BroMikey's Capacitor Dump Circuit

[TinselKoala]

It seems pretty clear from those posts that BroMikey already "knows" all he is ever going to know about mosfets and cap dump circuits.

[SeaMonkey]

Aye, one could easily come to that conclusion after reading his
postings.  I suppose we shall see in the coming days, weeks and
months whether this Old Dog is up to learning some new tricks.

Being stuck inside the "box" puts a real damper on progress...

[Farmhand]

Well I decided I had some time so I set up a dump circuit using one mosfet from my LF power control board and dumped 26 volts into a 12 volt battery from a 45,000 uf cap as shown. The smoothing bank should have been at least three times larger.

I got about 13 amps peak current and the current sense resistor at 0.8 ohms got real hot but my mosfet stayed cool  to touch.

I'll put some shots here and the circuit for those who cannot log in over there.

Circuit
then applied voltage in blue and current in yellow across the 0.8 Ohms.

then The voltage on the two cap banks with the inductor between them

Second shot shows gentle waves but the first shot shows sudden violence. 

Oh and a rise time shot. Slow mosfet.

The 14 Amp hour motorcycle battery I'm charging could do with the experience of some decent current it hasn't been used to start the bike for a long time.

Took out the 0.8 Ohm resistor and all is going well, mosfet cool. Battery voltage bouncing and increasing. 

[SeaMonkey]

Very good FarmHand!  That is how it should be done.

The "charging choke" with diode isolation will accomplish
resonant charging to some extent.  No waste of energy.
(Well, perhaps a tiny bit - but not excessive as would be
the case with a resistor.)

What value of capacitor did you use across the Gate Driver
Chip?

Excellently documented by the way.

[TinselKoala]

Mm-hmmm.

The peak current shown is a bit over 13 amps, I estimated 13.7 or so. But the average current during the pulse seems to be about 3 or 4 amps, and taking the 33 percent duty cycle into account that means that the average power dissipation  I²R in the mosfet itself is ((4 amps)² x 0.01 ohms) x 0.33 = a bit over 50 milliWatts. And since it's being switched cleanly by a good squarish pulse of 12 volts from the gate driver, it's not adding much switching loss to that. So no wonder it stays cool!

If you now make the capacitors bigger (more capacitance) and charge to the same voltage and use the same duty cycle, you will get higher average current (the exponential decay time constant will be longer) and you will get more energy transferred into the battery per pulse. The _peak_ current will still be the same.