Since MOSFET's are inherently thermal protected by themselves (their internal resistance rises with temperature) it acts as a sort of stopgap measure that it otherwise wouldn't have.
...Also I'm kind of in shock at the moment, because if what i just tested turns out to be correct, then this driver is going to blow everything else away, permanently. I double checked my connections, my oscilloscope, everything to make sure what I was seeing wasn't just a trick.
Alright. So. It boosted to 7 amps.
Seems that the supply is truly what determines how much power this can push out (I had to parallel two panasonic cells to get this, with one it can still shove 5 amps out at 5 volts.)
Here's the real, true, kicker, though.
I had to short all of the diodes in my test load, to allow all of the current to flow through just the resistor.
And because of ohm's law, 7 amps through a 1 ohm resistor means....
It boosted to 7 volts.
And, additionally, the new design and all of the little changes I made to the board and components seems to have made the new boards highly highly efficient. The ability to heatsink the mosfet directly also has a huge impact as well.
Compared to previous designs and whatnot the board doesn't seem to produce nearly as much heat, even while pushing out 7 amps into a 7 volt load. Because of the increased efficiency the driver seems to be capable of doing some ridiculous feats, since less heat produced also has a pretty big impact on performance overall. Additionally, improved efficiency also means reduced load on the batteries, meaning you don't have to have such ridiculous cells nor wire gauge to achieve good results.
To give you an idea of how important the supply is for this driver...
Here was my first test.
Battery: 18650 3400 mAh Panasonic Cell charged to 4.2 volts.
Upon driver connection cell voltage drops to 3.4-3.5 volts. Driver output is steady around 5 amps at 5 volts.
When I connect an additional cell...
Cell voltage drops to 3.73 volts. Driver output is steady at 7 amps at 7 volts.
I'm still blown away by how much power this driver is handling. I could NOT believe what I saw when I connected the second cell, seeing the current fly up on my oscilloscope awestruck me.
I'm going to test a second identical driver board tomorrow, hopefully to reproduce my results. If I can reproduce this reliably...
After doing some real basic math after a hard day of rockhounding and hunting I realize that there MUST be something amiss with my measurements somehow, or my 1 ohm resistor is no longer up to snuff somehow.
Reason I say this is because if it TRULY is doing 7 volts at 7 amps, then it's somehow handling 49 watts of power.
At 3.7 volts that means that the cells would have to supply AT LEAST 13.25 amps to break even (this is assuming 100% efficiency)
It may be possible still, of course, but I'm a little more skeptical now.
I'll try and do some more tests tonight to see if I can narrow down anything in specific, and try to do some more measurements.
i'm using NCR18650B's, and from a little looking around they're rated to around 10C?....
I would just measure the current directly but my dang multimeter introduces too much resistance to get anything accurate.
6.8 amps max discharge rate for these cells. Not 10C, don't know where the person I read that post from got that number from. They actually seem to be a hybrid chemistry battery, and are suited more for lower drain purposes (they have higher internal resistance and don't handle heavy loads very well)
So, okay.... Two cells in parallel, means I should be around 13-14 amps of supposed safe discharge? Hm.
So it definitely seems like there was something amiss, and I was very much correct. :undecided:
I went back and redid my testing, and found that when I power the driver, it starts out at the initial 5 amps mark. After a few seconds it "jumps" to 7-8 "amps" of current.
After disconnecting the power I noticed I could hear a small "tick" sound coming from the test load's resistor.
Seems to be that, when I'm testing it it heats up so greatly that it changes resistance quite a lot (more heat than can be carried away from it at any time) so when it cools back down again, I'm probably hearing something internal changing size.
I had a feeling something was up since I was hearing this, so to prove that it was the test load resistor behaving funny, I hooked up the driver to a 10 watt 1 ohm 1% resistor for some brief testings.
Measured 5.5 volts, so I was getting 5.5 amps. Great. There was NEVER any change. No sudden increase, no jump in current, no huge change with differing cell counts.
Heat definitely seems to be an issue though, however this may be due to improper diode heatsink contact. And, additionally, proper cells are a must as always.
So, I apologize for the initial hope for the ridiculous capability of before. I knew it seemed off, and I DID want to test it more yesterday but I needed to sleep.
But, still, 5.5 amps at 5.5 volts isn't anything to sneeze at least to say. This was boosting from 3.7 volts sagged cells too.
I've done a little more of a redesign with the board, not nearly as extensive.
However the new one utilizes an interesting MOSFET package... it has an exposed metal pad on the TOP of it, allowing for INCREDIBLE heat transference through the top, vs. just heatsinking to the plastic case. I've also consolidated the capacitors all onto the top (again, it'll be even EASIER to heatsink!!) and are using higher capacitance caps to compensate. Hell, because of the consolidation the total capacitance on the output will actually be slightly higher than before, reducing ripple a little.
The new board design'll be here in about 2 weeks from now. Until then! :san: