First battery pack build

[LiFeCycle]

https://imgur.com/gallery/VtoVF


This is my first battery pack build, now complete!

Im using a rewound mot spot welder with SSR and arduino. For the new secondary winding I used the copper from some jumper leads, folder over on itself twice, so that it's 4x the thickness of the original jumper lead - about 10mm diameter total, like a rope of copper wire. The primary winding gets hot way before this does.

It's switched with an SSR by an arduino nano on veroboard, on for [strike]15ms, off 15ms, then on 85ms[/strike] 10ms first pulse, 10ms wait, 30ms second pulse. Tested spot welding a piece of nickel to a stainless steel knife.

The welds arent perfect, there were some blowouts of the nickel strip when the electrodes weren't placed firmly enough into the nickel. Electrodes are now seperate (not taped together), so it's easier to firmly place each electrode independently onto the surface of the battery, and this means a good weld every time.

Once I got the hang of that, it was easy enough, just took a really long time. I had fun with this join:

Overall I love the rewound mot concept - free materials anyone can find, $10 worth of electronics. [strike]I still need to optimise it a bit obviously.[/strike] The main thing is to have the windings joined up directly to the electrodes with the minimum possible resistance. Then it works wonderfully.

At the end of the day, how safe is the pack? And what can be done better next time? Thanks

 

[Arty Marty]

may I ask what SSR you are using? Brand, type, where you purchased ... I'm not having a lot of luck... killed 2 already..

As for your battery pack, I read you should avoid the centre of the battery... but otherwise that looks good.

Marty

 

[LiFeCycle]

Thanks Marty. The SSR I used was the fdtek ssr-40 da. Couple of bucks on eBay, and it's supposed to handle 380v up to 40A, so should be well overkill for this purpose. I'm simply driving the input side directly from one of the 5v arduino digital outputs, and the output side is switching the 240 on the way into the primary coil.
https://imgur.com/gallery/RTlMV

 

[Arty Marty]

Dammit. That's exactly what I'm using.. Perhaps I was just unlucky... I have two left... I'll try one more and order some 80a ones... Thanks for letting me know.

Hey, you don't remember how many watts the microwave was that you pulled apart do you?
Mine was rather large, that might be an issue. I know that it shouldn't get close to 40amps, even 20amps, but in the microsecond that it first turns on things can happen that take an eBay $2 ssr out of its comfort zone. (there's a reason a good ssr costs $100+)

Cheers

 

[Arty Marty]

Well, well, well...
I was just looking online and I saw that the photos of those Fotek SSR varies rather a lot. I realised mine doesn't say "made in Taiwan" so I came back here to look at yours..
Not only does yours say made in Taiwan, it's got the bottom right corner clipped...

I think I have solved my mystery...
https://goo.gl/photos/MUstw1worY3UUoV79
Interesting eh?
Marty

 

[LiFeCycle]

Interesting Marty - you reckon you picked up a fake? My consolations.
I guess I would have switched this one ~1000 times now, so far so good so it must be the real thing. I wouldn't be able to tell you sorry on the size of the microwave, other than to say it looked pretty standard. Yeah the startup current must huge on these, I'd love to know how much, but don't have the gear to test it.

I've improved my electrodes by connecting them directly to the output coils, rather than via a 30cm piece of single jumper lead. Having done that, the resistance is massively reduced, and I only need a 30ms pulse duration to get a better weld than with 85ms before. My min time between pulses is 1second, so I'm now running a max 3% duty cycle - and you can weld for ages without it heating up.

 

[Arty Marty]

That sounds great. You'll have to post some photos of your creation.
I got 5 of those 40a SSR's, used a few to turn lights etc on and off. No problem. They are probably good for 10a. I ordered a 100a SSR so even if it's a fakey mc fake face, it should be good for 40a.. Fingers crossed.

Oh, you don't know the seller on ebay that sold you yours do you?

Marty

 

[LiFeCycle]

Yeah will do. The seller of my SSR was 'access-market'
Cheers

 

[Arty Marty]

Well, my 40a SSR Turned up today. Haven't tested them yet. Also got a 100a one from AliExpress (god I love that site)
About $23 Inc shipping
Tested the 100a one, works like a dream

https://goo.gl/photos/qqrgJLbTg3cNcqRp8

 

[LiFeCycle]

Batt complete! Added a few picks of my setup above. The pack is working pretty well - at the start it was about 5% unbalanced, but i've used some 10ohm 10w resistors over a few days to balance it to within .01v, now i'll let the bms take over.

The CA reports a battery internal resistance of 0.102ohms which seems pretty high.

Eg running 5kw at 80v, motor current is 62A, motor equiv resistance r = v/i = 80/62 = 1.28ohms
My batt IR is 0.102ohm, which is 7.8% of this. So i'll be dissipating 380w of heat in the batt when running at 5kw. Seems pretty high to me, is this just the way it is?