Brace yourself, because, if you haven’t already tried it, and you are like most people, you probably think this is the worst idea, ever. I was one of those people. Then I built a bike that simply had to use a backpack battery as its power source. I held my nose, gritted my teeth and just did it. I dreaded the result right up until I rode it for the first time.
Look at the two pics below. Where’s the battery? Nowhere. Nowhere in the picture, at least. I was wearing it. In the image at right, I have used subtle visual cues to highlight the silicone-insulated XT90 connector I plug into.
By the way, that is a Cyc X1 Pro Gen 1 motor. The little bag houses a BAC800 controller that reached 60 amps of continuous output before I chickened out and lifted.
What problem are we solving?
A backpack battery should obviously not be your first choice, so why do one at all? When doing a DIY ebike build, there are some donors that just don’t have space for a battery.
Where the hell am I going to fit a battery on this bike? I will deliberately NOT answer that question here.
In an earlier draft of this post, I wrote up all the different things I thought of or actually tried, and abandoned because they sucked for one reason or another.
But that is going off into the weeds as this discussion is about backpack batteries, not build or donor choices. So lets table all that talk and just stipulate: We have this bike that we have to work with. we looked at alternatives (remember… I hated this idea at the time), we are left with one choice:
The battery has to be in a backpack
Once I accepted the fact I was stuck doing a backpack, all that was left were materials and ergonomic/mechanical choices. i.e. just make it and do it right.
If you listen to the experts on the internet (thats a joke in case you missed it), whenever the subject comes up you hear all about how a battery on the back of a rider is a bomb just waiting to go off. There is some truth to this. Flying off the bike and landing on your back on sharp rocks is a really bad thing made a whole lot worse if a li-ion battery is your crash bumper.
There’s also a lot of talk about how the world will end if you put your battery weight up on your back, but we’ll get to that one later.
The solution for safety is to use a hardshell pack of some sort, of the kind you see used on sport motorcycles. I picked a 20L Boblbee GTX from Point65.
Nope, it sure as hell isn’t cheap, but remember that unexploded bomb thing? Its for real and a hardshell pack solves that problem. It also provides you with spinal protection in case of a crash. And you also get something that addresses another negative the villagers are shouting about: A pack like this form fits your spine, hugs your body and never shifts – not even a little.
I suppose if you had to, you could use a soft pack and then stick your battery into a 30 cal or 50 cal ammo can. Drill a hole in a corner for the power cable exit and it would work, but that can is going to be a lot of weight to carry. Still, if you want a cheap, safe solution that uses a conventional pack… thats it. I’m sure you will figure something out on the shifting thing. I know I have packs that don’t shift. Much.
Really though… this is a problem you need to throw money at to properly solve. In my case I spent about half of retail by finding a vendor closing out an old model and blowing them out at a big discount.
QUICK NOTE: I take a lot of pics of my stuff, but for some reason, besides whats above I have never done any of my backpack setup. In the near future (the pack and bike are hundreds of miles away from me right now) I'll get pics of everything. Especially the wiring/keyring/ball stuff below. That is a lot simpler to just look at than it is to write up intelligibly.
Battery placement inside the pack
You do not want the battery bouncing around freely inside that hardshell pack. Each battery and backpack combo is different, but the core of the solution is to stabilize it with dense, closed-cell padding. I didn’t say wrap it tightly in foam so it overheats (put down that pitchfork). However, part of a smart DIY plan is to use cells that can take a murderous flogging without heating up in the first place. I used the old standby Samsung 25R cell for mine. For my pack, I have enough extra room to fit my pump and tire change tools.
Some judicious padding. Sprinkle some tool bags in there (so no little bags on the bike). Job done. Its not moving around.
Figure out the wiring / connection
This is the tricky part because if you get this wrong and stay aware of the cable, you will hate your ride. First off, I used a short 8ga XT90S extension directly off the main battery output. I pretty much do this on every battery connection on any bike so, when connecting and disconnecting the pack, I’m visiting the wear and tear on a cheap replacement connector and not a live cable soldered into the pack. I also use a pair of XT60 pigtails to make a similar extension cable for the charge connector. Same idea. I’ve had my bacon saved doing this and the experience of just being able to throw away and replace a cheapie extension made this a go-to for me on everything.
Next comes a long length of true 10ga power cord, made into a long XT90S extension cord. This is what will go from the battery to the motor and its several feet long. How long exactly? I measured out enough to exit the pack, run down my back, down thru my legs and still be long enough to never tug if I am standing on the pedals and bouncing around at the same time.
OK… great… what if I’m sitting down? A cable long enough to stand up with is going to be all kinds of awkward when doing what you do most: Sitting. I spent a fair amount of time trying to figure out what to do about this. A lot of others have done some sort of elastic bungie contraption. I tried that and felt it needed too much strain to extend, and carried a risk of pulling apart the connection at the motor. I needed something that reliably retracted my cable and extended it without much resistance.
And here’s the solution: The Key-Bak Super48 HD. This is literally the direct descendant of that chromed steel extendo keyring thing that every janitor in the United States has on his belt. Except they aren’t chromed steel anymore.
The model I bought has a 48″ extension length, with their lightest 8 oz pull and a kevlar cable. Its so lightweight, it doesn’t impart the same feel of indestructability that the old steel pucks had, but I have been using it since mid-2019 and so far it shows no sign of wear. You can see from the Amazon link above that there are other models of varying lengths and pull weights. You can even get one with a steel cable. Since I’ve been using mine, I can say its 48″ extension is plenty, and the light 8 oz pull makes its operation completely unnoticeable.
How do you make the Key-Bak work?
What you need is a ball attached to your power cable. The cable threads through the key ring and stops at the point where the ball – which is bigger than the ring – is reached. You place the ball at a point down your back and to the side, so there’s more than enough cable slack to let you stand on the pedals, but not so much it gets in your way.
When you stand up the keyring lets the cable extend until the ball stops it. When you sit back down, it retracts back up behind you. Simple and effective. You never have excess cable down around your legs getting in the way. If you need more, the light 8 oz pull lets it happen without your even noticing its there. In fact, you really don’t know its there at all because its placed where you can’t see it, behind you and to the side. Out of sight and out of mind.
Once I spent some time figuring out the cable length needed to do the job right, and where the ball needed to be, I built and positioned the ball as follows:
- A strip of leftover silicone handlebar grip roughly 1.5 inches long. Since I have used Wolf Tooth Fat Paw grips and ESI Extra Chunky XXL grips with my Jones bars on various bikes over the years, I have leftovers from grips that were cut off.
- Plenty of silicone X-Treme sealing tape.
- The silicone grip segment – since it was already sliced off a set of handlebars – already had a slit in it to let it slip over the cord. Wrapping silicone in silicone tape sticks instantly, and doing so – with overwrap onto the adjacent power cord, tightly affixes it so its not moving, ever.
- Silicone tape fuses permanently to itself and isn’t going to unravel.
The above is just one way to do this. In my case with spare stuff laying around in my garage.
What is it like when you ride it?
I wasn’t expecting a good experience. The idea of being tethered to the bike and having a power cord running down off my back… I hated everything about that. Boy was I ever wrong, and if I hadn’t built the solution and gotten on the bike and tried it, I’d still be just as wrong. This is something you have to experience to fully understand and appreciate.
You are still tethered to the bike. But the smart setup mitigates this so thoroughly its unnoticeable when you ride and requires very little extra effort to deal with.
Not having the battery weight on the bike makes it behave… like a bike. Internet experts will jump up and down and point to the higher center of gravity that comes from putting the pack on your back. But reality is that without the weight of the battery, smashing thru a rock garden or challenging singletrack is like doing it on an unpowered bike. Since in singletrack you usually only use (or want) power when going uphill, that means your ride everywhere else is exactly like you want it: Old school analog. Your suspension acts like it should… but with a rider who’s eaten too many cheeseburgers.
Having the battery on your back means you can shift its weight from side to side just as you already do with your body. See the above point, because that one and this one together completely undo the whole ‘center of gravity’ argument, and put the backpack setup in the ‘superior’ category when it comes to all-around performance. If you are wearing a 10 lb backpack… so what? You spent the money to buy a pack with a completely form-fitting back panel, that attaches firmly to you so its an extension of your body. No shifting of any kind whatsoever. You did that, right? Bought the really good pack? Cuz if you swiped your kid brother’s lunch pack or figured out some other way to cheap out… you’re screwed. Proper packs are not just ones that shield and pad the battery. They shouldn’t fidget.
Holy crap I totally forgot about that cable! I thought that was going to suck so hard, and I don’t even know its there! Thats you after your first ride. My first config ran the cable around my side and did not go thru my legs. I was concerned (and rightly so) the cable could flop away from my side and hang up on a bush. So I took the plunge and ran it between my legs like the experienced builders say I should. Sure enough it works perfectly.
We have addressed the safety/crash issue by using a hardshell pack, with some dense foam around it but not smothering it (and used a battery cell that doesn’t heat up under extreme load). That makes the battery safer than it ever would be in a ‘traditional’ battery bag.
You are still tethered to the bike. I never said a backpack was the best solution. Its just the only one sometimes. Its not the end of the world if you do it right.
When you stop the bike, you have to disconnect. Its not difficult, but you have to do it so it goes on the list. I keep XT90 safety caps in a little pouch and use them to cover the open connections on the bike and my battery cable. When I mount the bike, I first lower the dropper post all the way. Then I straddle the bike from behind, standing over the rear wheel. I connect the power and, since the seat is so low, I can just step forward and be right over it. I then raise the dropper and I’m on the bike. Dismounting I have some options. I can be standing and reach down, disconnect and just throw my leg over like usual, or do the reverse of the mount from the rear. In practice I’m about 50-50 as the rearward exit is easier but I need to think about it to do it.
And The Ugly
Whats ugly is I used that unoriginal cliche for those pro and con section titles. Lets take a break, sit down with a plate of spaghetti and enjoy the movie!