1DC:

I'm no expert, but the 28v battery is normally there just to keep essential
services such as flight instruments running when all other power generation
systems have failed. It would only have a fraction of the power capability
of the main generators, which may be 100's of Kva or more on a large aircraft.
If the battery is larger, then it means that there are more essential services
to keep alive, greater power consumption, or longer run time. I would assume
that the apu battery is just there for starting the apu and is intentionally
separate for redundancy reasons.

Quite interesting that they use a 270v dc bus for some systems and shows just
how much power is consumed. For equivalent Kva, increasing the rail voltage
decreases the current, which allows thinner cable to be used, saving weight.
Introduces a whole lot of problems for the designers athough and not something
you would want to get your hands on :-(...

RR_NDB, #33:

> and ultimately FIRE...

Cue Fawlty Towers sketch etc :-).

One of the things about Lithium Ion (spelling corrected, thanks) is that they
don't have much thermal mass, whereas both lead acid and nicad do and
will withstand high short term overload or overcharge because of this. While LI
batteries will take very high charge rates, the temperature must be closely
monitored to prevent runaway, which may have happened in this case.

Perhaps they should talk to laptop and cordless tool manufacturers, who have
10 years or so experience getting the best performance and life from li, with
few real problems http://images.ibsrv.net/ibsrv/res/sr...ilies/evil.gif...

One presumes they did, I rather doubt they started completely from scratch.

Key difference is if a laptop starts getting hot you just put it down and walk away from it, even if it happened on board an aircraft odds are you could keep it away from the fuselage until it burnt itself out, bit harder to do for a fixed installation battery buried deep in an aircraft...

The two are not the same though. Laptop batteries are made down to a price and
driven pretty hard, whereas aviation batteries are built and tested to a very
demanding spec and will be used well within ratings.

Looking at the spec for the charger, it looks like each cell in the battery is
monitored and is confirmed by the additional thin wire harness within the battery.
Not clear if this includes individual temperature and voltage monitoring for each cell.

Also looks like there is no separate charge circuit per cell, just the the single
pair for the overall cell group. While this isn't optimum in terms of battery
management, it is cheaper and less bulky. I would have thought that they would
have gone the extra mile with this, but they must have had their reasons...

ScareBatteries
 

syseng68k,

Should be. In theory yes. In daily use you know what may happen.

A battery today is a complex device. No longer cells inside a case. Even software algorithms could be responsible for the problems. :} In 787 Batteries and Chargers thread there is a graph on that (optimization).

It seems an absurd what we are learning on that.

That 270vDC is actually a variable square wave AC. Boeing call it +/-270vDC. It's used to power the big motors. CACs, Engine Starters, pumps etc. It saves a lot of energy overall as it allows hydraulic pumps and such to be 'throttled' as required. Creates a load of heat though during the transforming which is why the boxes that do it are liquid cooled. :eek:

RR_NDB:

Fwics, to get the best result, each cell should be monitored for voltage and
temperature, with the sensors and ic in a single package that could be
built in to the cell during manufacture. Then, a simple serial protocol gets
the data back to the individual cell charging circuits. For any new technology
that has serious safety implications, it should be if anything overengineered
to start with, while experience is gained out in the field. Would assume that
Boeing have done something like that.

TURIN:

Square wave suggests that it's coming from inverters, not from an alternator, but
perhaps it makes sense as it's easier to rectify that to provide dc for the subsequent
motor inverters, or any other load that that needs dc voltage. Someone mentioned
1.4Mw capabilty onboard. Serious amount of power and wonder what they use
all that energy for...

Innovative approach
 

TURIN,

400 Hz?

:confused: This is amazing. Why the heat? Ok, losses but where? :confused:

In our terminology a square wave IS NOT DC. Is AC! The household mains supply has a (sinusoidal) waveform with positive and negative peaks. And it's not name DC. Why? Simply because varies the direction of the current.

DC stands for just one direction of current flow. For example, (an analogy):

You are at FL 150 with turbulence. You may climb and descend but the mean level is 150. OTOH you takeoff and climb. There is a continuous "direction" until your cruise level despite any typical turbulence.

I am wondering why the designers used square wave. It's easier to generate (from a DC buss). Question:

What is the voltage and waveform that is supplied by the four gennies?

syseng68k

Inside Thales? Securaplane? Outsourcing has limitations on that, you know. :{


Part of it (not so small) is being liquid cooled. :mad: