Battery switching without tears

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