klehman(at)albedo.net
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 Posted: Thu Jun 28, 2012 3:08 pm Post subject: Engine start on Z14 with different size batteries |
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Bob Barrows I like your idea. Why not use all the capability available
even if the batteries are different sizes. I know I don't have a regular
capacity checking schedule for my Z-14 batteries. Flipping a paralleling
switch is safer than messing with a boost on the lake or hand propping
when all alone in the boonies. Unless you install a ground service plug,
carry matching cables, and have a vehicle handy, I can't say I've ever
enjoyed boosting an airplane with jumper cables and an open cowl.
That little battery can supply over 100 amps if the main battery is very
weak or failed so I would favor at least 8awg to get everything I could
out of it even if the difference in resistance is small over a short
length of wire. Heavier wire tends to mean heavier lower resistance lugs
as well. One of those little batteries will start my subaru all by
itself in the summer although mine do auto parallel during cranking. If
the batteries and xfeed contactor are beside each other like mine are
there isn't much of a weight penalty. Sure #4 is overkill but I used it
mainly because I already had #4 welding cable on hand for the run up to
the starter as well as the lugs and a method of installing them. It also
lets me compare battery cranking current by disconnecting them one at a
time and cranking the engine with the ignition off.
Ken
(Z-14 with two 8ah batteries and liking it)
On 28/06/2012 11:47 AM, Robert L. Nuckolls, III wrote:
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<nuckolls.bob(at)aeroelectric.com>
At 04:40 AM 6/28/2012, you wrote:
> In my architecture Bus 1 with the big battery is the sole starter
> cranking bus for normal operations. Accordingly all "starter current
> path" wires on Bus 1 are 2 AWG to reduce resistance during engine start.
Unless your batteries are in the tail, 2AWG is heavier
than necessary but certainly not an electrical performance
issue.
> By comparison I calculate that the largest wire on Bus 2 could be 8
> AWG for normal operations on that bus including Xfeed operations to
> bus 2 (for non cranking operations).
If your batteries are up front and the small battery
is not intended to crank the engine, then probably 12AWG
is sufficient on the aux alternator side . . .
> However it occurs to me that if my Bus 1 battery (Odyssey 680) was
> struggling to crank the engine in a remore location I would be tempted
> to switch on the Xfeed and get whatever current boost I could from the
> smaller Odyssey 310 on bus 2.
>
> My question is this: What size wire do I need to incorporate in the
> "starter current" path of bus 2 to facilitate assisted engine cranking
> on the odd occasion from the smaller battery. Presumably bigger would
> always be better....but what's reasonable.
We're talking seconds here. 20A through a 22 AWG wire
wont 'burn' the wire. 12AWG or perhaps 10AWG in your aux
alternator system is quite adequate to the task even
if you find yourself tempted to 'boost' the larger battery's
abilities with the smaller battery. But I'll suggest that
a ground power system capable of jump-starting from a
ground vehicle is a much better hedge against soggy batteries
in remote locations.
If you plan to frequent remote locations, then keeping a
sharp eye on the condition of the larger battery is
important. You need to replace it when it drops to 70%
or less of capacity . . . this isn't an endurance issue,
it's a cranking issue for ventures into higher risk airports.
Being able to 'stack' the 7.5 a.h. battery on top of a soggy
17 a.h. battery is poor insurance irrespective of wire
sizes.
Bob . . .
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