Thanks John for trying to keep the tread on topic. All the parts have arrived and it has kept me very busy the last couple days! No task has completely failed but nothing has gone smoothly either. The day the parts arrived I started my work on creating the boards. This proved to be a real nightmare. Since I was now trying to do 10x boards at once the board area was much larger than doing a single board like I had test. I did finally get one of my transfers to actually work well enough that I proceeded with the etching. Of course it wasn't until after I do all this that find out there is actually a paper specifically made for this type of transfer, Pulsar Toner Paper. And just to rub a bit of salt into the wound it also available from digikey for a fairly reasonable ~$1.2 a sheet. Apparently it makes crystal clear traces and as soon as it touches water the paper falls right off, no scraping or extending soaking.
The boards turned out pretty nice and all the traces seem to survive and make connections. Now even though I only have a total of hand full of components I still have to solder 36 leads per boards, so 10 boards became fairly tedious pretty fast.
The end product:
The final specs of a single board:
After that was finally complete it was time to get the heat sink all prepped for the mounting of the 10 boards, 10 current sense resistors, and bus bar. I laid out the part in cad then place them on the heat sink. This was a bit of a challenge since the sink actually already had a good 15 or so holes in it. I then marked all the holes on the sink and started drilling . With in a couple holes I broke a drill off deep in the sink. GRRRR. The bit wasn't the sharpest and had clogged. I then had to move that hole and a couple other holes around to make sure everything would line up. I had originally planned to tap all the 40 or so holes but gave up on that after my 4-40 tap bottomed out and broke off in another hole. Luckily I was able to extract the piece. I decided to go to a #4 screw. After all the holes were drill out some more I added a small chamfer and then sanded the sink up to 1500 grit (overkill). It did really shine up nice even with the million plus holes:
I went ahead and put together the bus bar which is 0.5 x 0.25 and created a fuse holder. I currently only have a couple fuses and will need to order some 300 amp ones for the real test.
Now it was time to assembly line the devices on the heat sink. I laid down the resistor with heat sink grease, put all the wires on the boards and then attached them to the sink. After some testing I found 2 boards were not working. After some testing I found the connector was missing a solder on the one and the other had a small trace bridge. Once I started testing all the boards I did notice their current varied by about 4-5% which I assume is due to the 5% 25w resistor. I should have incorporated a little trim pot on the board to compensate for that.
Now what’s left is soldering up the jumper wires that inter connect all the boards, building a case/fan holder, and integrating the microcontroller . I’ll of course post here once I do some full battery tests with manual control. I’m really excited to be able to do 100 amp tests on a couple cells I have around. I also wanted to test a couple of RC Lipo Packs and see how they hold up.
So if I was to do the process again I would go try to go with a TO-247 package FET so I could use less total devices. The other thing that would greatly simplify the process is getting away from a separate current sense shunt. I should have no problem using a wire based sense resistor though I would give up some resolution. I want to start messing around with surface mount stuff so I don’t have drill the boards but I think silk screens become much more important.
So got all the boards interconnected and have been doing some full discharge test with only one fan. I emptied my A123 to zero volts on accident
I pushed that little 12v pack to 140 amps or around 1200 or 1300 watts intermittently. I then did my big headway packs both about 450wh at 50v 20 amps. Everything worked absolutely beautifully. The sink was keeping nice a cool at around 44C to 52C (17C ambient) with the fets at around 60C on the one side and 82c on the other (fan was blowing on one end). With two fans blowing I'll get more balanced air and I'm sure warmer weather shouldn't be a problem even at 1200W. Holds the current perfectly too.
gotta hurry off to work, hopefully I can read over this tonight and figure out what I missed and doesn't make any sense