1S 1000mAh LiPo for £1.59

[Jeremy Harris]

Based on my experience with prismatic cell packages, I'd be inclined to vacuum test any cell you intend to use (sorry if this is stating the obvious and you're already going to do this). Bag leaks and puffing are the most common faults I've seen, and these would be exacerbated by being in vacuum.

It's worth noting that prismatic cells often work better when they are fitted in an enclosure that applies physical pressure to either side, squeezing the electrodes tight together. This invariably reduces the internal resistance and acts to help prevent the chain reaction failure caused by partial electrode separation from tiny internal vapour pockets.

 

[SteveT]

There is a small Lithium Ion button battery that an ESA study identified as suitable for space flight;

http://www.thebatterystore.com/powerdiscpd245037v200mahli-ionrechargeablecoincell.aspx

But max discharge current is only 400ma.

Stating the obvious but you could always parallel 2 or 3. What is the expected life of this satelite?

Just thinking you would need to make provision for a balanced charge if you do parallel them if you want the battery pack to last any length of time.

 

[srnet]

Stating the obvious but you could always parallel 2 or 3. What is the expected life of this satelite?

Probably no more that a couple of months .......

De-orbit time is about 20 years though, so you never know.

 

[srnet]

Based on my experience with prismatic cell packages, I'd be inclined to vacuum test any cell you intend to use (sorry if this is stating the obvious and you're already going to do this). Bag leaks and puffing are the most common faults I've seen, and these would be exacerbated by being in vacuum

Vacuum testing needs to be done to ensure that none of the components outgas, which happens to some plastics.

The Lithiums are vacuum packed anyway, so should not be a problem in ...... a vacuum.

Then there is temperature testing (-40 to +65), the PICAXE and Radio performed well incidentally, and vibration testing.

 

[Jeremy Harris]

The Lithiums are vacuum packed anyway, so should not be a problem in ...... a vacuum.

Correct, IF, and it's a very, very big IF, the seals are OK, with the proviso that cells aren't "vacuum packed".

Cells aren't evacuated during manufacture, as if they were the organic solvent carrier would boil off. The process used is to externally pressurise the pouch before it's welded. Seal and plastic laminate failure seems to be a surprisingly common failure mode in my experience. As the polymer electrolyte is carried in an organic liquid solvent inside the pouch, and as I know for sure that this leaks from time to time from defective cells at atmospheric pressure, I think it's reasonable to suspect that such a failure mode might be exacerbated in a vacuum.

BTW, the "chain reaction" plate separation failure mode may well be more critical in a vacuum, because normally atmospheric pressure acts to keep the electrodes in intimate contact with the polymer electrolyte. This is the reason that applying physical pressure to the side of the cells tends to improve both reliability and lower internal resistance. It might be hard to engineer this into the small space you have available, but if possible I think you may find that it will improve both low temperature performance and reliability.

The way the "chain reaction" failure happens is that a microscopic area of electrode loses intimate contact with the polymer. This then results is a local increase in internal resistance in this region. The increased resistance results in increased local heating, which then causes tiny amounts of the organic solvent carrier to vaporise. The resulting vapour pressure then tries to separate a larger area of electrode, leading to a rapid acceleration in failure. The end result is what many will have seen, a "puffed" cell, where the organic carrier has vaporised to a significant degree and expanded the pouch like a balloon.

My guess, as a humble scientist, is that such cell degradation is going to be more probable in a vacuum, where the external pressure will be way lower than the vapour pressure of the organic solvent carrier in the polymer.

 

[boriz]

Would there be any benefit to storing my LiPos under physical pressure, like under a heavy book or in some sort of clamp, when not being used or charged?

(Single cell 3.7v 500mAh 14 grams)

 

[boriz]

Or/and any benefit to charging them under same physical pressure?

 

[Jeremy Harris]

There's no benefit, AFAIK, in storing cells under pressure. There is for charging and discharging, particularly charging, as that's when there is the greatest risk of vapour forming inside the cell as a rule (I'd go so far as to say that I've never seen a cell "puff" during discharge, only during charge, but that might just be luck).

Pretty much all reasonable quality commercial applications that use prismatic cells put them in a constrained outer package to maintain pressure on the cell. Even the very small cells used in 'phone packs are often contained within a metal or plastic external case that provides a modest amount of constraint. Larger prismatic packs are often contained within robust housings that are designed to "squeeze" the cells, a good example being the Fiska Karma battery back (which we know a lot about because thousands of the original design have been sold off as surplus). The Fiska pack uses tension straps running around the outside,, with pressure plates at either end of each module to apply compression to the pack. This photo of several packs in the Fiska Karma chassis just about illustrates it (although this is a LiFePO4 chemistry pack it is still lithium polymer/lithium ion and has exactly the same issues as a prismatic pack as a LiCoO2 or any other lithium chemistry pack):

 

[boriz]

...which we know a lot about because thousands of the original design have been sold off as surplus...

Never heard of it. None on ebay. Where should I be looking.

 

[Jeremy Harris]

Never heard of it. None on ebay. Where should I be looking.

Many were sold on ebay last year, since then several sellers (of varying reputation) have been selling them on various Chinese sales sites from time to time (the cells were made in the Far East by A123, so the defective packs seem to have been re-sold out of China). I've not seen the whole packs on sale for a while now (this thread has info on when they first popped up: http://endless-sphere.com/forums/viewtopic.php?f=14&t=38901) but a seller on Aliexpress, Victpower (http://www.aliexpress.com/store/product/lifepo4-A123-20AH-system-AMP20/705561_688696675.html), has been selling single cells (of variable quality and provenance) for a while. The general suspicion is that these cells are the same as those used in the old Karma packs (which were a fairly generic A123 product originally). As A123 have never sold these cells on their own, where they come from is a bit of a mystery, but the general consensus is that they may be rejects or returns, see here: http://endless-sphere.com/forums/viewtopic.php?f=14&t=46244.