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.