the capacity, and therefore the voltage, of the power source to start the APU?

just a thought...FD :confused:

9th Apr 2010, 22:02
It can also damage the APU, if the battery doesn't have enough juice to keep spinning the starter after fuel gets introduced.

Easy Street
9th Apr 2010, 23:49
I'll add another vote to SNS3Guppy's point: starting the APU with a drained battery can lead to excessive charging current immediately post-start. On my type this can lead to boiled electrolyte and a resulting acid-spill mess...

10th Apr 2010, 00:21
just thinking, when standby power is activated is the only time the main and standby batteries are connected electrically!

so, will activating standby power help in this situation by increasing the capacity, and therefore the voltage, of the power source to start the APU?

I'm not sure of the reason behind it, but when the APU is started, the APU ECU sends a signal to the Standby Power Control Unit which activates a Remote Control Circuit Breaker ("C1212") which stops the two batteries (Main and Aux) operating in parallel.

Ref: AMM D&O 24-31-00


10th Apr 2010, 00:34
Modern aircraft should have a battery temperature sensor circuit which will disable the battery charger if the internal temperature of the battery gets too high. Also, they should have sophisticated charging control circuits to prevent excessive charging currents and to help prevent thermal runaway.
The battery will not be recharged if the voltage (of the battery) is too low.

Boeing aircraft, including the 737NG, have these features.


10th Apr 2010, 03:07
Most all of the aircraft I've flown had minimum battery voltage requirements for APU Start or Engine start (when applicable). Why would you guys want to start the APU when the voltage is outside of the published (Approved) parameters?

Swedish Steve
10th Apr 2010, 17:00
On the A330 the APU battery is disconnected when it falls below 23V. You can't start with a flat battery. But Airbus, unlike Boeings that I know, will start the APU from ground power.
And during an APU start, the battery will disconnect at 9-12V remaining.

10th Apr 2010, 17:23
737 classic will start the APU from normal AC ground power too, with a low battery.

Flight Detent
11th Apr 2010, 03:54
Lots will start the APU with ground power,

some I recall are Lockheed P3 Orion, Classic 747 and 737NGs.

and certainly there are considerably more than that!


Swedish Steve
11th Apr 2010, 09:51
I have been thinking about the B737 Classic starting off ground power, and I can't see how it works.
I remember, many years ago having a B737-200 with a flat battery. We used a DC ground power unit plugged into the E and E bay and started the APU. We were in a hurry to get it up the ramp.
But other times, on the line with a flat battery, I have plugged in AC ground power and charged the battery until it has enough volts to start the APU. I have never tried to start a B737-400 APU off AC ground power.
Now I need to hunt out a maint manual.

11th Apr 2010, 11:59

Classics can have a dc plug in the e&e for an external supply.


Starting an apu with a low battery can be risky.
Sure the battery will suffer but the apu is at greater risk.

flt detent..
Not across all variants but the NG's I am familair with (-800's) have just the one battery for powering the buses.(barring the backup in P6 to close some valves )

11th Apr 2010, 19:48
Apart from the DC plug connection in the E&E bay, the 737 classic (300-400) will start also from normal AC external plug... I was puzzled too the first time that happened to me, because one would think, reading the manuals, that it doesn't, but it does start the APU like that!!

11th Apr 2010, 23:49
The 737-300 and 400's I have worked on (many dozens) cannot start the apu using a.c. power.
In normal service they all must have a battery connected and (relatively) serviceable.
The battery is charged using the a.c. via the charger but there is no facility to start directly from a.c. on the classics I have worked on .
It has a d.c. starter only.
The new gens are a different story.

12th Apr 2010, 02:28
I think it ia a limition issue,so why touch it.