Battery switching without tears

On Sunday, April 5, 2020 at 7:02:13 PM UTC-7, 2G wrote:
On Sunday, April 5, 2020 at 11:25:37 AM UTC-7, jfitch wrote:
On Sunday, April 5, 2020 at 10:03:31 AM UTC-7, kinsell wrote:
On 4/5/20 9:45 AM, jfitch wrote:
On Saturday, April 4, 2020 at 6:23:44 PM UTC-7, 2G wrote:
On one of my flights last year I had to switch between my avionics battery and engine battery when the avionics battery voltage dropped too low (I had left the master on after the last flight and could only partially charge the avionics battery before launching). The switch over seemed to go okay, but then I noticed that my LX9000 was giving me unbelievably short glide distances. It turns out that the QNH altitude had been reset to the altitude at the time of switching. This was unacceptable, so I resolved to do something about it before this season. The simplest solution was to add a capacitor to the avionics power bus. The capacitor supplies power as the power selector switch is moving, and breaking, from the avionics battery, and connector, or making, to the engine battery (this is called a "break before make" switch. But how big of a capacitor to use? The basic equation involved is:

V = I * t / C or C = I * t / V

where V is voltage, I is current and t is time.

Translation: the bigger the capacitor the smaller the voltage drop.. If the requirement is to keep the voltage drop to 1 V, the current is 2 A (my situation) and t is 0.1 s, then C = 0.2 F (200,000 μF). The capacitor would also have to be rated for 16 V, min. That is a pretty big capacitor, so I decided I could tolerate a larger voltage drop (4 V), which cuts the size of the capacitor to 50,000 μF. I ended up finding a suitably sized 39,000 μF capacitor rated for 25 V. A smaller capacitor could by used if the current drain is lower, which is likely for most gliders.
https://www.digikey.com/product-deta...301-ND/6928303

I installed the capacitor yesterday and monitored the bus voltage during switch-over with an oscilloscope, which was anti-climatic: there was no detectable drop in bus voltage. Apparently the bread-to-make time is very short, perhaps a millisecond. Haven't had a chance to fly with it yet, but should be able to soon. The scope waveforms and capacitor installation can be seen at:
https://flic.kr/s/aHsmMo9rN7

What is the inrush current when you first switch the power on? Must not be enough to blow the fuse, but that'd be something I'd want to O'scope with a current probe.


Yep. High enough current might eventually erode the switch contacts, or
even damage the capacitor.

The commonly used rotary switches are already being run over spec in modern panels. Schleicher and most others are typically using something like the NKK MRY106 or equivalent, these are called a 2A switch but that is the AC rating, DC rating is 1A. A modern panel uses something like 1.5A, and that is before you key the PTT switch.

When I redid my panel I used an MRT23 which is 3A, and paralleled the contacts for a 6A total. Rotary switches with a higher rating are normally much larger physically.

Schleicher parallels two sets of contacts of that 4-throw, dual pole switch.

Tom

That gets you to 2A - still less than the panel draws with the PTT pressed I'd bet. The time constant of the Klixon is quite long compared to the inrush current (if the impedance is low). Do you have a high bandwidth current probe for your O'scope? The peak current may depend mainly on the arcing characteristics of the contacts in the switch.

It's too bad they can't figure out how to make an LX9000 that can take a momentary interruption in power - many others have figured out how to do this..