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).