Too Busy For Words - the PaulWay Blog

Sun 20th Feb, 2011

A crimp in my plans

Well, yesterday I hooked all of the cells up and installed and wired up all the BMS modules. That's 76 wires about three centimetres long, 76 crimp connectors and 76 solder joints - two per cell module. I used crimp connectors because I had a bunch of them from hooking up the controller cables - Jaycar has 100 packs of connectors for about double what a pack of 12 cost - and so that I could disconnect and rearrange every module if necessary (i.e. I wouldn't have a mess of cables when I disconnect it).

After a while I got a technique up. I borrowed a friend's crimping tool (I tried pliers and they either didn't get enough force or cut straight through the connector), but because it's fairly cheap it tends to bend out of line when you squeeze it as the normal hand motion applies sideways force (due to the way your fingers curl). Solution: put the bottom half in the vice. This way I could exert all my force straight on and the tool itself wouldn't twist in my hand. This reduced the number of bad crimps down significantly. I also got into a bit of a production line - cut a number of segments of wire, trim all the insulation with a stanley knife, then crimp all of the segments and arrange them. This meant less tool swapping and allowed me to get a good technique on each operation.

I worried a bit as I was soldering everything up, but not because I thought that I would accidentally adger the circuit on the BMS modules. They're copy-protected in a crude but effective way - a blob of translucent epoxy on the centre of each module. You can see the LEDs shining through but nothing of the circuit or components. It wouldn't stop a dedicated counterfeiter for a second, of course, but it's enough to stop me making my own. I don't really care about this - if I need one I'll buy one, and if the company that makes them goes out of business I'll just cut the epoxy off one and figure it out on my own - or just buy a new circuit. I like the ones Elithion make - expensive compared to an analog BMS but you get so much more neat information!

No, I worried because I was hovering over a bunch of exposed metal connectors that could easily kill me if I was stupid enough to make a good circuit between two ends, or even parts, of the battery. It's only 130V, but if it gets a low resistance connection it can do 180A for as long as you please - about a quarter of a second will do it to stop your heart permanently. Fun, eh? The problem is in getting too used to handling them, too blazé about the accidental touch, until I do something stupid.

Anyway. There was a fraught moment getting the charger working. The charger takes a feed from the battery that goes through the relay in the BMS, it recommended hooking it up on the positive side (why?). I then proceeded to attach the other lead of the charger to the positive terminal, and wondered why it wasn't charging. Moving the connection to the negative terminal started it up just fine.

The only minor annoyance at the moment seems to be that the BMS turns off the charger as soon as the battery reaches 3.65v per cell. This means that about seven or eight of the modules show their red "I'm diverting current around this cell to stop it overcharging" before the BMS turns charging current off. I figure I'm going to have to this cycle about half a dozen times to get all the cells up to roughly equal charge - I can see cell voltages of between 3.34v to 3.44v at the moment. It only takes about two to three minutes for it to go from starting the charge to stopping it, though, so I'll do that a couple of times this afternoon and monitor the cell voltages and see what happens. The readout on the charger isn't getting beyond around 137V anyway, so I don't think there's any damage being done.

Now to start getting the controller inputs wired up and see if I can starts the motor up!

P.S. Here are the pictures.

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