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Capt Nomad
9th Feb 2004, 00:55
Just had an arguement about the two different types.
I was of the opinion that you could boil the acid of both types of batteries if you had electrical/battery problems.
But my friendly examiner said that you can't boil the acid in a lead acid battery. I thought this was wrong.
Can someone please correct my ignorance:lack of knowledge.
Thanks

Daysleeper
9th Feb 2004, 02:01
I used to fly a biz jet where we had upgraded the bat from the standard Ni-cad to a (heavier) lead acid. Reason being Ni-cads can get thermal runaway. Where, if my shakey memory serves, the battery keeps drawing current and overcharges itself to the point of melting at some huge temp.

Intruder
9th Feb 2004, 04:07
You certianly CAN "boil the acid" in a lead-acid battery! If the charging voltage is too high, or too high a charging current (unregulated) is used, the battery temp increases and the water in which the acid is dissolved will boil.

OTOH, the Ni-Cd battery may be subject to "thermal runaway" in similar conditions. When it heats up, the internal resistance is reduced (instead of increased, as with other battery types). This allows it to draw even higher [unregulated] charging current, further increasing the temp and decreasing resistance...

I don't think thermal runaway is very common any more, with modern, regulated charging systems...

Blacksheep
9th Feb 2004, 12:51
There's no acid in a Ni-Cad battery it is Potassium Hydroxide - an alkali. To avoid complications lets call the liquid in batteries electolyte.

Nickel-Cadmium batteries got a bad name for themsleves in their early days because badly designed installations combined with poor maintenance practices often led to 'Thermal Runaway'. This is where a battery overheats, damaging the plate seperators in the cells and leading to higher current flow, more heat and so on in an increasing cycle until the battery spews its electrolyte out of the vents and virtually melts internally. The result, apart from a knackered battery is structural damage to the airframe.

The cause of the breakdown was most often due to lack of servicing. It is important to adjust the electrolyte level regularly and shop visits must include cell balancing to avoid voltage reversal in individual cells during heavy discharge. Poorly regulated constant voltage charging circuits were another contributory factor and the phenomenon was more common in installations where an original lead acid battery was replaced on a one-for-one basis with a nickel cadmium battery. Correct installation includes a specially designed battery charger that provides properly controlled charging current. Nickel-cadmium batteries are more expensive to buy but provide greater energy storage per unit weight, and have a lower overall cost of ownership. There is no reason to reject nickel-cadmium batteries on the basis of the possibilty of thermal runaway as long as they are installed by a proper modification using an approved type of charger for the battery in use. Regular electolyte level checks and bench servicing are essential for Ni-Cad batteries, but that is also true for the Lead-Acid variety. Neglect a battery at your peril regardless of what type you use - they'll always let you down at the worst possible moment, right when you really need them.

Cameronian
9th Feb 2004, 17:10
Do you all really nickel-cadmium batteries or do you mean nickel-iron (NiFe)? Surely nickel-cadmium batteries are rechargeable dry cells?

forget
9th Feb 2004, 19:22
THERMAL RUNAWAY.

Batteries are capable of performing to their rated capacities when the temperature conditions and charging rates are within the values specified. In the event that these are exceeded “thermal runaway” can occur, a condition which causes violent gassing, boiling of the electrolyte and finally melting of the plates and casing, with consequent danger to the aircraft structure and jeopardy of the electrical system.

Since batteries have low thermal capacity heat can be dissipated and this results in lowering of the effective internal resistance. Thus, when associated with constant voltage charging, a battery will draw a higher charging current and thereby set up the “runaway” condition of ever-increasing charging currents and temperatures.

In some aircraft, particularly those employing nickel-cadmium batteries, temperature-sensing devices are located within the batteries to provide a warning of high battery temperatures and to prevent overcharging by disconnecting the batteries from the charging source at a predetermined temperature

Aircraft Electrical Systems.
E H J Pallett.
ISBN. 0-582-98819-5.

mini
9th Feb 2004, 20:43
'Fraid your instructor is dodgy on this one...

As has been mentioned, you most certainly can "boil" a lead acid battery. The most common reason is an excessive charge rate, caused by an excessive voltage difference between the battery and the charge source, this "pushes" too much current into the battery, causing heat build up and electrolyte gassing - its hydrogen and its explosive...

Some lead acid batteries have gel packs instead of liquid electrolyte.

I thought NiCad batteries developed short memories when used in shallow cycle applications?

:ok:

Intruder
10th Feb 2004, 12:45
I thought NiCad batteries developed short memories when used in shallow cycle applications?
True at least for early generation batteries. Not sure about current generation...

Blacksheep
10th Feb 2004, 12:52
Nickel-Cadmium batteries can develop a 'memory' that causes loss of capacity in service, but you are referring to conditions of poor maintenance, mini. Providing batteries are maintained in accordance with the instructions there are no problems. Aircraft MPD's provide for regular battery maintenance and once in the shop the CMM's for Nickel Cadmium batteries approved for use in aircraft include such essential elements as electrolyte level adjustment, capacity testing and deep cycling, that restore the battery's condition. If the conditions that lead to thermal runaway or apparent loss of capacity exist, then the aircraft and its systems are not being maintained in accordance with the legal requirements.

In the current situation in UK, the certification of electrical systems (including batteries) is being passed from the E & I avionics specialists to the Airframe and Engine trades without proper training and experience requirements. It may be that one consequence of this policy will be a new spate of exploding batteries.

I mean, what could be complicated about looking after batteries eh?

radeng
11th Feb 2004, 00:14
Blacksheep

>I mean, what could be complicated about looking after batteries eh?


I know you asked tongue in cheek, but being 'in the trade' so as to speak, I'd say 'practically everything!', thus I guess, strongly agreeing with you!

I find that keeping up with modern battery technology, especially the charging systems required, takes more time than I have to spare. Lead acid are about the simplest: yes, you can boil them by overcharging. Nickel cadmiums are getting less popular because of disposal difficulties; as I remember, they need a constant current charge for a given number of hours, although I believe a very small trickle charge is acceptable afterwards.

cirrus01
11th Feb 2004, 17:43
Blacksheep wrote....

"In the current situation in UK, the certification of electrical systems (including batteries) is being passed from the E & I avionics specialists to the Airframe and Engine trades without proper training and experience requirements."

Surely you mean JAR- land (EASA) and not just the UK ?

:confused: :confused:

Blacksheep
12th Feb 2004, 09:22
I only speak for UK, cirrus01.

As I understand it, in most of Jarland (EASA) initial aircraft engineering training for the B1 licence specialisations covers electrical systems, including things like wiring systems and batteries. In the UK, people trained in Airframe and Engine maintenance are being converted to cover electrical systems on the basis of answering a few multi-guess questions to which the answers are easily available for memorisation. Indeed the Engineers and Technicians forum on this BB regularly features question trading and information on the sources of such question banks.

Knowing as I do, the level of technical knowledge possessed by the majority of A & C licence holders in matters of electrical engineering, it scares me to think that they will soon be certifying that which they know not.

As I hope we have demonstrated here, even apparently simple matters such as batteries and battery maintenance procedures are not well understood, yet neglecting to employ the proper procedures can have potentially disastrous results. Even though a thermal runaway ought never to occur in an aviation environment, it is not an unknown event and the new licencing regulations will do nothing to eliminate the phenomenon.

Capt Nomad
14th Feb 2004, 22:07
Thanks for all the information.
I'll make sure I ask the examiner how he thinks it is impossible for lead acid batteries to boil next time I see him. I thought I was right on that score.:ok:

exeng
16th Feb 2004, 02:01
An old VW Beetle I used for a while seemed to need regular top ups of distilled water. One one occasion it wouldn't start so I checked the levels and found it nearly dry (and very smelly). For some reason I decided to disconnect the terminals (whilst the cells were open) - the result was a very loud bang as the battery exploded! I emerged from behind the drivers seat as a very shaken idiot!

The problem was that the regulator was delivering a full charge to the battery at all times.


Regards
Exeng

Sheep Guts
17th Feb 2004, 09:42
Batteries and maintenance really get up my nose. All the posts here are very factual and I dont have that much to add but the following.

Bad maintenance leads to failure in components and possible catastrophy.

Lead Acids bats need to be checked for fluid level and and electrolyte quality on a regular ba checked for good continuity, then no problems should occur. Remmembering that this is most critical in hot climates. lead acids do it tough in the tropics.

Nicads are more expensive usually 4 times more than Lead acids but then with good care deep cycling and checks they should last longer allthough they do suffer and Ive witnessed thermal runaway once in Twin Otter the other in a a King Air.

The one in Twin Otter was due to lack of deep cycling by the maintenance.


Basically whatever you choose it depends on the maintenance and availability of spares. Ie. if you live in the US Nicads are the go but if your in Sub saharan Africa then maybe a lead acid is the go.



Cheers Sheep

avioniker
20th Feb 2004, 07:19
Aside from the fact that lead acid batteries can (and do) explode and NiCad have experienced Thermal Runaway problems, One of the major considerations in choosing a battery is power delivery over time.

The NiCad tends to keep giving its power on an even output scale where the voltage and current stay constant until it has reached the end of it's service capability. If it isn't "deep cycled" (drawn down to almost zero volts multiple times over a specific period of time) on a regular basis that service life shortens with each charge cycle. At the end of the period the lights go out, period. That's the memory problem.

The Lead Acid battery tends to discharge on a linear scale where the voltage and current available will steadily decrease until it's flat. They can be recharged numerous times without developing a memory and until there's an internal problem they just keep on putting out. The problem with the lead acid type is that in solid state applications many components rely on a constant voltage or current to operate correctly so once the battery can't provide one or the other the system ceases function although the lights may still be on.
The new gel cell type of battery is much better than the older liquid cell batteries but the problems still remain.

The designers' question is: do you want long gradual loss of use or instant-off loss as your failure mode?

Blacksheep
20th Feb 2004, 08:53
New Gel Cells? They've been around for at least as long as me - they were called Varley batteries in the good old days and they allowed proper exciting aeroplanes like the Hawker Hunter to fly upside down without acid running out of the vents...

mini
20th Feb 2004, 17:36
Have the "newer" solid state batteries i.e. Nickel Metal Hydride etc (spelling?) made an appearance yet, NiCad dissapeared from most things years ago.

Is there a technical reason for not using newer technology stuff that doesn't have the memory problem or such detailed maintenance requirements? or is it a cost issue?

ROB-x38
22nd Feb 2004, 18:53
It's funny you mention that mini because the whole way through the thread i was preparing to ask my mobile phone question...

Do NiMH batteries have the memory problems mentioned with NiCads? I often stick my phone on the charger when it's nowhere near empty. Should i be 'deep-cycling' it? :ok: I've found the battery life to be getting worse. If so i'm going to have to warn all my mates about the dangers of not deep-cycling their phones and the possibility of a greatly reduced service capability :confused:

Blacksheep
24th Feb 2004, 12:31
Deep cycling aircraft Ni-Cad batteries: -

[Please don't try this at home with your mobile phone battery! Its very, very dangerous! ]

The battery is placed on controlled discharge and the individual cell terminal voltages monitored. As each cell voltage falls to 0.5V a shorting clip is placed across the terminals and left in place. When around two thirds to three quarters of the cells are shorted out it is no longer possible to maintain the discharge current and 5 ohm resistors are placed across the terminals of all the remaining cells until they fully discharge to zero volts. The resistors are then replaced with shorting clips. The battery is left in this condition for not less than 24 hours then it is placed on charge using a low current to get the cells started up to 0.5v per cell. Once all the cells are established in charging, the battery is fully re-charged at the full rated current, the electrolyte level adjusted and a capacity test is performed. After a deep cycle the battery is quite unable to remember anything that went before, which is the whole object of the exercise.

That and preventing individual cell voltage reversals during heavy discharge such as during engine starting. This occurs when an individual cell develops a low apparent capacity in service (due to lack of deep cycling). The heavy discharge current then takes the weak cell down to zero volts and recharges it in the oppposite direction, damaging the plate seperators and leading to cell breakdown. The cell then develops local hot spots and we all know what that can lead to don't we?

A deep cycle is only one part of the battery reconditioning process, which varies according to the model and useage. I won't go into the pros and cons of reflex charging v. constant current charging methods - aircraft electricians can argue the toss for hours and often do....