Frans, As always there is much in what you say, but an
Odyssey is a gas recombinant lead acid battery and has
many characteristics quite different from conventional
lead acid batteries, such as the ability to withstand many
cycles of deep discharge, to have a significantly longer
life, to work normally at very cold temperatures and more
specifically to charge to 97% on a voltage of just 13.5v
and to retain 80% of its rated capacity until the end of
its useful life.
None of that of course makes it a good idea to be
in the habit of flying around assuming that it will keep
the pumps going for hours!
Regards, David
Frans Veldman <frans@privatepilots.nl> wrote:
><frans@privatepilots.nl>
>
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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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