Hi Paul,
 Is it suitable to replace standard Renesis lead battery (12v
 70Ah) with Li-Polymer one?

 Expert from engine starter company asses that working power of
 Renesis starter is around 2kW and starting current (duration max
 0.5ms) is around 700A. By the factory standard starter has to
 manage 3 times 30 seconds without cooling pause. So battery
 capacity has to manage it - my condition for dimensioning

 I calculate that 45C 11.1v 15Ah Li-Polymer battery would be
 strong enough to fulfill demands. I really do not know if such a
 small and light battery is really suitable for starter. It
 weighs around 1.3kg/3lb comparable with 18kg/40lb for lead one.
 Weight ratio between technologies is around 1:12 and somehow it
 is difficult to accept replacement. If this really works it will
 be excellent contribution to aircraft weight reduction. Li-Poly
 lifetime is about 1/2 lead one and is approx. 3 times more
 expensive.

 Do you know obstacles for usage of higher voltage around Renesis
 engine? I would like to use 28v DC for whole aircraft.

 Viktor

 Slovenia, Europe

 I am no expert but as I recall Lithium Ion batteries require
special
 computer controlled chargers. Lead Acid batteries do not. Otherwise
 you risk blowing up the battery.

 Back in the old days (1960's) we used Nickel Cad aircraft batteries
 in our Chaparral race cars successfully with no special chargers.

 Doug Fir in Japan knows far more about this than I do.

 Paul Lamar


 http://www.powerstream.com/li.htm

 "The basic algorithm is to charge at constant current (0.2 C to
 0.7 C depending on manufacturer) until the battery reaches 4.2
 Vpc (volts per cell), and hold the voltage at 4.2 volts until
 the charge current has dropped to 10% of the initial charge
 rate. The termination condition is the drop in charge current to
 10%."



Not worth it for the airplane.  I just installed Li Po batteries
in my
skateboard, replacing the SLA batteries it came with.  (36Vx16Ah
in a 2x10
configuration)

In the skateboard it cut 10 lbs off the weight and more than
doubled the
range - BUT, the battery upgrade cost me more than the whole
skateboard
- even with a killer deal on batteries and I did the installation
myself.

The first problem is that you cannot easily keep Li Ion (liPo
etc) connected
to the electrical system of the aircraft.  The initial charge
would be too
high and if that didn't make them blow up, They would either
over-charge or
undercharge if you left them connected to the system.

It would be OK to have them connected to the system if you over
sized the
pack in both size and volts.  So 15v would not over charge...
4.2vx4 for a
max tolerable voltage of 16.8v.  Nomimal voltage would be
3.7x4=14.8v. These
batteries really dont like to be charged at more than 0.2-1C,
*maybe* 2C for
short periods if you have a very high current type battery but I
have not
seen any rated for 2C charge.  So if you figure 80A max charge
rate, you
would need an 80Ah pack at minimum.

That would get very expensive and weight would be coming up on
that of an
SLA battery.  You might say - but at least I would have 80Ah for
reserve.
But since you will only be charging to 14.5V or so, you will not
be using
even half of the battery's capacity - more like about 30Ah of the
rated
80Ah.  So what you just ended up with is a $2000 30Ah (effective)
battery
that weighs almost as much as a 30Ah SLA.  (along with other
downsides as
below)

Or you could keep the LiPo separate from the rest of the
electrical system.
That would mean a separate charger and an inverter (or a DC
charger that
probably just has a built-in inverter). And you would still need
to have at
least a small SLA connected to the system for alternator noise
and sudden
power loss.

Other important factors:
LiPo Li-Ion batteries are at high risk for fire. (remember all
those laptop
and cell phone batteries catching on fire?)  The risk is
mitigated with
proper management but there is still always the risk.

You need a controller board to manage each cell individually.  No
single
cell can be allowed to charge or discharge at an excessive rate, over
charge, or over discharge, lest you risk a fire.  (And with an
80Ah pack
that would be a pretty big fire).  That controller board is
another cost and
failure point. And, all of the boards I have seen, simply shut
down the pack
if one of the cells is out of range - not good for aircraft use.
 Also, you
need to periodicly balance the cells.  A good controller board
will do that,
but they do it at the end of a full charge.  Since you would not
be reaching
full charge they would not be balanced and you would need to pull the
batteries every 20 cycles or so and manually balance them
(requiring time
and equipment).  What you would really do, is build a custom
controller
board for aircraft use.

They are rather heat intolerant.  There are stories of LiPo's for
RC cars
being charged up, then put in the trunk where they get hot,
raising the
voltage above the upper limit for any one of the cells, then
burning down
the car (the big people car - not just the RC car).  At least 2
guys have
burned down their houses - probably by incorrectly setting the
charger.  The
RC cars themselves burn down all the time from overheat and/or
too much
current.  Even without fire, repeated heat cycles (like sitting
on the
flight ramp) are damaging and will degrade your $2000 battery pretty
quickly.

So, these batteries are not going to replace lead acids in cars
OR airplanes
anytime soon.

The only application I can think of would be a racing rotary
aircraft where
there is no charging system.  The LiPo would be a light-weight
way to power
the fuel pumps, coils, etc for the duration of the (short) race
and be
re-charged afterward (like at Reno).  For that you would probably
use an
11.1v (nominal) LiPo pack with 40 or so Ah. (weighing in around
12lbs)


David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net


--
Hi everybody and thank you very much for advices. In meantime I was "hunting" for real performance data for LiPoly (experience from users). I had a long discussion with developer from company which produce high quality engine starters and alternators for automotive and airspace applications (also for world known U.S. aircraft piston engine producer). I hope that gathered information might help others in their projects:

New improved LiPoly high current battery can supply 45C long time current and in short circuit even 90C (of course for short time before they explode). In praxis use limiting to half of presented performances makes battery long life. So they have really low internal impedance. This property is important for "piston" type engine starting. For first millisecond (literally - measured) starter is indeed short circuited and LiPoly could manage it with excellence. So they can be really much lighter with a much lower capacity and still produce same short circuit current as much bigger and heavier lead one. After critical millisecond starter will move - rotate with draining just (for renesis) cca 2kW from battery.

Low temperature affects LiPoly and lead battery in similar manner. Internal resistance jumps up more than twice which at most affect critical starting current. There are standard for temperature influence measuring but as all standards - extremely boring stuff defined by clever engineer but written by st...d lawyers.

Lead battery is extremely tolerant; just do not cross charging gassing voltage 14.4V and it is excellent capacitor for voltage ripping damping till 1kHz. LiPoly will explode if you overcharge or undercharge them (temperature instability). Now day's technology allows LiPoly charging till 4C of course this is on edge of performance (2C for long life). Regarding charging rate lead is almost champion.

Lifetime of lead battery is much better than LiPoly but with last improvement already available on market as mass product this difference is much smaller (approx. 1 to 2).

So conclusion is simple: If you are not critical on mass use reliable, safe, cheap, long life lead type battery. If you are critical on aircraft mass (like VTOL project) then low mass is first and lot of difficulties just stepped in to the project.

We thinking about problems over increased mass. Even more in project we plan to have two rotary engines almost 25 feet apart and use just one battery located in the middle (procedure: start first engine - wait a little - start second). To decrease (quite high) weight of starting cables (problem of first most important millisecond of engine starting) we have to switch on even higher aircraft voltage. Right now we are thinking about 2×28/24Vdc. Hazard limit (touch with hand) is by standards 60Vdc and with tree phase alternator + AC-DC (maximal ripping sqrt(3)/2) we are still within limits and can charge multi cell 4.2V/3.7V LiPoly package. Of course a new device - ultra-capacitor has to deal with voltage ripping, because LiPoly is quite allergic to voltage ripping. By the concept of electrical solution we are not far away from hybrid car regenerative braking.

We are thinking of new voltage controller which will be so agile that it will be capable to adjust alternator rotor excitement current (DC + small AC contra sinusoidal current) in such way that alternator output voltage sinusoidal behavior - ripping will be partially compensated. Developers from alternator producer company are skeptical about this idea because of voltage regulator instability (danger!!!) but try and check (talking about increasing regulator input low pass filter frequency) can make thinks better (also bad). With lead battery this (problem) was not an issue. Today's alternator with just a small rotor improvement (adapted magnetic resistance extremely nice patent) has decreased ripping from cca. 13% to the half. That is more than enough for lead battery long life.

Wish me luck.


Viktor

Slovenia


Vicktor,

Dave outlined the issues very well.   I  personally have had trouble with a LiH device leaking, coming in contact with moisture when the control box cooled down, resulting in, as you know, the production of  hydrogen gas.  Although the PCB in the box was solid state, a spark from somewhere caused an explosion.  It probably was from an outside source near the vented control box.  This happened with an experimental 1 Farad super capacitor that we were driving very hard with a peaky power wind generator.  About LI Poly batteries, you are correct though.  The batteries can be cooled and vented, and charge controlled and probably more importantly not drawn down past their lower voltage limit.  But they can still  leak from mechanical contraction caused by difference in extreme temperatures found in an aircraft though.  The issue is not so much, are the individual cells robust but the fact that there are so many of them, the chances of one having trouble is higher.  One cell takes the system down.    Compare this to the large area plates on a lead acid battery.  As Dave mentioned, the trouble in such a situation wouldn't be when you were flying,  but back in the hanger and more likely when you went home for the night.   Of course safety systems can be designed for that scenario as well but it is just one more issue you have to tackle to achieve an energy storage system that, I would think, the KISS principle should be applied to.   But even the Yunteec all electric airplane removes the battery when they want to recharge it.  If I may digress a bit, IMHO the electric car is going to get bashed, not for the limited mileage but for the number of houses that burn down when the battery charging mechanism screws up.

In electric or hybrid electric craft LiH based batteries make more sense as their control and safety systems comprise a smaller percent of the total energy pack weight.  I would tend to argue against the LIPoly as being a lighter choice for your intended purposes though.  For starting the engine and supporting the navigation and lighting equipment  it is hard to beat the 'usable' energy density of a AGM (absorbed glass mat) deep cycle sealed battery.  Concorde Corp. makes them for GA  aircraft specs and as well as a military spec. one labeled the LifeLine series. All major military craft in USA use Lifeline.  Since they are deep cycle you can repeatedly drawn them down to a low state of charge.

Looking at your application and system voltage I would say the high amp cables needed  between the battery and the two engines would be extremely heavy and the number one item in need of a diet. Wiring runs to assure less than 1% voltage drop over the 25 feet, between the battery and engine, (for dc that is calculated as round trip) you would need something on the order of welding cables size or stated in USA standard as AWG 0000 size fine strand wire.   I calculate it to be about 16 lbs. per 25 feet.  That 32 lbs. total. You will also need a heavy bus bar for the those connections near the battery, and robust high amperage  contactors near the engine.

You may find that  if you crunch the numbers, your electrical system would be 60% lighter if you go with al 28 vdc system.   Yes, certain navigation equipment will run less efficiently because of the higher DC/DC step down conversions, but that is only one factor in the big design picture.   Concorde also makes 24vdc nominal battery in one case. You will be able to drop the amp/hour needs in half so the weight stays about the same.  The ultimate solution would be to find a way to start the engine  using a small positioning motor and a fuel ignition method.  Additional weight savings could then come from down sizing the battery weight.  The deep cycle battery delivers more when lower amps loads are drawn out over a longer time.  Finally  look into  gathering the engine monitoring data in a black box  near the engine and delivering it to the cockpit via redundant RS-232 shielded telephone cables.

Cheers

Doug in Japan