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On 10/20/2013 04:59 PM, David Joyce wrote:
> Creighton, If you look at the Odyssey technical manual for the p680
> you will find they give 8 hrs for a 2amp discharge rate and 5hrs
> for the 3amp discharge.
This is for optimum conditions:
1) Perfect temperature
2) Perfect charge state
3) Fresh battery
In reality you can't count on the advertised capacity.
1) In cold weather the battery capacity can easily be 50% of the
advertised capacity. Elevated temperatures (like under the cowling)
are detrimental as well.
2) A full charge van only be obtained when charging with at least 14.5
Volts. My experience with the Rotax "rectifier" is that it tapers of
at around 13.7 Volts. This is good for battery life, but it means you
will roughly only get 70% of the capacity.
Furthermore, the startup sequence can consume quit some capacity, and
it takes a while to recharge this loss. If you get the engine failure
soon after the take off, the recharging might not have been completed.
Last but not least quite some capacity can get lost between the time
of the alternator failure and the pilot taking appropriate action. If
a "low voltage" warning is your first sign of trouble, it means you
have trouble indeed because the battery has already given away most of
its precious amp-hours.
3) Batteries age. Some people keep the battery for 4 years or longer.
Much of the capacity is gone by then, but you won't notice a thing
until you really need it. Voltage does not sag due to aging, and
starting will remain as usual. As soon as starting capacity is
noticably going down, you have lost about 90% of the capacity.
It might be a good thing to measure the capacity of the battery once a
year. Put on the fuel pump and a few other necessities, and measure
how much time it takes before you hit 11 Volts.
Note that such a run down test is bad for the battery but if you do
that only once a year it doesn't matter too much.
> Perhaps what you say applies to ordinary lead acd batteries but not
> to the superior technology of the Odyssey.
It is still a lead acid battery and all the physics apply to it
nevertheless.
Anyway, I wouldn't count on an advertised capacity; I would land as
soon as possible if the battery is the only current source to keep the
fuel pump going.
Frans
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