Battery switching without tears

If you run with both batteries connected, a failure that should have blown one battery fuse with either blow both fuses or none at all. Both results are bad.

Cheers,
...david


On Wednesday, April 15, 2020 at 11:04:01 AM UTC-4, kinsell wrote:
Mark can use his procedure if he wants, or better yet just keep both
switches on an forget about flipping switches. Unless he's a former 747
captain, who likes to fiddle with lots of switches.

On Tuesday, April 14, 2020 at 9:28:12 PM UTC-4, 2G wrote:
On Tuesday, April 14, 2020 at 6:38:00 AM UTC-7, wrote:
On Tuesday, April 14, 2020 at 9:31:13 AM UTC-4, wrote:
On Tuesday, April 14, 2020 at 1:25:55 AM UTC-4, Andy Blackburn wrote:
I used Shottky diodes plus power resistors plus capacitors. I'm no EE but I took enough circuits courses to handle this problem. The Shottky diodes keep the batteries from cross-discharging each other, the capacitors keep the instruments powered when the switch is disconnected from battery 1 and before it is connected to battery 2 and the resistors keep the capacitors from drawing too much current when you power them up since they make the circuit (even with the diodes) look like a direct short initially.

Andy

KISS. Just the two diodes (and no switch) should be enough. Whichever battery is stronger (higher voltage) would take the load. Automatically.. No switching needed. With the higher voltage of LiFePO4 batteries (relative to lead-acid) the voltage drop in the diode is acceptable, especially if it's the Schottky type.

Or, if you really want to remove the voltage drop in the diodes, add an SPDT switch (perhaps one with also a center-off position) IN PARALLEL to the diodes. No matter which position that switch is in, both batteries will still be connected. But the battery the switch leads to will feed the avionics with no voltage drop since the switch bypasses the diode on that side. The other diode will meanwhile prevent current from going INTO the other battery. The middle-off position (or no switch at all) is the safest though, since if either battery develops a shorted cell (or shorted or loose wiring, blown battery fuse, etc) without your knowledge, it won't affect the other battery and the avionics, thanks to the two diodes.

- Clarification: I meant a diode between each battery and the avionics bus as a whole. Not separately for a specific instrument.

I measured the inrush current once again and found that the vertical of the scope was set for a 1X probe instead of the 10X actually being used. This meant that the peak current was 90A instead of 9A, which is a bit high. I added a 1.1 ohm resistor and the peak current dropped to 6A. A simulation shows that a 2 ohm resistor drops it to 3A. This is a good value to use if you have a 1A current drain as the voltage drop will be 2V. The wattage of resistor is unimportant because so little energy is being dissipated by the resistor. The energy transferred remains constant regardless of the resistor value as it is the energy required to charge the capacitor (the current pulse lengthens for larger resistor values).

Tom

Tom,good plan to increase the value of the resistor to limit the current inrush. One could even make the resistor 100 ohms and have a reversed diode in parallel with it so that when the capacitor is needed to sustain the instrument during switching, the current would flow in the reverse direction through the diode, bypassing the resistor. You now have the best of both worlds - slow charge of the capacitor when the power is turned on, and a fast discharge to support the instrument without IR drop across the resistor, instead the drop would just be the bias voltage of the diode.

I've seen a lot of recommendations to keep both batteries connected and not bother with switching. One reason I don't want to do that is that I have a couple of slim 2.3 Ah SLA batteries in the tail of my ASW 24 as a backup (yes, they fit--barely) and the usual LiFePO4 battery up front for the primary source. Ignoring the issues of batteries with different capacities and chemistries running in parallel, pulling out the tail battery pack every night and charging it is just another chore. At a contest, I never do unless I've had to go to it (only a few times in many decades). I'm lazy.

Chip Bearden
JB

Chip how much loss do you experience getting that juice from the tail all the way up to your panel?

On Wednesday, April 15, 2020 at 1:25:50 PM UTC-4, wrote:
I've seen a lot of recommendations to keep both batteries connected and not bother with switching. One reason I don't want to do that is that I have a couple of slim 2.3 Ah SLA batteries in the tail of my ASW 24 as a backup (yes, they fit--barely) and the usual LiFePO4 battery up front for the primary source. Ignoring the issues of batteries with different capacities and chemistries running in parallel, pulling out the tail battery pack every night and charging it is just another chore. At a contest, I never do unless I've had to go to it (only a few times in many decades). I'm lazy.

Chip Bearden
JB

Chip, your situation sure does call for a switch. Or two switches. Or two diodes. But certainly not simply in parallel. Note that with two diodes the "switching" would be automatic: as long as your LiFePO4 battery up front is not almost completely discharged, there would be no draw on the tail batteries since the diode between them and the "bus" would have a voltage on it in the "wrong" (non-conducting) direction.

Chip, your situation sure does call for a switch. Or two switches. Or two diodes. But certainly not simply in parallel. Note that with two diodes the "switching" would be automatic: as long as your LiFePO4 battery up front is not almost completely discharged, there would be no draw on the tail batteries since the diode between them and the "bus" would have a voltage on it in the "wrong" (non-conducting) direction.

I use two switches. Switch on the tail battery before switching off the front battery. Since I hardly ever need it and the front battery has never been terribly discharged, I haven't experienced big inrush currents and fuses blown. I'm looking at adding a capacitor since that is apparently what my LNAV had for 25 years when I had a single DPDT switch to go back and forth if needed and never had anything powercycle.

I'm also a little less vulnerable than some--maybe. My PowerFLARM is a portable and I keep batteries in it that would be good for the whole day if necessary. Likewise my glide computer is TopHat running on a Kobo (with an internal battery) that gets its GPS data from the FLARM with a backup of a Dell Streak that is fully self contained and can be connected to a big USB battery if I need to run it all day. So in primary/backup battery failure, what I would lose would be the radio (of course) and the ClearNav vario (a big loss because I have the LCD screen with thermal assistant). But I also have a Winter running on a triple probe. If I lose the triple probe, the ClearNav uses electronic TE.

I try to think about redundancy because over the years I've lost (at contests) primary battery power (loose fuse holder), pitot pressure (hole in the ASI diaphragm, which also affected the LNAV), the LNAV as well as the ClearNav (forgot to connect tubing I disconnected during troubleshooting--operator error), and a bad battery (didn't check capacity in the spring--laziness).

There is some voltage drop from the tail battery due to the wiring but the current draw isn't as much as some guys have (e.g., no transponder). I haven't flown this year to note how much and I can't recall what I saw a few years ago when I stuck a voltmeter on it uninstalled and then again at the buss in the panel with and without keying the mike.

There are so many things that CAN go wrong these days that I try to protect myself against as many as possible. As I mentioned in a another thread a day or so ago, the most dangerous words in technology are "It SHOULD work" followed by "THAT shouldn't happen" (or "Haven't seen THAT before") followed by "Works on/in MY machine/glider". All true. Throw in interface problems and software bugs and it's wonder we can fly a contest without a major failure.

Chip Bearden
JB

This whole discussion is a living example of the old saw:Â* Better is the
enemy of good enough.Â* I flew for years with two batteries, two fuses,
and two switches.Â* When battery 1 gets low, flip on battery 2 then flip
off battery 1.Â* Never had an issue.

On 4/15/2020 10:12 AM, NM wrote:
On Tuesday, April 14, 2020 at 9:28:12 PM UTC-4, 2G wrote:
On Tuesday, April 14, 2020 at 6:38:00 AM UTC-7, wrote:
On Tuesday, April 14, 2020 at 9:31:13 AM UTC-4, wrote:
On Tuesday, April 14, 2020 at 1:25:55 AM UTC-4, Andy Blackburn wrote:
I used Shottky diodes plus power resistors plus capacitors. I'm no EE but I took enough circuits courses to handle this problem. The Shottky diodes keep the batteries from cross-discharging each other, the capacitors keep the instruments powered when the switch is disconnected from battery 1 and before it is connected to battery 2 and the resistors keep the capacitors from drawing too much current when you power them up since they make the circuit (even with the diodes) look like a direct short initially.

Andy
KISS. Just the two diodes (and no switch) should be enough. Whichever battery is stronger (higher voltage) would take the load. Automatically. No switching needed. With the higher voltage of LiFePO4 batteries (relative to lead-acid) the voltage drop in the diode is acceptable, especially if it's the Schottky type.

Or, if you really want to remove the voltage drop in the diodes, add an SPDT switch (perhaps one with also a center-off position) IN PARALLEL to the diodes. No matter which position that switch is in, both batteries will still be connected. But the battery the switch leads to will feed the avionics with no voltage drop since the switch bypasses the diode on that side. The other diode will meanwhile prevent current from going INTO the other battery. The middle-off position (or no switch at all) is the safest though, since if either battery develops a shorted cell (or shorted or loose wiring, blown battery fuse, etc) without your knowledge, it won't affect the other battery and the avionics, thanks to the two diodes.
- Clarification: I meant a diode between each battery and the avionics bus as a whole. Not separately for a specific instrument.
I measured the inrush current once again and found that the vertical of the scope was set for a 1X probe instead of the 10X actually being used. This meant that the peak current was 90A instead of 9A, which is a bit high. I added a 1.1 ohm resistor and the peak current dropped to 6A. A simulation shows that a 2 ohm resistor drops it to 3A. This is a good value to use if you have a 1A current drain as the voltage drop will be 2V. The wattage of resistor is unimportant because so little energy is being dissipated by the resistor. The energy transferred remains constant regardless of the resistor value as it is the energy required to charge the capacitor (the current pulse lengthens for larger resistor values).

Tom
Tom,good plan to increase the value of the resistor to limit the current inrush. One could even make the resistor 100 ohms and have a reversed diode in parallel with it so that when the capacitor is needed to sustain the instrument during switching, the current would flow in the reverse direction through the diode, bypassing the resistor. You now have the best of both worlds - slow charge of the capacitor when the power is turned on, and a fast discharge to support the instrument without IR drop across the resistor, instead the drop would just be the bias voltage of the diode.

--
Dan, 5J

Chip,

I solved that problem in my LAK-17a by tapping a charging pigtail into
the wire from the tail battery.Â* The charging connector (power pole) was
in the baggage compartment.Â* It was simple to charge it without removing
it.Â* It was a pain to access the wires to do that, but I only had to do
it once.

On 4/15/2020 11:25 AM, wrote:
I've seen a lot of recommendations to keep both batteries connected and not bother with switching. One reason I don't want to do that is that I have a couple of slim 2.3 Ah SLA batteries in the tail of my ASW 24 as a backup (yes, they fit--barely) and the usual LiFePO4 battery up front for the primary source. Ignoring the issues of batteries with different capacities and chemistries running in parallel, pulling out the tail battery pack every night and charging it is just another chore. At a contest, I never do unless I've had to go to it (only a few times in many decades). I'm lazy.

Chip Bearden
JB

--
Dan, 5J

Dan,

Thanks to so many of us showing up crewless and to CDs who call tasks with the intent of getting most or all pilots home, it's relatively rare these days that I have to hook up the trailer to my van very often during a contest: once in the past five nationals, IIRC (not counting several aero retrieves at Elmira).

And at sites with favorable weather and my new polyurethane finish, I'm tying out at night much of the time at national contest sites unless storms threaten. As I said, removing the tail battery is no big deal, although if I'm tied out, then I have to remove the elevator to get access. I'm just lazy. It's easier not to put that battery on the line unless I have to--then it's good for the whole contest.

Hence, my two switch (or my DPDT switch before it got shot up at TSA) approach makes the most sense for me. For those using batteries with wildly different capacities (e.g., motorgliders) or avionics drawing a lot more amps than mine do, a different approach might be in order.

Chip Bearden
JB

On Thursday, 16 April 2020 02:05:45 UTC+3, Dan Marotta wrote:
This whole discussion is a living example of the old saw:Â* Better is the
enemy of good enough.Â* I flew for years with two batteries, two fuses,
and two switches.Â* When battery 1 gets low, flip on battery 2 then flip
off battery 1.Â* Never had an issue.


Less components means less problems with bad components, less solder joints that eventually break with vibration, all of this is better than good. I run same setup, on-off switch for both batteries, usually only one of them online, except when getting my engine (turbo) out in case battery runs out of juice right then. Zero problems ever. Glider came out of factory with both batteries wired parallel btw.