There have been a few very frustrating setbacks lately, involving the two most important systems: the battery and the motor coupling.

I'll start with the motor coupling. We had started off with a simple set screw coupler, but that introduced a fair amount of wobble with the single setting pin. Plus, the other half of the coupled that was welded to the flywheel was slightly mis-aligned, further increasing the wobble. We then had a machinist cut out the old hub and add in a new one for a taper-lock style coupler. He also made sure that the hub was centered and aligned properly before welding. This is a much better system, but it also changed the way the flywheel sat on the motor shaft. We were using a 1 1/2" spacer for the bell house adapter, and with this thickness, the taper-lock bushing would slide fully onto the shaft. We tried tightening it as much as possible to see if it would hold, but as soon as the clutch was depressed, the clutch/flywheel assembly slid down the shaft instead of releasing the pressure on the clutch disc. I have now removed a 1/2" of the spacer (luckily we're still using 1/4" plates of aluminum instead of a single solid milled piece) and the taper-lock sits fully on the shaft. I'm still worried that the assembly may slide on the shaft. One of the coupler kits online has a piece that threads into the end of the motor shaft and holds the bushing in place. With the 944 shaft though, it has a tip which needs to slide into a pilot bushing at the center of the flywheel - which now will slide right into the end of the motor shaft.... So there really isn't any room for a threaded bolt to hold the flywheel on. If the taper-lock isn't strong enough to prevent the flywheel from sliding, it'll be back to the drawing board, and probably a new flywheel. I'm not sure how many times it can be cut and welded without becoming weakened or warped.

Also, the BMS has been causing problems again. I thought that it was the resistors that we'd been having problems with before, but those checked out ok. Now, the mosfets that switch on/off the bypass resistors are burning up. We had made a few changes to the bypass resister values during our initial testing, and now the current going through these mosfets rose from about 1.5A to 4 or 5A. With the voltage drop across the mosfet, this is now about a Watt which the little switch needs to dissipate. It's designed to handle up to 12A, but not for sustained periods of time. Plus, they are located very close to the bypass resistors, which reach upwards of 80 degrees C under bypass! Yesterday Darius found a larger mosfet which had much better heat dissipation, and replaced a few of the old ones. We then did a test see just how hot these components were getting. The bypass resistors hit 75-80 degrees pretty quick. The new mosfets reached about 60, which is within their limits. The old mosfets however quickly got up to over 120 degrees C! No wonder they were burning out on several boards.

Clearly, the by-pass resistor design has its flaws. We'll replace the mosfets on all of the boards anyway so that I can start using the batteries, but Darius and I will start designing a new BMS starting next week. 

I'll be testing the flywheel coupler today, and hopefully I won't have to redesign that too.