Davtron M800 Chronometer

Cruzinchris

Pre-takeoff checklist
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Jul 18, 2017
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Cruzinchris
I got a used one that I am going to install in my plane. It needs a "keep alive" voltage. This could be provided by a "hot" lead from the battery, fused of course. This would be a long run for my plane. There is presently nothing on the panel that is not activated by the master relay.

The other way is the M800 can be ordered with a battery for keep alive. Mine doesn't have it. I've done some test and the keep alive needs to be at least 8 volts. I put a 9V battery on it and it works okay, but over the past week the voltage is dropping and it projects to go below 8V in months.

Ideas:
Build a charge circuit to top off a rechargeable battery when the master is on. Hesitant to use lithium because of the fire hazard. Revert to Nicd? Any other thoughts?
 
There’s an instruction somewhere on the davtron site about constructing a keep alive battery pack. Supposed to be good for two odd years.

There are also davtron battery packs available on spruce - at least for the 811 model. Might see if you can interchange
 
Bradp was the lead it needed. Davtron suggest 4 AA's for the backup. That would be almost 5x the capacity of a 9V. However 4 x 1.5V = 6V. Seems a little low from my experiment. But I am going to try it.
 
The military version is the M880. It is identical, but it only needs one AA battery to stay alive for years...

You could probably find one for cheap online...
 
Almost all modern radios need a "keep alive" lead. The normal clock in most aircraft needed a keep-alive circuit. The FAA allowed a 5 amp fuse/breaker directly at the battery for this. If this fuse/breaker is there, then there is a keep alive wire somewhere on the instrument panel. That should make the installation rather trivial.

Jim
 
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