Battery pack inside bike frame

[NicDekker]

Hi everyone,

Im new to the group so pardon my stupid questions while I embark on this journey.

Several years ago I designed and built a titanium tandem bicycle with SS couplers to take the frame apart and everytime I disassembled the bike I would always think all that empty space in the frame tubes would be good for smuggling illicit substances and potentially a great place to put batteries for a sleeper ev build.

The tubes are quite oversized and I have 40mm internal diameter to play with. I was hoping you could shed light on whether building or buying a battery pack that would fit such dimensions is even feasible.

Many thanks for your time!

 

[gromike]

You probably can. Be a fun project. You would want to figure out the motor's electric demands, i.e. voltage and current. The planned use of your ebike would strongly affect your choice of motor, that, and how to fit the motor to the bike.
Have you built batteries before?

 

[NicDekker]

No I haven't built batteries before, I live a bit of a nomadic life so not sure building a pack myself is possible for me in the short term.

I'm very heavily considering the Bimotal Elevate system to switch between the various bikes I have.

 

[amberwolf]

there have been a few in=frame packs, some successful, but there are some potential pitfalls.

if there is ever any water intrusion, it can be 'trapped" in there with the cells, and cause problems.

if the pack isn't secured from vibration, that can wear away at it's insulation or casing and eventually reach something inside the pack you don't want touching the frame.

If you have cells that swell up for any reason, they can trap the pack inside the frame where it can't be removed to fix it.


it's likely that the only cells that will fit are cylindrical types; afaicr the useful prismatics or pouches are wider than the space you have. the numbered types use their dimensions as the numbers, so 21700 is 21mm wide and 70mm long, 18650 is 18mm wide and 65mm long, etc. with only 40mm you probvably can't put any cell sside by side, just a single line. technically 18650s can fit two (possibly four in a cloverleaf/diamond shape) side by side in 40mm, but that only gives you 1-2mm for insulation and casing, etc., and no room to allow for variation in pack diameter, etc.

so to see if you have room for a pack in the frame:

how high a voltage do you need? that will tell you (with the cell chemistry) the number of cells in series you need.

how many ah do you need? that will tell you (with the cell capacity) how many cells in parallel you need.

how much current do you need? that will tell you (with the cell capability) how many cells in parallel you need potentially in addition to the number you need for capacity.

do you need a bms? will it fit into the space available?

 

[NicDekker]

there have been a few in=frame packs, some successful, but there are some potential pitfalls.

if there is ever any water intrusion, it can be 'trapped" in there with the cells, and cause problems.

if the pack isn't secured from vibration, that can wear away at it's insulation or casing and eventually reach something inside the pack you don't want touching the frame.

If you have cells that swell up for any reason, they can trap the pack inside the frame where it can't be removed to fix it.


it's likely that the only cells that will fit are cylindrical types; afaicr the useful prismatics or pouches are wider than the space you have. the numbered types use their dimensions as the numbers, so 21700 is 21mm wide and 70mm long, 18650 is 18mm wide and 65mm long, etc. with only 40mm you probvably can't put any cell sside by side, just a single line. technically 18650s can fit two (possibly four in a cloverleaf/diamond shape) side by side in 40mm, but that only gives you 1-2mm for insulation and casing, etc., and no room to allow for variation in pack diameter, etc.

so to see if you have room for a pack in the frame:

how high a voltage do you need? that will tell you (with the cell chemistry) the number of cells in series you need.

how many ah do you need? that will tell you (with the cell capacity) how many cells in parallel you need.

how much current do you need? that will tell you (with the cell capability) how many cells in parallel you need potentially in addition to the number you need for capacity.

do you need a bms? will it fit into the space available?

Thank you for taking the time to go into so much detail, I really appreciate it!

The system I am looking at has a smallish battery pack as standard (300wh 52v according to the website ) a user of the same system posted the following details found printed on the battery pack itself (14S2P18650 6Ah 50.4Wh 302.4Wh XE31)

I'd not need more capacity than the provided battery pack as my current commute would only involve partial motor assistance for max 10km for the return trip home up a steep windy mountain road.

I'm not sure of the current requirements but the motor is 750w if that helps.

I'm not sure about whether a bms is required.

Thanks again for sharing your valuable time with me.

 

[amberwolf]

The system I am looking at has a smallish battery pack as standard (300wh 52v according to the website ) a user of the same system posted the following details found printed on the battery pack itself (14S2P18650 6Ah 50.4Wh 302.4Wh XE31)

given that, it's 14 pairs of 18650 cells in series. the cells are each 3ah. don't know which cell, but you just need to find one that can provide a minimum of 3ah capacity, and is capable of the required current.

so that's 28 cells. if each cell is 65mm long, and you have ot put them in single file, then you will need 28 x 65 = 1.82 meters of tubing length space for just the cells, not including the casing, cell holders / caps / wiring / etc.

if you can put htem in pairs, then you need half that length. if triples, 1/3. if quads, 1/4.

etc.

I'm not sure of the current requirements but the motor is 750w if that helps.

what is the controller current limit?

assuming it is matched to that motor, then 750w / 52v = 15a (rounded up). so the cells need to be able to suppply a continuous 7.5a.

at 7.5a continuous rate, if they are 3ah cells, they will be drained in 3 / 7.5 = 0.4h or 24minutes. this matters because:

I'd not need more capacity than the provided battery pack as my current commute would only involve partial motor assistance for max 10km for the return trip home up a steep windy mountain road.

jsut for numbers' sake, if you use the full power of the system on that section, then unless you are riding faster than 25km/hour the whole way, you'll run out of power before the end of it.

and that is only when the pack is new. as it ages it will become less capable of providing current, and it will lose capacity, so there will be less and less assistance available for a shorter and shorter time.

i'd recommend sizing hte pack so that you have at least 25-50% more capacity and capabilty than you actually need, so that over time it will still do what you need it to.

I'm not sure about whether a bms is required.

a bms (or something that does that job) is required if you don't want to take the packs out of the frame and test them periodically (possibly frequently, if the cells are not perfectly matched in characteristics) for "balance", and balance them manually. you'd also want to verify somehow that no cell drops below it's safe empty limit, or rises above it's safe full limit. (per the spec sheet of the cell).

a bms, if you get one with balancing built in (nto all have this) will handle the balancing for you during charge, usually at the end of charge, if you leave it on the charger long enough for this to happen. it will also do the checking for balance, and disconnect hte pack from the system / charger if cells go too high or too low, etc,. to protect them from damage that can lead to a fire.

 

[Tony01]

It’s easier to build the bike around the battery rather than the reverse. Especially since most bicycles are not designed to have a battery in the frame in the future. Then you can engineer every part of it to your specifications.

 

[squared]

I looked at the elevate "system". $2k for an add on motor and battery? Great idea, but the target market seems to be a bit small from my perspective. Yeah, I could throw down a couple grand without too much of a sweat, but am I putting that on my $300 WalMart special, or my $2,000 Specialized special? $500-700 bucks, I might be willing to give it a shot but...no not at the current price. I mean, I might as well buy a motor from China, put a sprocket and chain an use one of the sprockets on the stack for significantly less. Am I missing something here?

 

[E-HP]

Thank you for taking the time to go into so much detail, I really appreciate it!

The system I am looking at has a smallish battery pack as standard (300wh 52v according to the website ) a user of the same system posted the following details found printed on the battery pack itself (14S2P18650 6Ah 50.4Wh 302.4Wh XE31)

I'd not need more capacity than the provided battery pack as my current commute would only involve partial motor assistance for max 10km for the return trip home up a steep windy mountain road.

I'm not sure of the current requirements but the motor is 750w if that helps.

I'm not sure about whether a bms is required.

Thanks again for sharing your valuable time with me.

There are two or three companies who have done the in frame thing. This video repairing a battery from a Vanmoof ebike may help to picture how their battery was designed.

 

[NicDekker]

Very small target market indeed.

For me it's about the versatility. I've ridden a variety of ebikes over the years and generally dislike them except for the highly specialised MTB versions but they're very expensive.

I also live a nomadic lifestyle in which I've moved to a new country every 2 years for the past decade. Flying with an ebike is an issue.

So I'm looking for a system that can run on all my and my wife's bikes.

 

[A-DamW]

The tubes are quite oversized and I have 40mm internal diameter to play with. I was hoping you could shed light on whether building or buying a battery pack that would fit such dimensions is even feasible.

This may be a good candidate for larger format cylindrical cells, some common size like 32650, or Headway cells in 38120:
Headway Power Collection

 

[99t4]

Let's get your requirements listed:

• Battery pack to fit inside frame tubes?
• Entire e-conversion system swappable to various bikes in your stable?
• Ability to travel via air?

That's a complex recipe!

 

[Chalo]

everytime I disassembled the bike I would always think all that empty space in the frame tubes would be [...] potentially a great place to put batteries for a sleeper ev build.

Please don't. That's one of the worst features of commercial e-bikes, it was the most annoying feature of the first e-bike I ever built in the late '90s, and it will make you regret it. Don't marry yourself to a single cell format and total volume; sooner or later it will come back and bite you, and before that it makes routine maintenance and troubleshooting much more of an ordeal than it needs to be.

Better to put your drinking water inside the frame. And please don't do that either. Just stuff the frame full of weed and pat yourself on the back for being so clever and resourceful.