Monday, 8 September 2014

Planning the RX8 build

The first Monday of my hols I visited the Mechanics to find out when they could get the RX8 in for its strip down and motor fitting, it is booked for mid-September so I paid Steve for the Kostov 11 inch and Soliton 1 controller and advised Brent of the need to get Steve’s MX5 in for the motor & controller removal, Steve has assured me that he will cover any additional “removal” costs although I might well take some other parts from his MX5 EV for my RX8 EV.

So it’s chocks away for the start of the RX8 build and I am seriously looking forward to it, The battery is bought and paid for as is the motor & controller, I have been planning the battery compartment layout it will be laid out in rows of 5 (5S) and blocks of 5 (5P) I will arrange the 70S in 14 rows of 5 starting in one corner of the battery box and going to the other side up or down a row and back again, continuing on until I reach the other corner, The reason for this configuration is to simplify the buss bar manufacture as it will mean that all I will need to do to fabricate the buss bars is cut (or get cut) some 3 mm copper plate into squares and rectangles in order to join all the +ve’s and –ve’s correctly no special shaping will be required as I can modify the plastic cassettes by removing some small plastic tabs that currently separate the cells +ve’s and –ve’s bridge them with a plate that joins 5 +ve’s together in parallel same with the –ve’s and then make the bar wide enough to reach over to the next 5 +ve’s in that row, when I reach the end of a row of 5 I will make a buss bar that reached up or down to the next row and orientate that row in the other direction so I can continue the series arrangement of the 70S string.

I will knock up a diagram to explain this, I will also be modifying the cassettes in another way as Steve suggests that the latest advice from the manufacturers themselves is to hold the battery pouches in compression, not to actually compress them but to avoid any potential swelling from separating the layers in the cell pouches, Steve suggested putting balloons at each end to put them under a little pressure but I think I may just go for a tight fit in the boxes as in the Honda.

Steve has also suggested I should lose the aluminium separators on the cassettes and replace them with a sheet of silicone between every cell, I would be inclined to agree with this good advice as the cell pouches are made from Mylar which is itself a conductor and aluminium is also a conductor so the whole battery bank is only very slightly protected from shorting by the small air gaps between cell pouches, a sheet of silicone as a separator would provide both an insulator between cells and also a method of holding the whole cell block in compression.

The only issue I have wit this current layout plan is that the cassettes seem to support the pouches vertically and the blocks of cells are arranged with the tabs at the top so other than the supporting plastic cassette the cell pouches would be hanging from there tabs.

I was planning on cutting off the cassette frame completely just retaining the top piece that provides support for the screw terminals and allows the top tab parts to be bolted in blocks and separate the cells using silicone sheet, however this would appear to mean that the cells would literally be hanging by their tabs (not good) so the obvious answer would be to arrange the battery pack so that the pouches lay flat rather than tabs at the top, this will complicate the wiring arrangement but should not cause a major problem, I will try and get photos and drawings of the proposed arrangement uploaded soon.

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