You usually move to higher voltage to transmit greater power at the
same or lower current which reduces wiring weight. I suspect in
light aircraft the power requirement for the electrics is driven by
systems other than the avionics i.e electrically driven hydraulic
pumps or simply electrically driven flaps, landing gear retract etc.
There is consideration of going to 42 volts for cars for this reason.
B50s work down to 10 volts, B300/B500/600/800 a little less. B400 and
B700 down to 4 volts.(internal boost regulator cuts in). A 12 v
nominal SLA battery is about to die at 11V terminal voltage when
discharged at typical glider rates. If you are losing more than 0.2
volts or so between the battery and the instrument you need to fix
the wiring, fuses switches etc.
There doesn't seem to be any overwhelming reason to go to 24 volts in
gliders. I'll bet most "24 or 28 volt" avionics has a switch mode
regulator to get 14 volts before going to the rest of the gadget or
simply to the 5 volts or so required by the logic circuits.
If we go to electric flaps and aircon like the Duckhawk this may
change. I think it has something like a 54 A-H battery.
Mike
At 08:13 AM 28/02/2013, you wrote:
Content-Type: multipart/alternative;
boundary="----=_NextPart_000_0056_01CE1593.E2557430"
Content-Language: en-us
My contribution to the battery aspect of this thread is to question
why we are so infatuated by 12 volts? [I once wrote Standards, and
Standards often impede innovation]
I also agree that the electrical systems must be designed and
implemented taking into consideration volts, amps, temperature high
and low, wiring, insulation, noxious gases, fusing, short circuit
and thermal runaway, G load, weight and many other factors
At Oshkosh 2006 the Blue Mountains Avionics presentation said for
light aircraft the move to 24 volt systems was a no brainer, just so
logical. Most instrumentation and radio's require 10 volts and a 12
volt system decays to 10 volt reasonably fast. Microair's need
probably 10.5 before the transmitter goes garbled, Cambridge falls
over at about 8 volts. Mike could perhaps comment on the minimums
for Borgelt instruments. PDA's and XCSoar have a USB 5 volt input
so may work longer on a 12 volt system? Some avionics are designed
for 35 volt DC maximum input [but XCOM and Microair apparently have
16 ~ 17 volt maximum input specification]
But starting with 24 volts gets much more out of the battery before
avionics fail. Even moving to say 16 volt to keep within radio
specification could lead to increased useful battery life.
Cranking amps for starter motors is at the high drain end of the
drain spectrum but arguably is early in a batteries discharge cycle
in the glider application.
So why not have 16 ~ 24 volt systems in gliders?
Alan Wilson
[ARMIT Comm Eng]
Canberra
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: Wednesday, 27 February, 2013 8:58 PM
To: Discussion of issues relating to Soaring in Australia.
Subject: Re: [Aus-soaring] LiFePo4
Nice one Arie.
You do bring a bit of costing perspective into the argument here.
The YouTube footage is interesting (horrific?), but I suspect
basically irrelevant.
I think that any glider pilot who knows anything about the problem,
does not want to experience an inflight cockpit fire UNDER ANY
CIRCUMSTANCES. I know of one example, where the pilot was VERY happy
to have survived the experience - without having to bail out. He
reported that fire - as in burning - was NOT the problem. He
reported that the amount of fumes and smoke generated in an
incredibly short time from the ignition of the plastic wire covering
due to shorting of the electrical system was in fact the primary
problem. For this unfortunate pilot, there were really two
problems: first he had to be able to breathe, and secondly he had to
be able to see what you are doing - basically impossible in a
cockpit filled with smoke!
Re your statement "willing to replace a couple of batteries each
year ...." Probably a slip of the pen: As Bernard has pointed out a
high quality SLA gel cell type battery MAY last up to 9 or 10 years,
but this is hardly likely to be the norm. Five - seven years seems
to be much more realistic estimate. As far as I can tell, the life
of the LiFePo4 batteries is not claimed to be any better than this
latter figure.
At this point in time, LiFePo4 batteries are MUCH more expensive.
However I expect that in a few years time the price will have
fallen, and many glider pilots will be using these "new fangled" devices,.
I suspect that I will need to replace my current glider batteries in
the near future. I do not have a max AUW of the non-loading bearing
parts problem - one valid argument for lighter batteries - and my
power consumption figures - high power consumption requirements are
another valid argument for using LoFePo4 - are relatively modest, so
I have no intention of using LifePo4 batteries for the replacement.
What this whole discussion has crystallised for me is the
requirement, regardless of the type of battery selected, is to
always use a high quality battery manufactured by a reputable
factory who stands behind their product. [I have in the past tried
"cheap" batteries, and regretted it.]
Regards,
Gary
----- Original Message -----
From: <mailto:[email protected]>Arie van Spronssen
To: <mailto:[email protected]>Discussion of issues
relating to Soaring in Australia.
Sent: Wednesday, February 27, 2013 6:56 PM
Subject: Re: [Aus-soaring] LiFePo4
Hi,
When people start talking of glider batteries I laugh at their
logic. We have a toy that cost anywhere between 2 - 10k and upwards
to keep in the air each year (not including getting to and from the
airfield and actually getting it in the air) and they are not
willing to replace a couple of batteries each year for well under a $100.
These fancy batteries may be ok but in the vast majority of gliders
the simple still works best and is cheap and safe.
Yes I do play with these fancy batteries in my radio control toys
but with great care and they are always stored in a lipo safe bag.
You only have to watch this video to agree
<http://www.youtube.com/watch?v=mw8jb1KmAG8>http://www.youtube.com/watch?v=mw8jb1KmAG8
yes I know the newer ones are better but these are only small and
look at how they go up and even the newer ones can still have problems.
regards,
Arie
On 27/02/2013 12:52 PM, Future Aviation wrote:
Hello all
It just occurred to me that I have omitted to thank John Parncutt
for his research and his willingness to share the findings with us.
Of course, in this context Mike Borgelt's professional advice must
also be mentioned. Both contributions are extremely useful to many
of my gliding friends including myself. Many thanks to both of you!
Believe it or not, the last set of SLA batteries powering the engine
circuit in my ASH 25 lasted for 10 years. At the time I opted for the
most expensive SLA batteries I could get my hands on and now it appears
that the old saying holds indeed true. You only get what you ........
Kind regards to all.
Bernard Eckey
-----Original Message-----
From:
<mailto:[email protected]>[email protected]
[mailto:[email protected]] On Behalf Of Mike
Borgelt
Sent: Wednesday, 27 February 2013 11:22 AM
To: Discussion of issues relating to Soaring in Australia.
Subject: Re: [Aus-soaring] LiFePo4
As I said yesterday, do proper engineering on your battery installation.
If you don't have real numbers for temperature limits, discharge curves at
various rates, charging characteristics etc etc you aren't doing
engineering, you're just guessing.
A battery designed to start a racing motorbike and then be recharged by the
alternator and floated at that voltage likely has quite different
characteristics, design and longevity from one designed for charging and
deep discharging over several hours then recharging. You can also just stop
and get off the bike when the battery catches fire.
Li batteries all need individual cell monitoring during charge and use or at
least when charging after mostly charged.
The problem with Ni MH batteries is the number of cells (10 for a nominal 12
V system). You will have at least one weaker cell which will deep discharge
more than the others and will be undercharged or more likely the other cells
will overcharge resulting in reduced battery life. Individual cell
monitoring would help but with 10 cells vs 4 for Li it is a pain.
3 years isn't bad for a NiMH battery pack.
Some people have a problem with max weight of non lifting parts and a few
kilos saved may make the difference between flying in or outside the weight
and balance envelope. For these LiFEPO4 may be worthwhile but use the
correct cells.
The cylindrical Tenergy cells sold by these people have engineering data and
are Underwriter Labs tested. The tests are published there too. Start
here:<http://www.all-battery.com/lifepo4battery.aspx>http://www.all-battery.com/lifepo4battery.aspx
I've dealt with them and they did what they said they would.
Also these people may be of interest: The batteries seem to be the same as
the Tenergy cells but with a different colour outer sleeve.
They have battery monitoring/cutoff circuitry available also. You must use a
low voltage cutoff at least.
<http://lithbattoz.com.au>http://lithbattoz.com.au
The old sealed lead acid batteries are OK. They are heavier although in many
installations that may not matter. The capacity is usually quoted at the 20
hour rate. In modern gliders 1 amp continuous is not an unusual load so
that's the 7 hour rate. Likely it is a 5 A-H battery at this rate. Give it a
couple of dozen charge cycles and it is a 3 or 4 A-H battery and you begin
to have problems. Note also the number of cycles you get is non linear with
depth of discharge. Small % discharge you'll get lots of cycles. Large %
discharge many, many fewer cycles.
Size the battery to handle the longest flights and then use two batteries.
Use one routinely, keep the other charged then when the first battery dies
due to low capacity you have a known good battery.
Put that one in the first position then put a new one in the standby
position. You should always have a good battery available then.
If using some new type either learn enough to do an engineering analysis or
find someone willing to do one for you. Otherwise these things may get
needlessly banned or we have fires in gliders. It would be embarrassing to
explain to your insurance compny why you had to bail out of your burning
glider.
Mike
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