That means you cannot check the DME in the ground, unless the engine is running. What model DME is it?
Peter wrote:
That means you cannot check the DME in the ground, unless the engine is running. What model DME is it?
No halfway decent avionics tester would do a test without a proper 28V ground power connection. It’s a King KDM 705, shoebox sized with 3 vacuum tubes. A very fine analog computer. It’s working but I would never trust its readout 
Instead of relying on a power supply to keep the voltage perfect there is always the option of charging the battery while you mess with the avionics. A lead acid battery is a very effective regulator and will inherently absorb many nasties.
Indeed, but many/most planes have a “ground power relay” which disconnects the battery when external power is plugged in, so you don’t get the option. That’s why on the two power units I built there are two separate overvoltage protections; one in the switcher itself and then the external one implemented with a huge MOSFET in series.
achimha wrote:
Not really.
Well, I still think they should (should, not will). Otherwise you lose them when you switch to battery. Which can happen in flight, I’m sure we all know that. I would really want avionics to function well below 24 V. Unless there is a good voltage regulator.
Martin wrote:
Well, I still think they should (should, not will). Otherwise you lose them when you switch to battery.
Lead acid batteries have more than 24V.
achimha wrote:
My King DME is one of them.
Your specific pre-world-war-I DME model might not be too representative, there are doubts it works at any voltage, and furthermore if you switch it on it will interfere with just about any radio equipment within a 100km radius 
It’s good for legally cover you for “DME required” procedures and airspaces, but not for much else
Lead acid charge termination voltage should be 14.4 or 28.8V, if I remember correctly.
achimha wrote:
Lead acid batteries have more than 24V.
How much more and for how long? As I wrote, the voltage will drop – as you load the battery (discharge rate matters) and it discharges. I know that some people added a cell to get 14 V (6+1 lead-acid cells) in sailplanes because their VHF comm died too soon while there was still energy left in the battery. These days you can buy a unit that can go (as per datasheet) down to something like 9 V.
