Aaron Sliski's Blog

Electric Longboard Battery Box

Posted on Octorber 24, 2016

So here’s the Idea. I have a lot of 18650 cells thanks to my friend Austin. He goes to MIT, for some reason they were chucking these cells because "they weren’t good enough". They don’t have a high C rating, they don’t have a low internal resistance, they aren’t anything fancy but they will do the trick. As you can see I have the block of wood setup in the mill, it’s about 6in wide and 8 inches long. I laid out the batteries on the wood just to visualize if I had decent spacing. The extra room on the board is for my motor controller.

Well after programming the mill to do a simple rectangle pocket I was off! The reason why I was using a 1/2in end mill was because it was already in the tool holder and it was center cutting. The larger end mills we had in the tool holders weren’t center cutting, and would have not gone well with wood. So after one pass or so (about .3in) the machine turns off, and it says there’s a fault. So I am scratching my head...... nothing had happened... I made sure the Y axis wasn’t bottoming out on the mill, I made sure I had the right speed and feed. Well It used to be that if you used the vacuum and the mill at the same time the breaker would trip. BUT I got this amazing nice vac at the dump and it didn’t draw nearly as much as my father’s cleanroom vac with dual motors. Well this little vac tripped the breaker...... So I went to the panel and reset the breaker, walked back over to the mill and the mill computer was rebooting which only takes a year because its a 486...... So that means I lost my 0 as well as my program. Urg.... So I reprogram the mill, but aha I will move the vac plug to what I thought was another breaker, well it wasn’t...... So the breaker trips again. This time I had a plan. I have had the power flicker before and my monitors would turn off but my computer would stay on. So through this thought process, I would have to turn the breaker off and on fast enough to have the computer not reset. And by god it worked........ I was able to flip the breaker on and off fast enough to reset the breaker but not have the computer turn off.

Ok I kind of lied about the limits of the mill..... So you see that area towards the bottom of the work piece? Yea that’s because the vise runs into the mill at -7.75 inches, but, there is always a but, if I had grabbed the wood in the other orientation it wouldn’t have been good because the two sides of the wood aren’t parallel. The sides of this wood are rough sawn, yes I like the look. I want my board to look rustic and something hacked together, even though it’s been carefully thought-out most of the time. Anyways I went 1.00in deep even though the wood is 1.5in thick, because the 18650's are only 18mm thick i only need to go 18mm deep at the least. Since the bottom of this block will be facing down I am worried about things hitting it and possibly breaking through and doing something fun to the batteries. So I made the bottom surface .5in thick, I thought was plenty, but then again we will find out the hard way wont we.

So as you can see I put seven cells in there, when originally I had six up there. There is a reason for this. I bought a Chinese 7s BMS ((battery Management System) so I plan on doing 7s2p or 14 cells. I just ordered a buck boost converter so I can get 30v because it’s hard to find a 30v portable power supply. There will be some extensive testing but each cell is about a mile of range. Austin and I get about 10wh/mile with our boards, each cell is 3.7v nominal and 2.6ah so if you multiply those together you get 9.62wh/cell. So with this pack I hope to get 13.5 miles.

*Disclaimer* I don't use word editing software so there is a 100% chance of mistakes, we are all human. Also I try to write my blogs as if I was talking so if I use certain language its part of who I am. If you know me, you can probably hear my voice when reading this.