Battery switching without tears

CH.........I have switched on the fresh battery just before switching off the low battery for over 40 years and counting. No resisters, no capacitors, no nothing!
I documented this with my little test! Now, I’m using the same batteries, same capacity and same type batteries!
Hope this helps,
JJ

JJ, unfortunately, the previous owner installed a 3 position switch in bat1/off/bat2 so the switiching takes just a fraction of a second longer than if It was just a 2 position. And nothing else in my panel is affected other than the powerflarm and then only once in a while ( last time was in a nationals

On Thursday, April 16, 2020 at 1:35:35 PM UTC-4, wrote:
I got so mad the last time we fought this battery switching battle, I went out and hooked a fresh 12v battery to a low 10v battery with the parallel circuit completed with an amp-meter! I saw no spike and only 300m/a current flow as the fresh battery tried to charge the low battery.........the switches didn’t melt, the amp-meter didn’t explode and the fuses didn’t blow!
I have recently solved the issue by only using one battery............15a/h lithium iron battery............have t seen voltage below 12.2 v, even after 4 hours at 2a average current flow!
Please don’t stop arguing this issue, it’s so entertaining to watch the EE’s chase imaginary amps around!
JJ

Batteries don't keep the same voltage once you start charging them - the voltage jumps up very quickly. The remaining voltage difference driving that current is small (*** assuming the two batteries are of similar chemistry ***). Divided by the internal resistances of both batteries, the resulting current is reasonable. That is why nothing bad happens, usually, during the short period that both batteries (the strong one and the weak one) are connected.

Meanwhile my single 12AH lithium iron phosphate battery seems to have infinite capacity, in the sense that any flight I've done with it, even 6 hours, didn't discharge it too deeply. Unlike lead-acid batteries, these newfangled lithium batteries retain most of their capacity for quite a few seasons of use, output a voltage well over 12V until almost fully discharged, and can be discharged deeply without damaging future capacity or longevity. Much superior tech, and now quite affordable (probably lower cost per year than lead-acid).

On Thursday, April 16, 2020 at 10:35:35 AM UTC-7, wrote:
I got so mad the last time we fought this battery switching battle, I went out and hooked a fresh 12v battery to a low 10v battery with the parallel circuit completed with an amp-meter! I saw no spike and only 300m/a current flow as the fresh battery tried to charge the low battery.........the switches didn’t melt, the amp-meter didn’t explode and the fuses didn’t blow!
I have recently solved the issue by only using one battery............15a/h lithium iron battery............have t seen voltage below 12.2 v, even after 4 hours at 2a average current flow!
Please don’t stop arguing this issue, it’s so entertaining to watch the EE’s chase imaginary amps around!
JJ

I will add that Jon's concern about arcing is misplaced; arcing is associated with inductive loads when the current is interrupted. What happens in an inductor is that there is a magnetic field that is built up that has to have a place to go when the current is suddenly interrupted. The voltage in the circuit goes to very high levels as a result and will cause an arc in a mechanical switch. This is usually dealt with by a fly-back diode that allows the current to continue to flow and die off gradually.

If arcing had been taking place I would have seen it on the scope waveforms as a series of spikes in the current waveform. This did not occur in any of the many times I tested it, and it should not occur as capacitors are effective in minimizing or eliminating spikes.

Tom

On Thursday, April 16, 2020 at 12:07:53 PM UTC-7, 2G wrote:
On Thursday, April 16, 2020 at 10:35:35 AM UTC-7, wrote:
I got so mad the last time we fought this battery switching battle, I went out and hooked a fresh 12v battery to a low 10v battery with the parallel circuit completed with an amp-meter! I saw no spike and only 300m/a current flow as the fresh battery tried to charge the low battery.........the switches didn’t melt, the amp-meter didn’t explode and the fuses didn’t blow!
I have recently solved the issue by only using one battery............15a/h lithium iron battery............have t seen voltage below 12.2 v, even after 4 hours at 2a average current flow!
Please don’t stop arguing this issue, it’s so entertaining to watch the EE’s chase imaginary amps around!
JJ

I will add that Jon's concern about arcing is misplaced; arcing is associated with inductive loads when the current is interrupted. What happens in an inductor is that there is a magnetic field that is built up that has to have a place to go when the current is suddenly interrupted. The voltage in the circuit goes to very high levels as a result and will cause an arc in a mechanical switch. This is usually dealt with by a fly-back diode that allows the current to continue to flow and die off gradually.

If arcing had been taking place I would have seen it on the scope waveforms as a series of spikes in the current waveform. This did not occur in any of the many times I tested it, and it should not occur as capacitors are effective in minimizing or eliminating spikes.

Tom

Arcing will occur on break in an inductive circuit and on make in a capacitive circuit. It is quite easy to strike an arc with a 12V battery. It will happen without the capacitor - the increased capacitance will increase the energy and/or duration. You are very unlikely to see that on an oscilloscope - it will not create the voltage spike that an inductive load will. Open the switch body up and switch it on in a dark room - you'll see it. I'll be convinced when you show me the manufacturers spec that says 90A is ok on a 2A switch "as long as it doesn't last too long" or recommending it to switch large capacitive loads.

By far the simplest solution is to get a 12AH LFP and fly all day with the voltage above 13 the whole time. (By the way, you do get a warning near the end of the LFP's charge, on my 12AH it takes about 1.5 hrs to go from 12.5 to 11.5V at about the 9 hour mark).

On boats we use make-before-break switches to switch between battery banks (to preserve alternator diodes). The banks are hundreds of AH, with many thousands of Amps available. There are no problems with current when switching, even if a 10V bank is continuously connected to a 14V bank by selecting "Both" on the switch. The reason is the charge acceptance rate on LA batteries is quite low, even from a constant voltage, unlimited current source. Typically less than 1/2C, so in a glider with 12AH batteries, you might get around 6 amps for a few minutes between a fresh and completely discharged battery worst case. At about 1/2C charge rate, the battery terminals will be at max charge voltage even starting from completely flat. The IV characteristics of an LA battery are very different than capacitors. Switching LFP batteries this way may create problems - it depends on the particular batteries. Switching a charged LFP battery onto a discharged LA will again hit the same 1/2C current limit. Switching a charged LA battery onto a discharged LFP could create high current, depending on the batteries.

So it isn't a mystery why this has been done for 40 years with no problems. But those gliders don't have huge capacitors on the bus.

On Thursday, April 16, 2020 at 10:46:57 PM UTC-7, jfitch wrote:
On Thursday, April 16, 2020 at 12:07:53 PM UTC-7, 2G wrote:
On Thursday, April 16, 2020 at 10:35:35 AM UTC-7, wrote:
I got so mad the last time we fought this battery switching battle, I went out and hooked a fresh 12v battery to a low 10v battery with the parallel circuit completed with an amp-meter! I saw no spike and only 300m/a current flow as the fresh battery tried to charge the low battery.........the switches didn’t melt, the amp-meter didn’t explode and the fuses didn’t blow!
I have recently solved the issue by only using one battery.............15a/h lithium iron battery............have t seen voltage below 12.2 v, even after 4 hours at 2a average current flow!
Please don’t stop arguing this issue, it’s so entertaining to watch the EE’s chase imaginary amps around!
JJ

I will add that Jon's concern about arcing is misplaced; arcing is associated with inductive loads when the current is interrupted. What happens in an inductor is that there is a magnetic field that is built up that has to have a place to go when the current is suddenly interrupted. The voltage in the circuit goes to very high levels as a result and will cause an arc in a mechanical switch. This is usually dealt with by a fly-back diode that allows the current to continue to flow and die off gradually.

If arcing had been taking place I would have seen it on the scope waveforms as a series of spikes in the current waveform. This did not occur in any of the many times I tested it, and it should not occur as capacitors are effective in minimizing or eliminating spikes.

Tom

Arcing will occur on break in an inductive circuit and on make in a capacitive circuit. It is quite easy to strike an arc with a 12V battery. It will happen without the capacitor - the increased capacitance will increase the energy and/or duration. You are very unlikely to see that on an oscilloscope - it will not create the voltage spike that an inductive load will. Open the switch body up and switch it on in a dark room - you'll see it. I'll be convinced when you show me the manufacturers spec that says 90A is ok on a 2A switch "as long as it doesn't last too long" or recommending it to switch large capacitive loads.

By far the simplest solution is to get a 12AH LFP and fly all day with the voltage above 13 the whole time. (By the way, you do get a warning near the end of the LFP's charge, on my 12AH it takes about 1.5 hrs to go from 12..5 to 11.5V at about the 9 hour mark).

On boats we use make-before-break switches to switch between battery banks (to preserve alternator diodes). The banks are hundreds of AH, with many thousands of Amps available. There are no problems with current when switching, even if a 10V bank is continuously connected to a 14V bank by selecting "Both" on the switch. The reason is the charge acceptance rate on LA batteries is quite low, even from a constant voltage, unlimited current source. Typically less than 1/2C, so in a glider with 12AH batteries, you might get around 6 amps for a few minutes between a fresh and completely discharged battery worst case. At about 1/2C charge rate, the battery terminals will be at max charge voltage even starting from completely flat. The IV characteristics of an LA battery are very different than capacitors. Switching LFP batteries this way may create problems - it depends on the particular batteries. Switching a charged LFP battery onto a discharged LA will again hit the same 1/2C current limit. Switching a charged LA battery onto a discharged LFP could create high current, depending on the batteries.

So it isn't a mystery why this has been done for 40 years with no problems. But those gliders don't have huge capacitors on the bus.

Arcing is a chaotic process that produces erratic current spikes - go do your research and you will see. I saw none of that on the waveforms I recorded. Again, for what must be the FIFTH TIME add a series resistor to reduce the current level if you wish. I REPEAT, add a series resistor to reduce the current level if you wish. Do you got it this time, Jon?????

Give 'em hell, John!


On 4/16/2020 11:35 AM, wrote:
I got so mad the last time we fought this battery switching battle, I went out and hooked a fresh 12v battery to a low 10v battery with the parallel circuit completed with an amp-meter! I saw no spike and only 300m/a current flow as the fresh battery tried to charge the low battery.........the switches didn’t melt, the amp-meter didn’t explode and the fuses didn’t blow!
I have recently solved the issue by only using one battery............15a/h lithium iron battery............have t seen voltage below 12.2 v, even after 4 hours at 2a average current flow!
Please don’t stop arguing this issue, it’s so entertaining to watch the EE’s chase imaginary amps around!
JJ

--
Dan, 5J

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.

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?Â*
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.

--
Dan, 5J

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?Â*
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.

--
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