Battery switching without tears

Agreed.

On 4/16/2020 1:16 PM, 2G wrote:
On Thursday, April 16, 2020 at 12:09:32 PM UTC-7, Dan Marotta wrote:
Well, Tom,Â* I don't have a scope and you do, so I ask again:Â* Why don't
you measure it?

But I will say that it's absolutely impossible to have a 5 volt
difference between the batteries.Â* I've never seen a 12 volt SLA higher
than 13.6 volts and that's fresh off the charger.Â* By the time you plug
it into your system, it's closer to 12.2 or 12.4 volts.Â* An 11.4 volt
battery will run a variometer, but likely won't transmit over your
radio.Â* A DPST switch will switch over in mili seconds, not seconds.

We're not talking about bridges that carried heavy trucks for 40 years
with a design defect.Â* If your wiring is 40 years old, I'd suggest
changing it.

They also said that a jet fuel fire couldn't weaken a steel beam
sufficiently to cause a building to collapse.Â* Come toÂ* Moriarty and
have a look at the steel post and beam hangar that slumped to the ground
last week after a fire.

On 4/16/2020 11:23 AM, 2G wrote:
On Thursday, April 16, 2020 at 8:56:54 AM UTC-7, Dan Marotta wrote:
Really?

What is the shorted time when flipping the switch?Â* What's the voltage
difference between the two batteries?Â* What's the total circuit
resistance, including the internal resistance of the batteries?
Theoretical math and practical application do not always agree.Â* It
might be fun to set up such a demonstration and use your o'scope to
measure that current and it's time duration.Â* Compare that to the "blow
time" of any fuses.

Seriously, I've done it for years without any problems, but I recognize
that past performance is no guarantee of future results. I'd be curious
about the results and you have the equipment to do it.

On 4/16/2020 12:21 AM, 2G wrote:
On Wednesday, April 15, 2020 at 6:18:08 AM UTC-7, wrote:
While I am reading these posts with interest, I confess to being an electrical illiterate. I just use two batteries, each with a fuse, and two switches. When switching, I turn on #2 before turning off #1.

If these circuits with diodes, resistors, make-before-break switches and so on are superior, please explain why, and if the case is compelling, a circuit diagram would be appreciated so that I might take advantage of the information.

After all, in aviation "R & D" actually stands for "Ripoff and Duplicate."
A make-before-break is also called a "shorting" switch. If you use such a switch you WILL short the two batteries together, which could result in a large current flow from the battery with the higher voltage to the battery with the lower voltage. This large current could blow your protection fuse(s). This is especially the case if you have two separate switches.
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Dan, 5J
Dan,

The I-35W bridge in Minneapolis worked fine for 40 years before it collapsed. The problem was a design error that was there since Day 1.

How much current flows between your two batteries? A lot! The internal resistance of the batteries is probably 0.01 ohm apiece, 18 AWG wire is 0.06 ohm/ft and your switch contact is typically 0.01 ohm. The big unknown is the location of your batteries and the length of the wire. However, the longer the run the more likely they used a smaller gauge wire, so let's start with 10 ft. This totals 0.1 ohm. Looking at the worst-case scenario, you may have a 5 V difference which results in 50 A of current. Since you are manually flipping switches, this current could last a few seconds. The largest factor here is the length and gauge of the wire. If I were you I would measure the actual current the way I did: with a scope and current probe or shunt.

Tom
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Dan, 5J
I will measure it. The 5V, which I said was worst case, comes from a 9V battery being connected to a 14V battery. These voltages will equalize very quickly and impossible to see with an ammeter or a DVM because they don't have the bandwidth to observe millisecond events.

Tom

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Dan, 5J