Since the last post in this thread four months ago, Mexy has mostly been working fine. There has been the occasional difficulty in getting started, that seems to occur when the auxiliary battery is low. We really should have staggered the turn-on of the eleven EV200 contactors. We have since designed and built suitable turn-on-delay boards that we use in our Black Monolith solar power systems. We could publish the schematic and veroboard design if anyone is interested. But I haven't got around to installing any in Mexy.
There is a bigger problem with Mexy now. The battery is nearly 10 years old and a few cells have high internal resistance (which limits power) and only about 50% of their original capacity (which limits range). Our BMS tells us which cells these are, and it is blatantly obvious that they are the cells that have spent the most time at higher temperatures—those in the rear window and those directly above the motor and power steering reservoir. Replacing these should give Mexy a new lease of life.
Unfortunately I have not been able to find any place to buy LFP cells of the same dimensions. They are the Sky Energy 40 Ah cells (blue). Sky Energy became CALB and the cells became grey, but CALB no longer make the grey cells of that size either. However Greg MacDonald recently advertised a bulk buy of UBetter brand 50 Ah cells with almost the same dimensions. They are 10 mm smaller in their smallest dimension, 7 mm taller at their terminal contact areas, and exactly the same width. The terminals are a similar spacing, except they have M8 studs instead of M6 holes. But I can drill out the holes in the CMUs. So I bought 16 of the UBetters to replace the worst of the old Sky Energys.
Because our BMS' cell monitoring units (CMUs) are designed as a single board that goes across 8 cells, I need to pack (or "shim") the new cells out by 5 mm on each broad face (or 10 mm between new cells), and I need to pack the old cells up
by 7 mm. Fortunately there is enough spare height for this.
Yesterday I decided I had to go and buy some material to do this packing. So I tried a crazy search strategy. I went onto the Bunnings website and searched on simply "5mm". Then I started slogging through the 2314 items that came up. At 432 I struck gold! Well yellow. No, not Yello (the Swiss electronica duo from the mid '80s that Coulomb introduced me to). But having typed that, I had to go to YouTube and find The Race
This kind of yellow:
https://www.bunnings.com.au/macsim-fast ... k_p0044303
Since the broad face of the old cells is 116 x 180 mm and these shims are 100 x 150 mm, they don't require any cutting or joining. Brilliant! Bunnings also have them in 10 mm (black) for between new cells, and 2 mm (blue) to go with the 5 mm (yellow) under the old cells. Like this:
What's that you say? The thing in the background? That's the hot buttered 15 mm fruit shim (brown).
One thing that concerned me was that the new cells are in aluminium cases, which are known to carry a voltage with respect to the terminals, and have only a thin blue plastic film to insulate them. In Mexy, the cells sit in steel angle. What if the thin insulation wore through with vibration over time and two distant parts of our 720 V battery were shorted together via the chassis?
So I decided to measure the resistance between the case and the terminals. I first noted that a fully-charged rested cell with a terminal voltage of 3.35 V had 0.61 V between case and positive, and 2.74 V between case and negative. I then connected various resistances between the case and one or the other terminal and discovered that for both terminals it behaves like approximately 180 kilohms in parallel with 1 millifarad. So there [Edit: should not be any fire danger], even if the insulation does wear through. And in any case, we have an insulation monitor that checks the battery every time we turn the key to start, that will raise an alarm.
[Edit:] There may be a shock danger, or a danger of cell damage, if the thin insulation should wear through. Greg MacDonald of Lithium Battery Systems tells me that they insulate their cells from any surrounding metal with 12 mm of polyethylene in the battery systems that they sell, and they encourage their cell customers to do the same.
In the MX-5 retrofit, the best we can fit is 1 mm of polyethylene or polypropylene in the form of these cutting mats.
https://www.bunnings.com.au/avanti-flex ... t_p0155338
One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).