How safe are large lifepo4 packs?

[Dan]

We know the benefits of dumping the lead and going lithium but what are the drawbacks? Are there any safety issues you must consider before going full lithium? The cells come with insulation but its not good good enough if you pack the cells tightly together so a divider must be used but what would happen if a divider fails?

Are there any particular safety precautions that should be taken?

[Irving]

Mechanically, chemically and electrically LiFePO4 cells are intrinsically safe. The only real risk is the one you've outlined. You need to protect from shorts. But that's easily done with some 0.75mm or 1mm polycarbonate sheet as a spacer between cells. A bit of spray-on contact adhesive and its sorted. Even if it cracks there's little risk of shorting between cells as it provides mechanical spacing as well as insulation - that's why I wouldn't use tape or vinyl wrap. Then good quality wiring, no different to lead.

[Dan]

1 mm Clear POLYCARBONATE Sheet Free Post A3 sheets 297MM x 420MM £90 for 30 sheets.
https://www.ebay.co.uk/itm/1-mm-Clear-P ... x_t77U_otw

I believe most chairs have a fuse between the two batteries where they connect in series. Would it be a good idea to do the same with two 12V lithium batteries? What rating of fuse?

[Burgerman]

You could. Better to add that at the pos end though. Same fuse rating as with lead. Or you could get some odd cell balance curves in the PL8 while charging. The fuse is to protect the wires. So as long as that blows first, before any fires then you are good to go.

[Irving]

1 mm Clear POLYCARBONATE Sheet Free Post A3 sheets 297MM x 420MM £90 for 30 sheets.
https://www.ebay.co.uk/itm/1-mm-Clear-P ... x_t77U_otw

I believe most chairs have a fuse between the two batteries where they connect in series. Would it be a good idea to do the same with two 12V lithium batteries? What rating of fuse?

You can buy it ready cut - a piece big enough for 1 side of a cell 130 x 210 is about 50p, the big piece along the side 310 x 200 is about 80p. Total cost under £15.

You can, and should, fuse it.

[Dan]

50P a sheet sounds really cheap. These guys will precut you 16 sheets 130 x 210 for £50 and three 310 x 200 for £6 and they do a price match. Precut would save me a lot of hassle.
https://www.simplyplastics.com

The reason I bring up the safety aspect is because if anything happened I would not be able to move, burning to death does not sound overly appealing to me.

[Burgerman]

Well you are not worried about sitting on lead.

If shorted, or overcharged they produce hydogen and oxygen, turning you and your bedroom into a bomb. And when they explode they shower you with his sulfuric acid, to make sure you go blind, and that your burns get some acid in them. Still, the windows will be gone so it should soon be breathable!

Take a look here. https://www.google.com/search?q=lead+ba ... 80&bih=618

Its all really about what you are conditioned to see as normal.

As it happens all the usual lithium ion chemistries burn like fireworks, or explode. FAST and very intensively. You dont want those on a chair. They are used on full sized cars, hobby stuff, phones, laptops etc.

So dont use lipo or lithium ion...



With the exception of LiFePO4 cells. Those if punctured, shorted, or overcharged drastically, wont burn or explode.*
They do produce vast quantities of evil smelling white steam/smoke if you really try hard. But never burst into flames.

*With one exception. If you add EXTERNAL heat, as in the middle of a fire from other sources, they can get hot enough to boil the oily electrolyte. There is not that much in there, but like engine oil, or cooking oil, if you get it hot enough it burns really well. So if theres a BMS with a bunch of cables on board, that can go short circuit (say a failed shorted mosfet) then the smaller wires will start a fire. That can set fire to the chairs plastics, and heat the cells, burn off the main wire insulation, and heat up the cells for a real oil based fire. Every time you see an LiFePO4 fire, it turns out to be caused by wiring or usually BMS failure. Not actually cell failure.

13 mins of very heavy cable short circuit. This is what you get.

[Irving]

50P a sheet sounds really cheap. These guys will precut you 16 sheets 130 x 210 for £50 and three 310 x 200 for £6 and they do a price match. Precut would save me a lot of hassle.
https://www.simplyplastics.com

Lots of places and materials to choose from. e.g. HIPS sheets (the stuff chopping boards are made from)

https://www.cutplasticsheeting.co.uk

£40 min charge, but if ordering enough for 4 packs it would be ~£18 each inc onward shipping. There's probably others with lower or no min charge

[Scooterman]

Because the cells aren't always perfectly flat sided (shirley mentions this). IMHO i think a flexible material with a bit of cushioning is better. (Although a few of my cells did get damaged in transit). But even if you put a straight edge on visually perfect cell it's not always dead flat. I don't mean they've bulged cos they're secondhand, but I guess like most manufacturing processes some are always better than other.


Rubber sheeting

[Irving]

Because the cells aren't always perfectly flat sided (shirley mentions this). IMHO i think a flexible material with a bit of cushioning is better. (Although a few of my cells did get damaged in transit). But even if you put a straight edge on visually perfect cell it's not always dead flat. I don't mean they've bulged cos they're secondhand, but I guess like most manufacturing processes some are always better than other.


Rubber sheeting

1mm adhesive backed silicone rubber sheet is £32/sq m. and you need 0.7sq m.

My only reservation is that it's only 60A hardness so won't resist a penetrating blow as well as the HIPS.

[Dan]

Thinks for the help.

After a little research it seems like Polycarbonate is a lot stronger than Acrylic. The Storm 4 xplore's battery box is not that wide, I have been unable to measure it but its at least the width of a MK gel battery, 25.9CM. So I need at least 263MM if I use 1MM Polycarbonate as a divider.

Long ways is not really a problem so I could use 2MM rubber. There is a brace on top of the batteries but that can be removed so I could use 3MM rubber on the bottom of the box.

My real question is, is the 1MM Polycarbonate up the the task of using it on the side walls of the battery box and the long single middle divider between the inline cells?

[Burgerman]

yes.

[Dan]

Thanks

[Burgerman]

Be careful. MEASURE first. Thats a very tight battery area on that chair.

[dewaj]

I've been contemplating some means to eject the batteries in case they start burning or something so that the chair is not destroyed.

as in, to save the ship, eject the warp core... etc...

[Burgerman]

You can still destroy a house.

You just need to use correct LiFePO4 chemistry and preferably no BMS and its safe anyway.

[dewaj]

You can still destroy a house.

Oh, I had in mind just letting the pavement melt. No way would I let it burn indoors. And those class E extinguishers are too expensive.

[Burgerman]

When you look at LiCo type cells, like laptops, tesla cars, phones etc and LiPo (lithiu polymer) then they are really energy dense but have around 1/4 the service life of the LiFePO4 cells.

And when you compare the very dangerous explosive/burn type fro those cells then they are OK in a small quantity like your laptop. Or outdoors in a tesla car with water cooling and warming, and very well controlled computerised management system. But they have no place in a wheelchair in 100 to 200Ah aounts... Either indoors, or outdoors. Outdoors OK if you can get out and run away, but most of us cannot.

LiFePO4 cells have around 2/3rds the energy density. But they do not burn and are safer than lead that you are used to. And they can do 2 to 5000 cycles at high discharge levels and so with a correct setup, and use, they may outlive you.

[Scooterman]

This is what I used. I'm not saying it's the best solution but it worked okay for me, and you got lots for the money (incl del).

A lot of alternatives suggested in this thread I'd never even thought of. I wished I'd thought of using spray adhesive for sticking the rubber to the cells. I bought a roll of double sided sticky tape from Wilco. But spray contact adhesive would have been easier and quicker.

I've still got some rubber left over, enough to make a gimp suit for the cat