Shaun Simkins wrote (in part):
> [Bob Nuckolls said] Rotax 912/914 series crank with low power requirements -
0.6KW ( about 50A )
> for a few seconds. 8AH RG batteries will crank a Rotax.
> ...
> I note that in Tech Talk #25, the factory suggested wiring diagram states that
the battery
> must be 16AH minimum. I don't see this anywhere in the installation manual.
But if this
> is the case, pretty much the front battery needs to be 17AH.
It's odd they use that ampere-hour stuff (mere capacity), whereas for
cranking engines, internal resistance is as important per Nuckolls, so
stuff like CCA - cold cranking amps - is the spec on batteries for
engine applications. Cranking amps reflect both capacity and internal
resistance, and the Odyssey PC625 I have is 265 CCA (440 HCA - "hot"
at 80F). Cranking amps is the amps it delivers for 30 sec. while
maintaining 10-point-something volts. "AH" is based on something like
a 20-hour discharge rate, but other specs suggest the PC625 is about
13AH. That's also why Nuckolls says even an 8AH batt, if RG and hence
low internal resistance, will work, but I would add not for too much
cranking nor allow for in-service degradation.
Also, the 50W starter draw would be average. If you've ever seen the
wild needle dance on those little induction ammeters placed on a
battery cable, it's the compression stroke that hurts, and quick
starting is oomph into that stroke rather than average cranking RPM,
else no one could hand-prop an airplane. And Bob H. notes the effect
of 50% more oomphs per revolution on his six-banger. That's what a
higher CCA provides.
Regards,
Fred F.
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