On 10/01/2013 01:45 AM, Fred Klein wrote:
> Are there any Europas flying w/ something other than an Odyssey 680, and would
there be a good reason to choose another brand or type?
My experience and insight:
First you need to establish how many amp hours you really need.
I have two Odyssey 310's. They each have half the capacity of a 680.
Although I have a switch to put both batteries in parallel, the truth is
that so far I have never needed it. My 914 has started fast and reliable
on just one battery (250 flight hours), and this includes the occasions
where I had to crank excessively due to the hot start problem (which I
have now solved by closing the fuel pump prematurely to empty the carb
bowl before the engine shuts down). I can't believe how much cranking
power there is in a 310.
So, for starting you don't need any larger battery than this.
The 914 is dependant on electrical power (and I believe your engine is
dependant as well). So, you may need to have electrical power when your
alternator dies. If this is the case then you need to do the math and
calculate how long you want to be able to fly with a dead alternator and
how much amp hours are needed for that (and multiply that with two
because of battery aging and other reasons).
Another way to solve this problem is to install two alternators in your
ship. Assuming that you don't want to go beyond this reduncancy, now you
only need your battery to start the engine. It might be that a second
alternator is lighter than a larger battery, so you will save some
weight and it gives some piece of mind that in case of an alternator
failure you don't have to worry about the remaining capacity and charge
state of the battery.
I decided to go that route and have a second alternator installed.
The reason to use two small batteries:
With two alternators and two batteries I could make two totally
independant electrical systems. I have two fuel pumps, each on a
different system. I have two GPS's, both on a different system. I have
CHT gauges and a coolant gauge, both are wired to a different system.
And so forth. In reality it means that one of the electrical systems may
give up the ghost completely, but I will still get home safely.
(I don't want to become stuck at the North Cape or to run out of options
above the Alps or over sea).
Another reason is that I wanted to be able to use the glider wings in
the future. While soaring I would just use one electrical bus (and not
worry about depleting the battery), and save the other battery for
starting up the engine again.
To safe some weight I decided to use only very small batteries. With my
system setup I only need the batteries to start the engine, and to prime
the alternator. I made a provision to connect both systems together in
case of starting problems, but I never needed it so far. (In flight it
is forbidden to interconnect both systems because it defeats the whole
redundancy).
Once in two years I take out the battery of bus B, put the battery of
bus A into bus B, and put a new battery in bus A. (Bus A is used for
starting up the engine). This means that I always have a relatively new
battery for starting, and the "old" battery will get an easy time for
the remainder of its usefull life and will only be used for powering up
the fuel pump before start. It is a cost effective strategy.
About the Lithium things: I have decided against it for various reasons.
Although I have not researched the subject once my ship was ready, there
are a few things I remember:
1) The cell voltage of lithium batteries is different thant that of lead
acid batteries. It means that you will get a somewhat different voltage
than all your avionics was designed for.
2) Lithium batteries have special charging requirements. It means in
reality that you can't connect the alternator and battery in parallel,
but there needs to be some piece of electronics in between. This will
become a weak spot in your ship with no redundancy.
3) Field maintenance will be more difficult. Batteries sometimes die. A
lead acid battery is easier to replace, a motorcycle battery can be
sourced everywhere and will get you home. Also field charging is easier.
4) The technology has not matured yet. Some batteries explode, others
have only few charge/discharge cycles, others require complicate
management systems, and others have a weird discharge curve (with either
too much voltage sag during discharge, or just too little so the end
will come as a sudden surprise).
So, in short:
1) If I want to build a light weight ship with a non electrical power
depenant engine, I would install just a Odyssey 310.
2) If I want to build a light weight ship with an electrical power
dependant engine, I would use two alternators and still a small Odyssey 310.
3) If redundancy is important, I would use two alternators and two small
Odyssey 310's.
In my scenario's there is no place for anything heavier than an Odyssey
310, unless of course I need to have some ballast in the rear of the
ship. Then I would favor a bigger battery over just a piece of dead lead.
Frans
|