JAR-VLA requires two sources of electrical supply (where electricity is
needed to keep the engine running; e.g. if "battery" ignition or electric
fuel pumps are used).
These two sources need to be independent and independently selectable so
that power can be supplied from each or both together. Bear in mind that it
is not a good idea to isolate the Rotax alternator from the battery while
the engine is running, which presents something of a challenge in then
meeting the requirement of JAR-VLA as explicitly stated. It can also be fun
designing-out single items of hardware that both systems rely upon (e.g.
bus bar bolts) as both systems have to be joined together somewhere. Done
all this and more for the BMW conversion.
If a battery is used as one of the sources, then its sizing will need to
allow for only 80% of its rated capacity being available (to be tested at
every "annual") AND for being 50% discharged (i.e. .50 x .80). A means of
establishing its remaining charge (e.g. in-flight electrolyte SG
measurement!! or an integrating ammeter (battery "fuel gauge")) is also
needed.
If two batteries are used, then these can be brought together for engine
starting. This will allow two smaller batteries to be used and/or provide
better system reliability by alternately renewing each battery at regular
intervals.
Has anyone measured the current-draw of the 914 fuel pump? I'll bet its
more than the pump manufacture's data sheet states!!.
Duncan McFadyean
On Wednesday, January 01, 2031 6:34 PM, david joyce
[SMTP:davidjoyce@beeb.net] wrote:
> I am a bit confused about the whole notion of needing two batteries for a
914 installation. The Europa manual wiring diagram doesn't show it or
mention it,
> and I have fitted just the one (Odyssey) battery in my innocence! As far
as redundancy goes it seems to me that you have an alternator usually
supplying the power to run one or both fuel pumps and if that or the
related kit blows up you are then relying on what is stored in your battery
to get you home or to a nice big field. Whether you have one or two
batteries then seems to be of no great consequence, compared to the
question of how much juice you have stored, and I guess you could store a
given (large) amount more efficiently in one larger battery rather than two
smaller batteries - or have I missed something? David Joyce,402
> ----- Original Message -----
> From: Steve Hagar
> Subject: Looking down the road at batteries
>
>
> I am not up to the point to needing any thing yet concerning batteries,
probably over a year off for this. However I haven't seen much written
here about the dual setup required when runninng the 914 with its electric
only fuel pumps.
>
> Of those who have this setup what size battery is being used as the
primary unit and what size unit is used for redundant or essential bus use?
How does this compare to the size of the single battery used on a 912
Europa. Were both battery located around the same location?
>
> Thanks for any input.
>
> Steve Hagar
> A143
> Mesa AZ
>
> --- Steve Hagar
> --- hagargs@earthlink.net
>
>
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