18650/21700 Case Material?

Ham

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Feb 18, 2016
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Hi All,

I'll be building a case for my next build soon and wondered which materials people have had success with.

I am happy TIG welding aluminium (probably 1.5 to 2mm) or stainless (around 1mm) so these were my first thoughts but wondered if either had a benefit over the other in terms of bash resistance and specifically heat resistance in the worst case scenario of a battery fire, I assumed stainless will hold up for longer in that situation?

Fibre glass moulds have also become a possible option, any opinions on that over and above a metal one aside from weight?
 
A metal case for cylindrical cells? Oh you brave soul. Have you tried an image search for 'ebike battery case'? There are some premade ones for sale. Also check out '3d printed ebike battery case'.
 
alu would be good. but i would put some foam on the inside to insulate the battery from shocks, heat/cold and so on.
i use this a lot:

https://www.amazon.co.uk/Acoustic-Soundproofing-Resilient-Tape-Isolation/dp/B07HXZSKMJ

its sticks like a mofo on alu.
it helps a lot to absorb sunlight, winter cold and the small shocks you get that might rub on things you dont want to rub.

ps: keep 2 holes in the bottom on each side pointing down. that way any moistiure can escape. dont worry about cells venting. in case a cell (or more) needs to vent you probably have a LOT more problems. they dont vent unless they are really crappy chinese fake cells or you hit something really hard at a speed big enough to crack your case AND the steel cells. i am extremely confident that venting is not a problem for you if such an event were to happen....
 
TrotterBob said:
A metal case for cylindrical cells? Oh you brave soul. Have you tried an image search for 'ebike battery case'? There are some premade ones for sale. Also check out '3d printed ebike battery case'.

Pre made means no custom shapes to fit the lines of a frame...aka..looks crap imo.

What don’t you like about metal casing?

Specialized levo etc have metal casings in the form of a frame, no?
 
flippy said:
alu would be good. but i would put some foam on the inside to insulate the battery from shocks, heat/cold and so on.
i use this a lot:

https://www.amazon.co.uk/Acoustic-Soundproofing-Resilient-Tape-Isolation/dp/B07HXZSKMJ

its sticks like a mofo on alu.
it helps a lot to absorb sunlight, winter cold and the small shocks you get that might rub on things you dont want to rub.

ps: keep 2 holes in the bottom on each side pointing down. that way any moistiure can escape. dont worry about cells venting. in case a cell (or more) needs to vent you probably have a LOT more problems. they dont vent unless they are really crappy chinese fake cells or you hit something really hard at a speed big enough to crack your case AND the steel cells. i am extremely confident that venting is not a problem for you if such an event were to happen....

Top stuff, yes vent/drain holes would be built in for sure. Plus the foam as with previous builds...top stuff. This build is having a potted battery for extra protection from impact and water which will help all round I think.

I have seen impact tests on high end cells and they seem to not vent at all, which is nice to see tbh

Thanks for the feedback.
 
Just concerned that your battery might evetually short out against a metal case. But if you think your build skills are up to par, go for it.
 
Ham said:
Top stuff, yes vent/drain holes would be built in for sure. Plus the foam as with previous builds...top stuff. This build is having a potted battery for extra protection from impact and water which will help all round I think.

I have seen impact tests on high end cells and they seem to not vent at all, which is nice to see tbh

Thanks for the feedback.
never.

ever.

pot.

cells.
 
I thought the same about potting until I spent some time chatting with an experienced builder in the workshop and after seeing their previous builds and current three year old 40ah pack that has taken years of delivery abuse with deliveroo riders powering mxus 3k hubs I have decided to give it a go for this next build...

Out of interest, why do you say never pot them: any bad experience with it?
 
Ham said:
I thought the same about potting until I spent some time chatting with an experienced builder in the workshop and after seeing their previous builds and current three year old 40ah pack that has taken years of delivery abuse with deliveroo riders powering mxus 3k hubs I have decided to give it a go for this next build...

Out of interest, why do you say never pot them: any bad experience with it?

well, i have only been building batteries for 10 years or so. i have done lots of testing and potting is always the worst in nearly every metric and makes especially humidity issues MUCH worse when the resin does crack but 1 thing that you dont need a degree for is to understand that potting over the frigging PTC is not a smart choice to make. if you are potting shut the emergency vent hole you turn your cells into a litteral grenade.

here is a link of vents working correctly on cheap chinese cells: https://imgur.com/gallery/BsnNkl5/ notice the venting happens on both sides depending on where the positive side is of the cell.

just imagine what happens if you pot over that vent and turn thsoe cells into little bombs.

just because 1 guy never had an accident does not mean nobody has or will. not to mentioin that the guy making the batteries is not sitting his arse on it every day....
 
Well now you have me re thinking the whole idea. I have seen vids like that in the past and the consensus seems to be that they must have been "cheap Chinese cells" or built poorly (I'll be using LG 21700 cells fwiw), that appears not to be your line of thinking?

"potting over the frigging PTC"...ermmm what is the PTC?

I am taking on board the information, it is appreciated.

Have you built potted packs and had bad experiences with them?

I have seen the massive triangle pack the builder has been using on his own bike for years and it appears to be holding up well with large abuse.

I am just trying to figure out the best way to build a solid pack that is protected from heavy offroad impacts and vibrations and sucky wet UK weather.

I have assisted in building two previous 18650 packs in the usual manner with a spot welder, cell holders, nickel strip etc wrapped in three layers of shrink wrap but am not confident enough to have a go on my own due to the risk reward. The potential downside of mis-welding and shorting something is too great in my mind right now, in part to videos like the above!...not to mention I am after a custom shape pack...

If there were a builder near me in the UK that was willing to help me gain confidence in my own build I would...

Thanks again for the time to share your experience
 
the PTC is the plastic disk that is underneath the positive terminal. its the fail safe of the cell. it pops under high heat or pressiure and breaks the connection and vents the cell. its what keeps the cells from becoming a bomb in case of a fire for example.

i have done loads of testing with potted cells and have seen/repaired quite a bit of packs from other builders that were abused for years and i have seen it all. mostly mostiure problems that gets trapped in cracks that happen over time and rusts the cells badly. overheating is also a fun one. potting traps heat, by..like..a lot. wich causes a LOT of fun issue and damages surrounding cells.
ps: we have the same weather, and most of my customers live less then 30 miles form the north sea... :wink:

its not hard to make a pack that can deal with daily driving on roads. you just need to work in layers and have each layer do a specific thing. if you need the right cell holders for starters.

what you want is not hard. but just lathering everything in potting is simply dumb and dangerous.
 
Custom fit metal case.
Holds all electronics: 12S6P (color printout is for 14S5P version) INR21700-50G Battery, 2000W controller, 12V DC/DC converter, fuse panel, fan.
Assembled using 1/16" aluminum plates, pop-rivots, 1/2" angle-aluminum.
Clam-shell style using piano-hinge allows easy access to case internal components.
Round-21700 cell matrix using interlocking plastic holders. 2X Krypton tape on "Battery end" sides.
3" x 4" x1/8" felt padding on all corners of battery pack block.
1/16" nylon armor lines inside of aluminum box protects battery from shorts if case gets crushed/penetrated.
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put a wall between the controller and the battery. it needs support on that side as well. and make sure you use thermal pads for the controller to dissipate heat. that thing will get hot if you mount it there.
and a "floor" for the battery to sit on. it needs to be "naturally" supported from all sides so it cannot move no matter the dirrection of gravity. and be supported on as much "floor" as possible.
 
Solid info here chaps, thank you. Love the battery box! Good work Sir
 
flippy said:
the PTC is the plastic disk that is underneath the positive terminal. its the fail safe of the cell. it pops under high heat or pressiure and breaks the connection and vents the cell. its what keeps the cells from becoming a bomb in case of a fire for example.

i have done loads of testing with potted cells and have seen/repaired quite a bit of packs from other builders that were abused for years and i have seen it all. mostly mostiure problems that gets trapped in cracks that happen over time and rusts the cells badly. overheating is also a fun one. potting traps heat, by..like..a lot. wich causes a LOT of fun issue and damages surrounding cells.
ps: we have the same weather, and most of my customers live less then 30 miles form the north sea... :wink:

its not hard to make a pack that can deal with daily driving on roads. you just need to work in layers and have each layer do a specific thing. if you need the right cell holders for starters.

what you want is not hard. but just lathering everything in potting is simply dumb and dangerous.

I've just been reading and watching about grins potted packs... they seem to be holding up well and still for sale.

The failure testing was also very interesting. Have you seen it?
 
Ham said:
I've just been reading and watching about grins potted packs... they seem to be holding up well and still for sale.
The failure testing was also very interesting. Have you seen it?

i have seen it and i have one of their batteries and tested it.

they are not made to handle dutch cobblestone roads nor the utterly fantastic sea climate that goes with it. its considerably different from the california climate to say the least.

potting only works if you have a LOT of foam padding and are 1000000% sure your cells are perfect and will never leak or go bad.
just 1 spot needs to crack from a hard impact (a curb for example) and the pack is done. moisture will get in, cant get out and will utterly destroy the cells.

still, potting serves no purpose, it only adds cost, difficulty and many other concerns that can be better solved in a more durable way.
 
Thanks for the reply, appreciated.

How would you create a pack that is fully waterproof and shock resistant?
 
Also I would really like to hear about the testing you carried out on the ligo pack if you get time?
 
Ham said:
Thanks for the reply, appreciated.

How would you create a pack that is fully waterproof and shock resistant?

simple, you create layers. first a enviroment case that protect against direct elements (aka: metal case) but that does allow for small ventilation and drainage. then a shock absorbing layer (aka: foam, different kinds), then a hard water layer, generally polyolefin on rolls (heat shrink film that works like cling film and contracts when heated) and just wrap the battery in it, and caulk the areas where wires come tru. and on the inside you have strong cell holders (preferably nylon or abs) and those are capped off with 3~4mm poly or acetal and glued on the sides to provide rigidty to support the cell holders. more can be done depending on the application.

Ham said:
Also I would really like to hear about the testing you carried out on the ligo pack if you get time?
sorry, that work was done under commision/contract. i cant go into detail. i gan only give you my impression that matched my own testing.
 
Nothing wrong with potting a cylindrical cell pack. In fact I'd argue it's the safest thing to do. The PTC plate will function just fine in most cases.
 
jonescg said:
Nothing wrong with potting a cylindrical cell pack. In fact I'd argue it's the safest thing to do. The PTC plate will function just fine in most cases.


only for specific use cases (i.e. diving gear) and with extremely careful selection of materials. and even then its safer, lighter and cheaper to just build a case that does the same job.

you dont see tesla just vaccum-fill their batteries with 2 part epoxy.

i tested bunch of setups triggering a ptc with a 2 part epoxy on the cell. the results were....violent.
 
The resin, so I am advised, will turn softer and much more pliable in a thermal run away event and thus not be an issue for venting.
 
Ham said:
The resin, so I am advised, will turn softer and much more pliable in a thermal run away event and thus not be an issue for venting.

your normal 2 part resin you buy at your local store will not do that.

please dont forget that the resin does not give you structural strength and adds a considerable amount of weight wich still needs to be supported and needs more shock absorbing materials and so on. its a circle that just adds more problems then solutions.

most people that think they need resin simply dont. its like the "fuse wire" BS that pops up constantly. people think they need "fuse wire" to protect cells when its just something they saw tesla do and want to copy it. completly ignoring it that its not even fuse wire in tesla packs. its just the cheapest and fastest way to have a robot make thousands of battery connections an hour. i talked to tesla people and they simply stopped correcting people when they kept calling it fuse wire.
 
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Unrecycleable lump in my area.
Pack still works great 5 years old vtc3 cells but the weight is up there too got to be 15kg or so its 130 cells total and the side has 3 mm alloy and same potting for protection it not failed me but ive never hit it or do i intend to, i keep it down my shed if it goes i won't lose my house and i suggest that rule for everyone.

My new pet hate is a trend for removable batterys and charge in the house but these are moped packs with hundreds of cells and one them decide times up while its on charge in the living room is going to ruin your day.

To top that off the alloy outer gets hot very fast been extremely good heat conductor so by the time you notice it no chance in picking it up get it outside without some gauntlets so i would never bother making an alloy removable pack.

Myself i like prismstics but i know they are not perfect theres alot less connections to deal with in exchange for some compression if carefully considered in a design it can be frame strength that provides this compress load and play a dual role in a design introducing little added weight but in reality its not always achievable, ive managed it with a scooter battery box but and ebike would need additional structure and thats weight penalty so its not ideal depends in your build as to what cells structure can suit.

Plastidip seems to perfom well as a cell barrier and is light give a decent wster resistance and any thermak situation would have liytle resistance blowing off through it.
 
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