Groundloop, Harvery, and I think WaveWatcher also,
It looks lo me like you guys are speculating on the fringe here. For starters, a way to really determine what's going on would be to simulate the circuit in PSpice. You just have to replace both batteries with 12-volt voltage sources, and perhaps add a little output impedance resistor to each voltage source to resemble batteries a bit more.
I stared at the circuit for 10 minutes and taxed my brain. Here are my thoughts.
Let's use the terminology primary battery (left) and secondary battery (on the right, the one that gets charged). We will say the secondary winding of the transformer is the 230-volt side, it charges the secondary battery.
For starters, the power stroke to charge the secondary battery is when the transistor switches on. All of the circuitry to the left of the transformer is an oscillator.
You can see how the dot on the 230-volt secondary is on the wrong side. In the schematic it's on the bottom when it should in fact be on the top. When the transistor switches on the "dot" on the lower primary winding goes low. So on the secondary winding the dot must be on the top, so that the other side of the secondary winding goes high and that forward biases the diode and charges the secondary battery.
And that's the essence of the whole design. The transistor switches on and while it is on you are in power stroke mode to charge the secondary battery. There is no magnetic energy in the core at the end of the power stroke at all. It all gets transferred into the secondary battery. It doesn't matter if the secondary battery is partially charged or fully charged, during the power stroke current flows through the secondary winding to charge the secondary battery.
Here is a big point that I think you are missing: During the power stroke, the lower primary winding is putting changing magnetic flux into the core. There are many turns on the secondary, the ratio from secondary winding to primary winding is about 30:1. Therefore, as fast as flux is being put into the transformer core by the primary, it's being "taken away" or cancelled by the secondary. The core is just an energy transfer medium in this case, it's not storing energy.
The top primary coil is just part of a feedback mechanism to keep the transistor on for a certain amount of time. It acts akin to what you see in a Joule Thief to snap the transistor on and keep it on.
When the transistor switches off, nothing special happens to the primary battery. There is never any energy from the secondary battery that makes it back to the primary battery.
If a very small amount of energy goes back and recharges the primary battery through the top diode, it's a moot point. Any energy that goes back to "recharge" the primary battery came from the primary battery in the first place.
Like I said before, the near-dead zombie battery on the primary is increasing in voltage because it is being "exercised," not because it is being recharged.
Anyway, that's my take on it. Standard disclaimers in that I could be wrong, but that's what I think I am seeing.
MileHigh
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