LiPo in a Backpack-- Can I make it Safe??

[theyerb]

Hey guys, it's good to be back on here! My next project, which I posted about a while back, involves putting the batteries and controller in a backpack with a tether going out to to the throttle, motor, and lights. This would be for a full-suspension, offroad ebike application. I've been thinking about what types of batteries I want to use (one of the most pleasurable activities for an endless-sphere member haha), and I think LiPo is the way to go-- anything else is just too heavy to carry on a back. I don't have any experience with LiPos, but I do have an understanding of the dangers involved. So I was wondering:

Can I strap LiPos to my back and make it safe?!?! I would, of course, have a BMS and LiPo charger, and I would make a special container out of Nomex/fire-resistant material to contain any... problems.

 

[vanilla ice]

A fire resistant composite plate maybe.. along with the bags. I've got a welding blanket in my work area for sparks. I'm not sure how useful that would be for this type of project, but they have got to be more mass-produced than anything for the R/C market.. Not sure how effective it would be but one could probably sew the whole backpack from this material without too bad a cost.

 

[hydro-one]

the good thing about a backback lipo is you can take it off in a hurry!! I melted a lipo in my backpack, didnt even get hot (to me anyways, it was minus 25c outside!!), the thing sausaged and liquified itself, but didnt blow up. I think the key to saftey here is not to enclose them at all, let them exotherm and swell, this releases some of the energy, and makes the plates inside come apart and slowing or stopping the current flow, and exothermic decomposition.

but now we have something even better than lipo,
How about 20c of cellmon cells, in a flat pack , 15ah foil . just ducktape pop in the pack and go!!!! mabye some edge protection in there!!! this would weigh 20p or so, and deliver all the power of lipo, without the exotherm risk.. AFAIK

 

[Little-Acorn]

I'm not a very good off-road rider. My few attempts, when it came to going up a fairly steep, rocky hill, ended up with me going one way while the bike went skittering down the hill another way. Since then, I've pretty much confined my riding to flat areas, mostly paved roads. This is probably better for both me and my bikes, as well as innocent bystanders.

I hope you're a better off-roader than I. I'd hate to see something like that happen on an ebike where the motor is mounted to the bike and the battery is mounted on the rider. Are you sure the sudden shock to the "tether" would result simply in the battery unplugging itself from the bike? Or might it yank the wires out of the BMS by the roots?

To answer the question in the title of this thread, No, I don't think you can make a LiPo battery safe, no matter where you mount it. Even if you charge it perfectly and discharge it exactly according to Hoyle, what happens when someone knocks your bike over and the LiPo hits the edge of the curb when you're not there? Or, if it's in your backpack, what happens when some klutz in the store bonks you from behind with a shopping cart, or with the 2x4's he's carrying in Home Depot? You'd look pretty funny, dancing around with smoke and flames emitting from your posterior area as you try to figure out what's going on and shed the backpack.

HobbyCity has some LiFePO4 batteries as well as their huge selection of LiPos. Same weight, same size, same power. Why tempt fate?

 

[vanilla ice]

For one the HC life packs cost more.. for another they suck.

 

[LI-ghtcycle]

I think the operative word here is C rate.

NOTHING comes close to the C rate of LiPo (unless your talking the various chemistries in the same family, i.e. Cobalt but they are all really the same volatility) and also, NOTHING quite compares to the "Ive got fire ants in my pants!" dance you will be doing to get a back pack full of orange road flare flame off in time.

Don't forget, nothing is fire PROOF, only fire RESISTANT. Also, just because you don't have orange flames yet, and just a smoke signal, doesn't mean it won't still be hot enough to fry your back-side. :wink:

I'd be extremely cautious about attaching anything with that potential to my bike at ALL much less to my body in a pack. I understand that your trying to keep the weight down, but I couldn't see putting something that potentially dangerous in a back-pack that I couldn't see, and even if you have mounted "externally" to your pack (with some kind of pack frame design) your still risking a very nasty situation, much like if you had a gas tank attached to your back-pack.

I think it's been said that LiPo is no more dangerous than gasoline, but I have never heard of anyone having a gas tank strapped to their body for obvious reasons.

If it was me, I would just figure out a way to solidly attach it to my bike's tri-angle, there are full suspension bikes that still would allow for a very substantial battery pack (even in LiFePO4 battery chemistries) such as with kfong's Motobecane:

http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=12654&start=15

 

[Lock]

Don't forget, nothing is fire PROOF, only fire RESISTANT.

Pyrogel XTF Flexible Insulation for Fire Protection
Standard product for moderate to high temperatures: -40°F (-40°C) to 1200°F (650°C)
Thicknesses: 0.20 in (5 mm), 0.40 in (10 mm) (Actually they only make the 10mm thickness at present...)
Data sheet here:
http://www.aerogel.com/products/pdf/Pyrogel_XTF_DS.pdf

tks
lLoC

 

[swbluto]

I hope you're a better off-roader than I. I'd hate to see something like that happen on an ebike where the motor is mounted to the bike and the battery is mounted on the rider. Are you sure the sudden shock to the "tether" would result simply in the battery unplugging itself from the bike? Or might it yank the wires out of the BMS by the roots?

This has been a problem for me but I found the solution is having some slack and also securing the wire to the backpack somewhere along the wire's insulation, so that a sudden jerk won't jerk the connection, only the pretty strong insulation (and thus nothing breaks). I know this from experience.

Anyways, as to LiPo safety - I think it's possible, but probably not practical. If it were in a backpack, it would need crush-impact-puncture resistance in the case of an accident. You can have that covered with a thick plastic box or some such. Make sure that it isn't "enclosed" so that any escaping gases can escape and not build up and explode. Another thing you'll want is short protection - while I've heard LiPo from hobby-city has inherent short protection, a fuse probably wouldn't be a bad idea. Another thing you'll want is the ability to shed the backpack quickly. Once you've got those things covered, the chance of bodily harm resulting from an accident from the lipo should be minimal, though not completely gone.

For this application, I might suggest cell-man's pouches. It has awesome performance, cycle life, and energy density (The gravimetric energy density is pretty much equivalent to HC lipo.) while also being a "safety chemistry". I'd still suggest impact protection because I'd hate to lose a not-so-cheap battery in a crash, but it's up to you. I don't really ride with impact protection with my battery, and I'm starting to think that's going to bite me one of these days.

 

[Lock]

Are you sure the sudden shock to the "tether" would result simply in the battery unplugging itself from the bike? Or might it yank the wires out of the BMS by the roots?

I would bind up the electrical wires w/stranded stainless steel cable. The stuff is mfgr'd down to as small as 2mm dia. V.strong. Then attach both ends of the wire cable to bike and pack so electrical wires suffer zero yank. 1x19 2mm breaking strength about 700lbs.
Lock

 

[LI-ghtcycle]

Don't forget, nothing is fire PROOF, only fire RESISTANT.

Pyrogel XTF Flexible Insulation for Fire Protection
Standard product for moderate to high temperatures: -40°F (-40°C) to 1200°F (650°C)
Thicknesses: 0.20 in (5 mm), 0.40 in (10 mm) (Actually they only make the 10mm thickness at present...)
Data sheet here:
http://www.aerogel.com/products/pdf/Pyrogel_XTF_DS.pdf

tks
lLoC

Point taken, however a quote from the same PDF:

Ideal for insulating piping, vessels, tanks, and equipment, Pyrogel® XTF is an
essential material for those seeking the ultimate in thermal efficiency....

Rapid Rise Fire Tests of
Protection Materials for Structural Steel:

12 mm 68 min
30 mm 132 min
48 mm 184 min
66 mm >240 min

I'm sure that these time frames would allow for a building to be safely evacuated, (assuming that 12 mm 68 min is referring to time before structural failure) YOU still might be a lot MORE uncomfortable than a steel structural support long before the time it takes for failure of said support. :wink:

Another thing I just thought of, say you out single-tracking and manage to knock yourself out cold, how then does one evaluate the safety of having a virtual bomb strapped to you back?

 

[Lock]

They sell this stuff also to be sewn into fire-fighting suits... Not sure how long you would stay unconscious with yer back on fire really...
Anywhooo, it's just the best spare-no-expense high temp insulation I have found so far (other than vacuum panels.)
Lock

 

[theyerb]

This idea is based on the concept of backpack-mounted batteries for a number of beneficial reasons that frame-mounted batteries cannot provide (given, they both have their points). I will start looking at safer chemistries... and I've never even heard of the cell-man packs, so I gotta check those out!

As for the tether... this is an engineering challenge, but something that can definitely be solved. I would want a quick-release solution-- when tugged on with a certain amount of force, it would disconnect the power supply from the bike for obvious reasons. The Apple Macbooks have a really ingeniously designed magnetic power connector-- this is the type of innovative thinking I like!

LiPos have about 1/3 more energy density in a much smaller volume than LiFePO4 batteries. LiPos are practically unbeatable but also something you don't want strapped to your back XD

Thanks for all of the input, fellas There are some incredibly bright people on here!

 

[TPA]

I think the tip from the pros with lipo around here is that when you make something that you think is lipo safe, that is when you are in the most danger.

Never...ever think that you have fool proofed your lipo. Or so it's said.

 

[hydro-one]

You'd look pretty funny, dancing around with smoke and flames emitting from your posterior area as you try to figure out what's going on and shed the backpack.

lol yes i agee you would look funny, but you would probably be ok. I can get a bakcback off pretty fast. and what works for me for the tether is duh, the bullets that come with hc batterys, just have some loose cord in the pack too for "short falls", they wont rip out, when its a "big fall" they will . oh and dont land on the pack, try and land on your stomach!!!! Oh and for those knockout dayz, I wear a fullface so i wont get knocked out on that lonely trail.

I may have been lucky so far, but luck is just opportunity meeting preparation right?
mike 15s zippy/ rm switch

 

[vanilla ice]

As for the tether... this is an engineering challenge, but something that can definitely be solved.

I feel the same way about the ignition problem. Its a challenge, but making an ignition resistant backpack can definitely be done. It might not be worth it time wise or fund wise, but that depends on the person and on how badly they want it. I wouldn't do it, but the materials and know how to do it are out there.

 

[TPA]

Design a backpack that has the ability to be jettisoned with the pull of a single pin.

 

[Lock]

There ya go. Just install an automotive airbag between your pack and the backpack, activate to blow the pack off your back...
:lol:
Lock

 

[hydro-one]

hahah just dont go into an airport with THAT one!!!!! youll never come back...hahaha but that is a good idea, some pins in the straps, kind of like a para-solider when they land!!