Battery Vehicle Society

Hi Mark,

Yup - They're not for heat dissipation. They're actually to compress the cells evenly using a metal banding

OK I'm beginning to learnstuff here so more questions, why do the batteries need to be kept in compression and what are the consequences of not comprressing them?

MT

marktime wrote:OK I'm beginning to learnstuff here so more questions, why do the batteries need to be kept in compression and what are the consequences of not comprressing them?

MT

Because (I hope I get this right!)....

Lithium Ion cells have a habit of expanding as they are heated and cooled during use and charging. The compression helps everything stay inside the cell not outside it!

I was wondering the same thing Mark. Trying to learn as much as I can about lithium, but it's hard to find much solid information.

I still don't quite understand though, Nikki. If the cells expand as they heat up, won't you increase the pressure inside the cells if you restrain them, and make them even more likely to leak?

And ... do you have to restrain the new generation of Thundersky LiFe batteries in the same way?

The plates in the cells need to be kept in firm contact with electrolyte. If the cases were stiff enough to do so they'd be quite heavy, so it's much better to rely on an external stucture.

From the Reap Systems website (www.reapsystems.co.uk)

>>Can I use the cells without compression?

Solid state lithium-ion cells have a very thin solid electrolyte. The electrodes need to be in contact with this solid electrolyte. In case of expansion, the solid electrolyte will not be able to settle down like a liquid electrolyte and so the electrodes may lose contact with the electrolyte. This would lead to a reduction in capacity. Compression is required in order to prevent this expansion, the ThunderSky cells PP case is not strong enough to provide this compression by itself, so that the pack requires external compression. So - the answer is NO - do not use the cells without compression.<<

My education continues, thanks Paul.

Cant think of any more questions yet!

Mark

marktime wrote:My education continues, thanks Paul.

Mark

Mine too There's several bits and pieces I've learnt from the threads here. All good stuff and adds all the more to the usefulness (is that a word?) of the BVS and the discussion forum

Well, work has progressed at speed this weekend with the batteries. By yesterday afternoon I'd wired up all the connectors to the loom (BMS side) and had installed it in the car. (That wasn't fun).

By mid afternoon I'd soldered up some of the wires to the back:



I probably stopped last night about 9pm but things continued this morning (sadly I wasn't ready to take the car to the Microcar show and had to press on as I wanted the car to use tomorrow!)

The BMS fitted a treat beneath the seat in the space which originally was taken up by the capacity meter (Booboo, if you ever do this to yours you'll probably remove your capacity meter anyway). I had to drill one extra hole to mount it all.



In a desperate last leg, I finally got all those temperature and voltage sense wires all connected up, along with the shunt wires, relay controls (I had to install a relay inside the transformer box for the charger so the BMS could turn the charger off when it reached the correct voltage) it ended up looking pretty good.

I'm glad the flying spaghetti monster heard my prayers though, as it all seemed to come off quite nicely.



After a quick cup of tea when Kelvin popped by he witnessed the car's first official charge with Liions in place. And lo!.... drum rolls....


Kate kept an eye on the voltages with the serial cable from the BMS going into an old laptop running a terminal program. It was working!



The laptop is only used to datalog while charging. When the car is driving I use a tiny LCD display I installed which keeps me abreast of the battery state of charge and remaining time before it's empty (at the current discharge).

With only two hiccups (one caused by wiring the relay across the mains not the output of the right part of the transformer and one incorrectly wired relay I'm quite happy. I had over forty cables in the end which were soldered and crimped in position. I'm glad it all works!

Stay tuned for some tests tomorrow!

Nikki.

wow , I can't wait to see it all working - fingers crossed you've had all the hiccups....just plain sailing from now on !

it looks fab !

I've got to say two things.

Firstly I was very impressed with what you've achieved. It was a wee bit nerve wracking when you threw the switch but wow! it all worked WELL DONE

Secondly Thank you for the refreshments and yes I did get rained on on the way home but fortunately most of it was right at the end just as I was a few miles from home though the rainbows were intensely vivid.

Good luck with the test run tomorrow. I'm keen to hear how it goes and also how the new power pack settles down over the comming weeks.

Oh and yes I'll post some pics from the microcar event shortly