Lipos: Better to gently cradle or tightly constrain?

[lumenpdx]

I'm going to be stuffing 8 4s 16Ah Mulitstar lipo packs in the aluminum box of a Luna fatbike. Neither the frame or batteries are in my hands yet, but should be any day now. If the published dimensions are correct, I should be able to fit them all in the box with roughly 10-20mm to spare in each dimension. The batteries are rated for 10c constant. I doubt I'll ever be pulling more than 3C from the pack, so in theory they won't be getting worked too hard.

My plan is to 3D print some spacers to secure the batteries in the aluminum box, but I still have a decision to make...

* I can support the packs away from the edges and each other, even use some low-durometer material to cradle each individual pack and leave an air gap around it . This should allow the best shock damping and impact protection.

* I can support the packs such that they're pushed against the side of the box, potentially helping to conduct any excess heat out of the battery compartment.

I've read of many lipo packs 'puffing'. Some only exhibit this when abused, others seem to do it regardless. Many folks seem to have the best luck by tightly constraining the packs to reduce deformation. I can't imagine these big packs, at these fairly low c rates, would deform much. Still, is it worth trying to implement some means of tightly confining them? If not, do you think it would be preferable to gently cradle each pack, or try to maximize surface contact to dissipate heat? Perhaps something else entirely?

 

[markz]

I bought the cheap hardcase lipos from Hobby King, I would buy the green ones on sale. They are getting cheaper all the time. Too bad they explode and burn down houses.

 

[Ykick]

IME, dimensions can vary somewhat from what they publish.

Myself, I wrap bricks/packs in a layer of clear plastic (SaranWrap, shipping film, etc.) and then a layer of reinforced filament shipping/packaging tape.

Gently supported in whatever enclosure I happen to be using but the filament tape is key to help bricks from puffing. Plastic wrap used merely to make it easier to remove the filament tape if ever needed.

 

[lumenpdx]

Gently supported in whatever enclosure I happen to be using but the filament tape is key to help bricks from puffing. Plastic wrap used merely to make it easier to remove the filament tape if ever needed.

I hadn't considered that, but I like the idea. Seems easy enough to implement. Have you noticed if it's effective? Does the tape get tight?

 

[Ykick]

Over time almost all seem to puff. Or, attempt to puff.

Ypedal has my credit and gratitude for posting filament packaging tape technique many moons ago. Cheap, easy, effective for hobby cells.

 

[dogman dan]

First of all, theres puff and theres PUFF. Every lipo pack I ever owned eventually puffed slightly. A true puff is a bad cell, and it will look like a balloon. Nothing is going to stop that.

Others report that compressing the cells helps lower resistance. So wrap it in something that does not stretch much makes sense.

I find that the real problem is not puffing, but dings, or other damage that can happen if there is any way the pack can wiggle as you ride. So combining the tape with something relatively rigid that keeps the corners from rounding is smart. Then if the pack tries to puff, the tape doesn't crush the corners. The fiberglass strap tape and Masonite sheets is reported to work good for both mild compression and keeping the corners square.

I've taken a bit more of a less strict approach myself. I just build a tight fitting box from coroplast sheet ( political signs) and clear packing tape. The idea is not to constrain, but to provide protection from chafing. A tight box when cells are new, will become a very very tight box as soon as the cells puff 1%. Then I can carry the battery in rack bags, frame bags, etc, and not worry about chafing or crushing a corner as I ride.

 

[MrDude_1]

Everything dogman dan just said.
The hardcase lipo fit in the box snug,and glued to ONE side..then they puff ever so slightly(like all lipo will) and they never move again.

 

[Punx0r]

Pouch cells should be compressed by their flat faces for best performance and longevity. The edges, however, must not be bent/squashed/folded/compressed.