The 16AH hour minimum quoted on the Europa factory schematic is Rotax'
recommended
minimum battery size for the 914 (section 9.4.14).
Note also that available power is not the full 18A; according to section
19.5, the maximum
loading from the fuel pumps, TCU, servo motor, and warning lamps is 8A.
Typical for each
pump is 2A; the TCU, probably about 0.15A; the rest are intermittent. So at
best, we have
perhaps 15A available to us. Airmaster CS prop loads are also intermittent.
Another consideration is the battery charging capacity of the system. I
believe the FAA rule of
thumb is that 20% of the alternator capacity should be reserved for
charging.
Your personal specs may differ, but this is another 3.6A off the top.
Thus, the 18A alternator is able to power about 11A of external loads with
charging reserves.
That's what basic electric gyros, VFR nav/comm, xnpndr, and engine monitor
will eat up.
Incidentally, this thread was last active in late 2000. Check out the
archives. Many are running
heavy electrical loads on the factory alternator, but probably with very
small charging reserves.
Shaun
----- Original Message -----
From: "Nigel Charles" <72016.3721@compuserve.com>
Subject: Re: Looking down the road at batteries
> Message text written by "Shaun Simpkins"
> >I'm worried that a 914 with an Airmaster prop will
> be rather nose-heavy, and thought that a RG battery of the minimum
required
> for cranking up front with a very
> tiny battery in the back for backup might work and possibly save cabling
> weight. Redundancy would also be desired
> for a motorglider restart. Since I haven't got to this point, either, I
> thought I'd throw out some of my findings for comment as well.
>
> I started by asking Bob Ruckolls (Aeroelectric connection). He said:
> 1. Rotax 912/914 series crank with low power requirements - 0.6KW ( about
> 50A ) for a few seconds. 8AH RG
> batteries will crank a Rotax.
> 2. 8AH is about all that's needed if you size your battery to allow
> comfortable termination of flight (1HR after failure)
> with essential electrical loads. And if you load-test your battery
> regularly to avoid suprises<
>
>
> I have a 912S with an NSI VP prop. Although the engine might be slightly
> lighter than the 914 the heavier NSI prop is more significant in terms of
C
> of G. Even with this combination and a heavy panel it is possible to get
> the trim towards the aft end of the envelope with a 17AHr battery in the
> aft position.
>
> Although 8AHr may well be adequate for engine starting all batteries
> progressively lose capacity with age. A 16AHr battery will therefore last
a
> lot longer in service for a small weight penalty. Having 2 batteries for
> redundancy is only of value for engine starting if each can be selected
> individually, otherwise if one battery has a cell go low it will pull the
> cranking voltage down with it. The RG batteries certainly give better
> cranking amps. Mine actually turns the engine faster than a standard full
> sized car battery.
>
> To protect your expensive onboard battery it would be helpful if the first
> start of the day (particularly in cold weather) was from an external
> battery. It is much easier and safer to have an external power connection
> aft of the wing and a rear mounted battery makes this simpler.
>
> As the 914 needs electrical power to keep the engine running (unlike the
> 912 and 912S) I think that a backup power source for the electric fuel
> pumps is a wise precaution. If this is not done I would consider a voltage
> crowbar unit essential to prevent a single failure becoming a total
> electrical failure. Provided this is fitted it is reasonable to treat the
> alternator and the battery as two seperate power sources. If this is the
> case it is important that the two fuel pumps do not share any fuses or
> c/b's (ie the main busbar fuse) as should it fail neither pump will run.
>
>
> Regards
>
> Nigel Charles
>
>
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