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