The vast majority of ebikes in the US have the battery mounted on the frame of the bike. With the exception of my commuter ebikes everything I ride in the woods ends up with the battery in a backpack. There are advantages and disadvantages to either setup, being educated about your choices will help you make a decision about what will be best for you.
Backpack batteries can be stealthy if you do it right. Nothing screams ebike like a triangle full of batteries.
Frame Mounted Battery
Advantages
- Not connected to the bike when you take a fall (if you’re not falling you’re just not riding hard enough) or if you have to ditch the bike because someone is trying to run you over
- The battery cools better on the bike than inside a backpack
Disadvantages
- Your ebike looks more a lot more like an ebike, less stealthy
- Battery takes more vibration from riding and connections could fail
- Bike is much heavier, trailriding feeling more like impossibly light motocross and less like mountain biking
Backpack Battery
Advantages
- Easier to have different sized batteries for the same bike that you can switch between based on how long you want to ride
- Because you never leave the battery plugged in when you’re done you don’t ever have to worry about killing your battery if it has no BMS from leech loads
- Battery doesn’t get cold because it is never left outside on the bike, it is brought inside to charge
- Preserves the integrity of the single-track trailriding experience
- Battery is protected from vibration due to the suspension in your legs
Disadvantages
- Battery is vulnerable to damage if you fall on it
- Connected to the bike when you fall, it can easily damage connectors or wires
- For long rides with a heavy battery on your back it can be fatiguing
A frame pack like this 25R pack from em3ev is pretty lightweight (8.15lbs) and shouldn’t get damaged very easily mounted on the frame. It can also provide 25Amps continuous.
For lighter weight batteries that are less than 10 lbs most people will probably not mind having the convenience of having the battery mounted directly on the bike. For heavier batteries I personally find that I have a lot more fun in the woods when the battery is on my back. The ebike is far more nimble and easier to throw around, the weight of the battery does not make it feel like I’m riding a dirtbike and instead just makes me feel like I am overweight and need to lay off the cheese. There is a huge difference between riding a 40lb ebike with a 15 lb battery on your back vs riding a 55lb ebike with the battery mounted on the frame. The weight of the whole package is the same, but the experience is completely different.
For Hobby King lipos and other smallish odd-shaped batteries a tupperware container ziptied to the frame of the bike in the triangle with a little bit of padding inside works quite well. You can spraypaint the tupperware to match the bike color if you want. For backpack cells I wrap them in a thermarest pad and cut large holes for cooling in the sides.
Looks like crap but it works. Holes cut for cooling and 2 layers of thermarest. 16lb battery with 100’s of hours of abuse\falls and no issues. 20Ah/48v will allow me to trail ride for about 2.5 hrs.
It’s quite easy to try most batteries in a backpack by making a long custom cable for it. Even framepacks or water-bottle batteries can be thrown in a backpack even if they are expressly designed to be mounted on a bike. Just cut the power connecting cord (with the battery disconnected of course) and then build an extra long 4-5′ cable with Anderson power pole connectors and throw that battery in your pack. You can also mount most batteries on a bike even if they aren’t designed to be by using a padded triangle bag or mounting a rack on the bike and bungie-ing the battery down. Bungied batteries on a rear rack will not survive any real trail riding, but batteries in a padded triangle bag should be fine if you take it easy on the jumps.
A very excellent article about the different mounting locations for battery packs was written by our friend Ron (spinningmagnets) over on electric-bike.com and can be found here. The best place to mount a battery on a frame is always inside the triangle.
If you don’t experiment and take risks you’ll never find out what is right for you. The Internet is full of people who claim to know what is best for you, but everyone is different. Something that works well for one person’s riding style might not be for everyone.
Ride On.
.
ElectricBike-Blog.com 
A couple more big advantages to having the battery in a backpack:
You take it inside with you to charge so it never is left sitting out in a cold garage in the winter (non-lipo batteries). Being inside when charging also means that I see when it finishes and unplug the charger without forgetting it overnight.
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You are right, Lithium batteries hate the cold. If it is below freezing I keep the backpack zipped up and the battery maintains a very comfortable temp. In the summer I leave the zipper open.
I added your suggestion.
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Hmmm…an interesting idea. I already maintain a bag in which I keep all of the accessories I have (lights, lock, pump, etc), primarily so I can spend a bit more on each one since I’ll be using them with all of my bikes rather than buying one set for each bike. Why not the battery, too? I may just have to try that out.
I also had a feeling the pack that came with my Fat Cat was generic, and now that you’ve provided a link (it’s the 25R pack pictured) I know for sure. So, thanks for that, too. 🙂
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The frame pack is generic, but different manufacturers put different batteries into the generic housing.
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Not connected to the bike when you take a fall (if you’re not falling … belectrici.wordpress.com
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Battery in a backpack?? Isn’it dangerous? I mean if you fall badly your back or neck could be broken due to anything stiff in the backpack, especially a battery… that’s a no go for me!
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Sorry for the laymen questions but:
A. Can Anderson power poles handle 52v 30A without issue?
B. Is there wire suitable for the lengthened power connecting cord which has the “tight springy” properties similar to old fashion telephone wires? Seems like that “springy” property be great for backpack batteries, but does the thick gauge of the wire prevent it?
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A. If they are the 45amp versions then yes.
B. Use standard multistranded 12 GA wire.
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Karl, What’s the benefit of using multi-strand 12 gauge wire vs. a solid single strand 12 gauge wire? Seems like the solid single strand would have more copper to carry the current better, but that’s just a guess.
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Multi stranded wire is much more flexible.
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I use a triangular frame bag (bought off em3ev). This way you take full advantage of the space (useful for storing other things too) and the battery is suspended within the bag via Velcro ties, so it doesn’t get any bad jolts. Mine’s a bottle battery, but I’ll probably get a triangular one with a few more amps when it comes time to upgrade).
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This is not a good idea. I tried it and the wires were just tangling everywhere no matter how hard I tried to keep them together. The cables kept becoming strained and disconnected to the motor.
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Is there high gauge wire that retracts like old phone cable?
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I see some doing the connection in the front by the handle bars and some doing it from the rear. Any advice which you prefer and why would be greatly appreciated, also if you have a favorite back pack, or connectors? thanks for the great site you have and sharing
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has anyone come up with some sort of a toggle switch or an A/B switch, to be able to switch between batteries while riding. would be super cool and convienient. maybe? 14 guage 40 volt toggle switch.
I want one for Xmas.
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