We chose a 25 Amp hr battery with the assumption that we would
eventually build long wings for our Europa (which we eventually did) and
that we would need plenty of battery capacity to run various avionics
for several hours with the engine off. We have since flown with the
engine off for about an hour max, and had the voltage to the instruments
drop to the point where they started dropping out (we were consuming
about 2.5 amps). Our problem is that there are small voltage drop at
many locations that can add up to as much as 2 volts. So when the
battery drops to 12.0 volts, the instruments see 10.0 V, and have
problems operating there. We have spent some time looking for the
voltage drops. They come from each connection (spade lugs, etc.), each
switch, and each circuit breaker. The biggest drops are in the
individual circuit breakers in the switch panel we bought from Aircraft
Spruce. We have reduced some of the drops by coating connections with
silver grease, but have not come up with a good solution for the drops
in the circuit breakers.
We have since swapped out the 25 Ahr battery for a 16 Ahr battery, to
save a little weight.
As far as charging the battery, we have only the stock alternator, which
has no trouble keeping the battery charged. We were very careful to
keep our power consumption low, using a mechanical master contactor
(saves 1 Amp), low power strobes (4A vs 7A), LED Nav lights, lower power
radio and xponder, etc.
Regards,
Terry Seaver
A135 / N135TD
-----Original Message-----
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of Frans
Veldman
Sent: Wednesday, April 01, 2009 2:32 AM
Subject: Europa-List: Battery capacity
<frans@paardnatuurlijk.nl>
Hi,
The Europa build manual states that for the Rotax engine, a battery of
16 Ah is recommended.
I have been thinking about this, and would like to share my thoughts.
1) For cranking power, the battery Amp-hours is a vague indicator, but
nothing more than that. A battery of 5 Ah could well be able to deliver
more amp's for starting than a certain 16 Ah battery. The Amp hours tell
you something about the total energy conserved in the battery, nothing
more. Just like the amount of fuel in your tank doesn't say much about
the power of the engine (but one could state that a small tank is
somewhat indicative of a small engine). If the battery has enough Amps
to crank the engine, the amount of Amp hours indicates how long and how
often one can run the starter before the engine starts.
2) The battery capacity usually allows for "starting problems", i.e. to
flood your engine, crank it dry, try again, etc, switch it off again at
the fuel station, start again after refueling with the same problems, do
some trouble shooting, etc.
3) In a usual setup the function of the battery is twofold: Producing
power to start the engine, and providing emergency power when the
alternator fails. In a dual alternator setup however, the second
function of the battery is not necessary. (OK, during an engine failure
when both alternators stop, the battery still has to provide power, but
this situation usually doesn't last for very long and even a small
battery is sufficient to issue a mayday call, to deploy the flaps, and
play your favorite song on the panel mounted CD player.).
4) With a 16 Ah battery, one should assume that it is possible to start
the engine with the last few Amp-hours left (the capacity is there to be
used, otherwise one could do with a smaller battery anyway), so it is
feasable one starts the trip with an almost depleted battery. The
alternator is then supposed to recharge the battery during the trip.
Now, we all know about the fabulous power output of the Rotax
alternator. To top off a 16 Ah battery, you need to charge the battery
for 1 hour with 16 Amp's. Or for 2 hours with 8 Amp's (100% efficiency
unrealistically assumed). But the Rotax alternator has just enough power
to keep the avionics powered in a typical setup, with only very few
Amp's to spare to charge the battery. With other words, a 16 Ah battery
will probably never get properly charged again once it's capacity has
been put to use (unless using a mains connected charger on the ground).
I have a setup with dual alternators, and dual batteries. Idea behind
this is to be able to fully deplete one battery during glider activities
(future MG), while still keeping a not used battery for restarting at
hand. And of course to have full redundancy, with a fuel pump on each
bus, a navigational instrument on each bus, etc, and to be able to keep
flying even after a total failure of one bus.
Why am I telling all this:
The battery capacity.
I plan to use one 5 Ah battery on each bus, nothing more... These babies
are able to produce 250 Amp's momentarily each, while the Rotax starter
needs 60 Amp's to crank the engine.
I plan to start on just one bus (so just one battery), and one battery
should be sufficient to start and even allow for a few failed attempts.
And then if all fails, there is yet another battery.
When an alternator fails during flight, I always have another alternator
to power the essentials and keep the airplane flying as long as the fuel
allows. The "dead" bus still has a small battery to power a secondary
fuel pump for about one hour, in case there are multiple failures.
The advantage of this minimalistic battery setup is of course the
weight: a total of just under 4 kilo's, (8.8 lbs), for both batteries
together. As a typical 16Ah battery weighs in at more than the double
amount, so this setup more than compensates for the weight of the
secondary alternator.
For less weight, more juice during flight, more redundancy, etc.
It just sounds to good to be true. Hence my suspicion:
Any comments on my reasoning?
Is my assumption that the Rotax usually starts within 10 seconds
acceptable?
Has any of you ever depleted a 16 Ah battery during starting?
If so, how many start attempts did you get out of this 16 Ah battery?
And did you get this 16 Ah battery recharged again with just the Rotax
alternator?
Has anyone else experience with a "minimalistic battery setup"?
--
Frans Veldman
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