Too Busy For Words - the PaulWay Blog

Tue 3rd Jan, 2012

Time to make something better

I've just managed to release the magic smoke from some or most of my battery management modules - the little boards which sit on each cell and make sure that it doesn't over-charge and notifies you when the cells are getting low. Unfortunately, EV Power's basic design has a fundamental flaw: they require two through-hole soldered connections which are very close to the cell terminal. With the large cell interconnect blocks, this makes it rather easy to end up with one cell's sensor wires touching the traction battery. When another cell somewhere else also has its sensor wires touch the traction battery, the tiny MOSFETs that do this switching end up with a potential difference of several tens of volts with over a hundred amps ready to gleefully emit magic smoke.

I consider it a basic flaw in EV Power's design. They could supply the modules with wires already soldered in and epoxied over. They could have supplied explicit instructions about how to wire them up which warned me of this possibility. The most recent design had an even worse flaw in my opinion - the holes which you feed the sensor wires through to provide some strain relief on the solder joint were on the wrong side. Earlier designs had it so the insulation of the wire would be the bit touching the terminals if things went wrong; the new design makes sure that you put the solder joint is right there to contact the traction battery.

In addition, the circuit board didn't go all the way across, instead using a wire and a lug (in some kind of supererogatory effort to save a fraction of a cent in circuit board cost, as far as I can see). This means that the sensor wire solder joints are literally pressed against the terminals unless you turn the board sideways. I did try to avoid this, but apparently not fervently enough, and the result seems to be numerous module boards with scorch marks and (in the next-to-most-recent design) burned-off heat-shrink.

Now I have to find another BMS, because there's no way I'm paying for any more EV Power products. I would strongly recommend anyone considering using them on 60AH Thundersky cells look elsewhere. I will now be ordering a Lithiumate Lite BMS from Elithion - the communications interconnect is top-soldered only. EV Power's BMS probably works fine for 90AH cells and anything larger than a 61mm between-centres connection. But for me this is definitely a dud.

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