Frans, A single Odyssey 16 amp/hour battery has done me extremely well for 6
years, without ever blinking. It also had ample in reserve when I had
regulator failure and stopped charging 1/2 way between Ostend and the Essex
coast. Regards, David Joyce, G-XSDJ, 914
----- Original Message -----
From: "Frans Veldman" <frans@paardnatuurlijk.nl>
Sent: Wednesday, April 01, 2009 10:31 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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