Dillemma with battery setup for 3kw MTB full suspension frame build

[Jimmydoerty]

Really sorry if this is incorrect subforum or has been answered but still kind of stuck...

Been taking on the idea of building a 3kw hub motor bike with a downhill mtb frame (orange five) however, the full suspension frames pose a bit of a dillemma with the battery pack since the frame geometry will change when it compresses the shock. So, I'm dwindling on what type / size / style of battery to use and not getting anywhere, just hoping to get some insight and food for thought from you kind folks.

• A cell pack (18650 / similar) would likely have to be 72v and so I feel it'd pose issues with size re: mounting it somewhere.

• A lipo pack setup feels like a better option for power output but I'm looking for at least a few hours of just cruising, or an hour of ragging on it hard; but feel like I wouldn't get enough ride time out of it

• A backpack battery setup with quick releases could be a way to go, but poses some safety concerns.

Photo of the frame below and what space I'll have to work with. Really appreciate any ideas and thoughts on this!

Thanks!

 

[2old]

At times (maybe not now), em3ev will build a custom pack. I talked to Joseph about it once (but they weren't doing it then. Great frame BTW.

 

[Jimmydoerty]

At times (maybe not now), em3ev will build a custom pack. I talked to Joseph about it once (but they weren't doing it then. Great frame BTW.

I see, things may have changed since Feb when I asked him so I'll try again. The size of the Jumbo or super shark is fine, but the issue lies with BMS as he told me they only use one type of BMS of 40A continuous and max 50/55, yet they use molicell p28a which he himself said is far more capable than what it's used for. I think they are just playing it safe which is fair. But 3000w continuous doesn't seem too much to ask from them

Thanks btw! I imagine this project will be far more work than I have envisioned but should be a great result.

 

[nicobie]

40A going up a hill is enough to melt most motors. 55A @ 80V and it will be hard to keep the front wheel down.

Seems like your option will be either on the top bar or on a rear rack (hard to do right)

 

[amberwolf]

Been taking on the idea of building a 3kw hub motor bike
<snip>

• A cell pack (18650 / similar) would likely have to be 72v and so I feel it'd pose issues with size re: mounting it somewhere.
• A lipo pack setup feels like a better option for power output but I'm looking for at least a few hours of just cruising, or an hour of ragging on it hard; but feel like I wouldn't get enough ride time out of it

How much power does the job you want it to do? Meaning, for the terrain conditions (hills, etc) and riding style, how many watts will it take, worst case? If you don't know, you can use online calculators / simulators to guesstimate what it will take, like those at ebikes.ca--you don't want to spend the time and money building a custom bike that won't do what you need it to.

If 3kw is all it will take to do what you're after, then to use max power for one hour will take some percentage more than 3kWh (because there is always wasted power in a system, the harder it's used the worse that usually is, let's just call it 25% (could be more, or less)). So call that 3750Wh, which you could round up to 4kWh to account for pack aging, etc over time to have enough capacity later on.

That's a pretty big battery pack. At 72v, thats about 52Ah for 3750Wh. At a guess, I'd say that's probably at least 40-50lbs including whatever protective casing and mounting it would have. Probably cost about a thousand dollars depending on who builds it (I'm sure there are places that sell them cheaper, but you should check them out thoroughly because they may not be as advertised).


How big a pack you will need for a few hours of cruising will depend on the amount of power it takes to do that at whatever speed you're after and whatever riding conditions / terrain /winds / etc you have. It could be greater than the ragging on it version, meaning an even larger pack.


If you need more than 3000w to do the work you want, then the pack would need to be larger to supply full power for an hour....

 

[scianiac]

Having had this same dilemma with my build I ended up going with lower discharge but higher energy density prismatic lipos. These can be a little annoying to find but they are out there. With these packs though you can more easily wrap the batteries around the frame so you can have some under the downtube, some on the sides of the downtube, some under the top tube, some on the sides of the top tube and some on top of the top tube if you wanted. Because these are fairly large bricks mounting and wiring all those batteries isn't that difficult although more difficult than one giant shark pack. For instance my pack is just 5x 4s 8ah bricks, you could probably use even larger bricks and easily use 10x wrapped around the frame. Like everything else you have to factor in the time, effort, skill, tools, and money you have available to find the best solution.

As for sizing the battery, I've found just using a battery I already have even if it's not quite correct and running it in a backpack for a few rides and then from there I can easily extrapolate out how big the battery needs to be.

 

[Jimmydoerty]

@amberwolf @nicobie @scianiac Thank you all for the thoughtful replies on this!

Being realistic with the power levels I think 3000w is just for fun and not sustained. I'm moving from a 1500ish watt ~70lbs full suspension bike and just wanting a little bit more agility + power on my new mtb build, as I have ridden some dirt bikes and higher power bikes, 8kw QS273 motor bikes and Surrons since I bought it, and been infected by the need for mooore

I think based on your replies I will stick with the plan of just start with lower capacity lipos, as realistically I will only be hitting on it in bursts, and knowing this I can take it easier with the lipo setup. 30mph cruising with the proposed setup and weight should hopefully be maybe 1250-1500w with your 25% loss so I think I will start with lipo and begin working on design for a cell pack enclosure that fits around the top tube.

Seems silly doing both and spending more money but, you've help me come to a conclusion of I feel like it's better to have both. a best of both worlds pack would be nice but I imagine there may be times were I will want to just commute or cruise, and times I will want to go trail riding for a burst; and the mxus seems pretty capable for short bursts of higher power so lipo seems the way to go for now.

Really appreciate the food for thought everyone

 

[999zip999]

Food for thought well I've cooked once or twice before. You may be able to fit the battery under tube from the bottom bracket up to the head set. If possible ???
Well I don't know how far you've gotten I guess you've already bought that frame ???
I used an older kona kalaapoo with a big triangle a little different shock setup. Or a trance frame or. Something with a big enough triangle you can manage to fit a battery in. I never buy anything too I figure out how wide my axle can be to fit in the frame. I do like mountain bike frames as they're easier to get away with things. Ask Chalo on frames for fit.
How fast and how far do you want to go what's the steepest hill how big of a controller 18 fet or 24 fet.
Find out what controller you think they're using and what battery and make a box of that size carry it around with you every time you look at a new frame.
Just how much I like Italian shoes 63 255 lbs I don't fit in a lot of Ferraris and I sure can't wear those Italian or French shoes all of those guys are under 5 ft 8 with a size 7 shoe so they can push down on the throttle.
That's why I had to buy a Bugatti.
I I asked a lot of questions I should have numbered them but I want you to answer as many questions as you can.
This just won't help me but help the others too I think Amber wolf has already asked quite a few of the questions I asked but do answer the questions that's the reason we ask. Good luck I like rear hub motors. And tell us what rear hub motor are you going to buy because they're all different. Good luck