Carrying an Ebike battery in your backpack safely?


New member
Local time
2:48 PM
May 8, 2020
There was a post about someone wanting to use a backpack to remotely power the bike (Battery in the bag). The reason stated was to manage weight on the bike.

I responded with "that seems pretty dangerous". While of battery chemistries, these are some of the safest. That should not imply there is no risk. And the difference between a bike-mounted battery and one in a backpack is you can just jump off the bike and run. Or you have to stop, remove the bag and then run. Bike mounted you will likely smell or see smoke, backpack no such warning. I thought I was not insulting or argumentative in any way. When these batteries fail they either vent like a blow torch or explode if proper venting is not available. I personally do not want a blow torch aimed at my spine even for a split second. A YouTube search will illustrate.

I could maybe understand if you were competing or something like that. Even then I would assume there would be some barrier between the battery and your body other than a thin piece of cloth.

I explained my experience as a 10-year ecig user and a flashlight collector has proven even the best name brand batteries can fail. Over-discharge or damage as small as a pinprick can create a runaway failure. I thought my post benign and useful, but it was removed from the site.

I expected E-bike groups to be advocates of safe use and care with their batteries?? I'm not trying to scare anyone just educate a little to potentially avoid both harms to individuals in this community and bad press for E-bikes in general.

They are pretty safe with proper care and reasonable safety measures.

Is discussing battery safety taboo when it comes to E-bikes??? I would have expected this to be something commonly addressed within the e-bike communities.
A lithium ion battery not treated right is literally a bomb. You'd have to be completely out your mind to strap lithium ion to your body... There is a reason they no longer ship bulk lipo's on planes. Minimize risk like a smart person not take large risk like an idiot. I personally have seen these things blow and take down homes.
A lot of guys run backpack batteries without incident. A backpack battery would see less vibration so it will hold up better.

Quote from Battery University:
With more than a billion mobile phones and computers used in the world every day, the number of accidents is small. By comparison, the National Oceanic and Atmospheric Administration say that your chance of being struck by lightning in the course of a lifetime is about 1 in 13,000. Lithium-ion batteries have a failure rate that is less than one in a million. The failure rate of a quality Li-ion cell is better than 1 in 10 million.

You're more likely to be hit by a car.
I don't disagree. And it did not go unnoticed by my you quoted Ohms law in a recent post. I suspect with smaller devices often it is never reported just tossed. I know I have done that with several e-cig mods that have overheated. Even considering that and conceding the numbers you listed. For me, it's less about the "Frequency" and more about the "Severity". Once they go they are pretty much unstoppable. Shark attacks are rare, we still try and reasonably avoid them. I expect you understand safety and care really does not involve much. Just understanding what the failure modes are and avoiding them. I think charging and discharging which relates to Ohms Law, or physical damage is about the end of the reasonable discussion in that regard. Or, an individual should know what you're doing when modding the electronics (really) for over-discharge. Removing from the charger when not in use for overcharge. And protecting them from physical damage. I think that would be about it?? I was never telling the OP he can't do it. I was saying I would not and why.
In the DIY world, there are some people running backpack batteries without problems. Personally, I don’t think it’s worth the risk, at least not long term. During testing of my first ebike, I put my battery in my backpack while I was deciding how to mount it. I didn’t like being tethered.

Long term, I’d worry more about falling and damaging the battery or cables than a spontaneous fire.
I landed on a 1250Wh homemade lipo strapped to my back, hard, very hard, hard enough to go to hospital, tear backpack, road rash back pack.

Battery is fine. It was wrapped in think plexiglass and that was the most damage ( crack). Still holds full capacity 400 miles later and power is good.

Since I have moved battery under tube to keep front end down. It was silly on my back, wheelie to much.
I think 18650 is one of the most dangerous form factors to build with, period. I have seen 10X the failure (and then the fire) from cylindrical energy dense or power dense cells.
I've been daily driving a 27Ah 52v battery in my backpack for 3-4 years now. And from my experience:

-Back fatigue after looong rides (to be expected with any heavy-is backpack)
-battery does not run hot (or even warm)
-I fell a couple of times, everything is OK.
-mtb is hella-fun with a light and balanced bike. The backpack is not noticeable in the first 2 hours but needs to be firmly strapped to handle jumps.
-I feel the battery is better protected in my packpack on rough terrain and jumps rather than being on the frame.
-it's a pain to be tethered to the bike. But I've gotten used to adding a couple of steps to unplug when getting off. The cable extends enough so I can get off, rest the bike down and not pull on it.
-used a keycard extender thingy to auto-retract my cable when unplugging or extend if random tuggs occurred.
-not really worried about explosions, it's not a pouch battery, it's an 18650 pack and the ends are pointing up/down not directly at my back. I did a fire drill a couple of times and can safely chuck my backpack reasonably fast.

Other notes: I stupidly put a jacket with a metalic zipper in the battery compartment and it shorted the charging leads (mini xt60). Nothing exceptional happened, there was a pop, part of the connector melted and the bms shut the battery down. All was fixed after I replaced the connector. No damage to self. I could tell there was a problem and took it off immediately.

If anyone has any questions go ahead :)

End note: I'm looking to add a small frame mounted bottle style battery to use on really short trips (store / neighborhood) when I can't be bothered to take my backpack.
I think 18650 is one of the most dangerous form factors to build with, period. I have seen 10X the failure (and then the fire) from cylindrical energy dense or power dense cells.
Which batteries would be safest to use? I'm currently looking for a downtube battery to go with a 750w mid drive bafang

Any help is appreciated
I avoid carrying anything on my body while riding, the bike itself is far more capable of carrying load, especially dense/heavy load.
If you need to carry an extra battery to extend the range, carry it on the rear rack.