Ni-Cad Aircraft Battery

[FieldTeam]

I know that this is not the Aircraft related forum, but my question is specific to batteries, so I seek battery professionals.
I am in a remote field location in support of a company operation and am experiencing a recurring problem on our aircraft APU starter batteries.
The batteries are Marathon Power 20 cell Nickel-Cadmium Aircraft Batteries. They are charged by a dedicated Charger/Monitor. This charger is on during flight.
The power units will start on the ground. After flying 8+ hours at 33,000 ft, the battery is not able to start the unit. After three hours, or so, the batteries are again capable of starting. No failure indication occurs except for no start.
This is occuring on multiple aircraft and has not been a problem in the past.
Have I got a bad batch of batteries or is there something else wrong? These batteries are rated for flight and I can not see how they could freeze, especially with the charger on. I am not able to properly check these batteries where I am and I have a long lead time before receiving replacements.
Any help would be appreciated!

 

[IRstuff]

Why are you assuming that they can't freeze? Most chargers are designed to avoid overcharging, so once the batteries are charged, only a trickle current will run into the batteries, reducing the heat dissipated within the batteries.

Can't you arrange to monitor the temperature at the battery case? A relatively simple heat transfer calculation based on the case temperature and the amount of trickle charge should reveal what the temperature wihin the battery is.

TTFN

 

[FieldTeam]

Freezing has occured to me but I assumed that they do not normally freeze because they never have in the past under similar circumstances. This is why I was thinking I got bad batteries.
I also read somewhere that the internal resistance of Ni-Cad batteries help prevent freezing.
If the battery is freezing, would this be considered normal or is the battery bad. I can not reconfigure an aircraft with test equipment without going through some extreme pain.

Thanks for your answer. I am getting a crash course in Ni-Cad batteries (something I have taken for granted)!

 

[IRstuff]

Are the batteries otherwise OK after their ordeal? e.g., are they otherwise fully charged?

It's also possible that something has changed in the manufacturing of the batteries to make them more susceptible to whatever happens in the aircraft. Since most materials also contract when they cold, there may be some dimensional changes that increase contact resistance in the discharge path. Likewise, there may have been process changes that affect the chemistry of the batteries themselves.

TTFN

 

[zeusfaber]

I assume you've already done all you need to to eliminate the possibility that the problem lies with the APUs, their controllers or whatever ropey fuel you're being supplied with in that part of the world.

Sorry, but somebody had to ask.

A.

 

[FieldTeam]

Thanks for the replies.
After a few hours, the batteries are fine. They are charged and work as advertised. If an external battery (one that has not flown) is dropped into the system, the APU will start. That leads me to believe that the system itself is good.
Could the electrolite level be serviced wrong to cause the battery to freeze in flight but work on the ground? And if so, wouldn't this level eventually reduce itself to a more normal operating level through the constant charging process?
When I get this figured out, I will report my findings, in case anyone else ever stubbles on something like this.

 

[zeusfaber]

Just to confirm then, you're seeing the problem while trying to start the APU on the ground immediately after it has flown rather than trying to start it in flight?

A.

 

[FieldTeam]

Yes....the APU is not operated in flight. It is turned off after engine start, before takeoff. The APU is charged in during flight. Charger has been replaced with known good one. No difference. Awaiting a new battery, but long lead time.
Thanks

 

[zeusfaber]

I'd misconstrued your original description.

I was going to suggest having a look at the aircraft's battery connector (having seen some odd problems with wayword dustcaps on the terminals in the past), but notice your problem is with multiple aircraft, so it doesn't sound like that.

Not much help I'm afraid. Good luck.

A.

 

[FieldTeam]

Last night, immediately after landing, read battery thermistor and found it open (high megohms terminal to terminal). Two hours later it read normal (3K). The battery charger uses the thermistor reading to stop charging below -25C and above +60C.
If this circuit faulted, and the battery continued to charge below -25C, would this be the cause of the battery freezing?
I have flown one aircraft with the battery charger disabled, and when it came back, the battery worked fine. I would take this to mean that charging in flight does not keep the battery from freezing.
During charging process, gassing of the water component of the electrolyte occurs. Maybe charging below -25C is causing freezing of this gas or the electrolyte?

 

[IRstuff]

Nope, that does not compute. There are no gases that a battery could produce that would freeze above 77 kelvins.

However, the thermistor issue re-raises the question about the mechanical integrity of the connections. Since the thermistor, being only a resistor, should have no failure mechanisms associated with a rather balmy -25ºC, the open circuit behavior that goes away with time suggests again that the cold temperature is causing one or more connections to open.

Immediately upon landing, can you measure the voltage of tha battery as well as the starting current when you try to start the APU?

TTFN