Battery switching without tears

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

Nice, I have the same issue and have been thinking along these same lines.

Thanks for the report. I had wondered about doing the same thing. Just curious, did you include a resistor across the capacitor to make sure it discharges when the master is switched off and no loads are left on?

On Saturday, April 4, 2020 at 11:45:25 PM UTC-7, Mark Morwood wrote:
Thanks for the report. I had wondered about doing the same thing. Just curious, did you include a resistor across the capacitor to make sure it discharges when the master is switched off and no loads are left on?

No, there is enough parasitic drain to take of that.

Tom

On Saturday, April 4, 2020 at 7:23:44 PM UTC-6, 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

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

On Sunday, April 5, 2020 at 3:36:09 AM UTC-4, John Foster wrote:

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery.. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

That often works, but the engine battery will be fast-charging the panel battery while both are connected in parallel, so you don't want to leave the switch in that position for very long. If there's a 5 amp fuse between the engine battery and the panel, you don't want to blow it.

John DeRosa's excellent tutorial details several good options for the two-battery issue. Starts at slide #95:
http://derosaweb.net/aviation/presen..._Practices.pdf

Cheers,
...david

On Sunday, April 5, 2020 at 9:22:59 AM UTC-4, David S wrote:
On Sunday, April 5, 2020 at 3:36:09 AM UTC-4, John Foster wrote:

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

That often works, but the engine battery will be fast-charging the panel battery while both are connected in parallel, so you don't want to leave the switch in that position for very long. If there's a 5 amp fuse between the engine battery and the panel, you don't want to blow it.

John DeRosa's excellent tutorial details several good options for the two-battery issue. Starts at slide #95:
http://derosaweb.net/aviation/presen..._Practices.pdf

Cheers,
...david

Actually, slides 119 - 123 provide additional options.

http://derosaweb.net/aviation/presen..._Practices.pdf


Page 97 shows the caps.

Page 98 shows why not to switch 2 in parallel for longer than it takes a fuse to blow. (Murphry says it will be the fuse on the good battery;-)

Page 120 shows the diodes. Note that the K2's have more volts to start with, so the diode drop is less of an issue.

I run like page 120, except with only one diode. The theory is that with the flat discharge curve of the K2's, I mostly run on the non-diode batt until is gets near empty and then transition to the second K2. It's automatic and I can watch the bus voltage to see how it's doing. SH wires the front battery in parallel with one of the mid batteries, so if the diode is on one of those, it runs with the vanilla ship wiring. There is a second connector on the diode battery for charging. (The second mid-battery gives me a switchable third, but so far haven't needed it.)

I also built an 'ideal' diode (page 121) but decided it I'd rather keep it simple and see the discharge progress in the bus voltage.

John's pictures make this easy. Aside from the one-diode, two-K2 plan, has anybody got another that is not in his collection?

On Sunday, April 5, 2020 at 12:36:09 AM UTC-7, John Foster wrote:
On Saturday, April 4, 2020 at 7:23:44 PM UTC-6, 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

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery.. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

The glider, an ASH 31 Mi, is already wired with a battery selector switch for the avionics, and is the way to go. You definitely don't want to parallel Pb and LFP battery's accidentally.

Tom

Tom,

Isn't the rotary panel switch in a Schleicher glider a make before break?

OK, I'll stop posting to this thread now.Â* My solution, which I've
relied on for years without ever losing a connection or blowing a fuse
seems to be too simple to be acceptable.

On 4/5/2020 7:45 PM, 2G wrote:
The glider, an ASH 31 Mi, is already wired with a battery selector switch for the avionics, and is the way to go. You definitely don't want to parallel Pb and LFP battery's accidentally.

Tom

--
Dan, 5J