Battery pack from used batteries

[shezza]

Hi all,

New here and new to e-bikes. Just spent a huge amount on one, but I believe the ease of getting through the hard parts of riding should ultimately get me on the bike more. So the bike I purchased offers a 2nd battery for $1200. Taking advantage of the proprietary battery of course... I have basic electrical skills and want to have a go myself at making a pack. I would probably place it in one of those traingle battery back holders as the battery for this bike slips into the frame and I would just have a wire running. Not ideal, but should do the trick. Onto my questions...

I repair tools for a living and get a lot of dud batteries that the client no longer needs. The catch is, the actual batteries are usually ok, it's the BMS that has failed. The batteries capacity is generally 70%+ I could theoretically just charge all the batteries and take the ones with the highest full charge. There is a correlation between full charge voltage and capacity in my experience.

As I am writing this, I have been doing my own research. A battery's capacity nowadays crosses 3000mAh, the batteries I have are mostly 1500mAh. So I would need double the batteries per cell or even more due to them being used, which is crazy really. I have some other batteries from an Electric skateboard, though they are two different batteries with two different capacities. Both are above 2000mAh, so I will probably make some really huge cells, maybe 8 or so per cell. Going back to my question, is different capacities per cell an issue? Or as long as the 8 in the cell are from the same original cell and are balanced, the BMS will take care of the rest?

Any other input would be appreciated.

Thanks

[73Eldo]

Anytime any batteries are connected together in any way you want them to be as identical as possible. When building packs from used cells the best practice is to test the individual cells by running them through a charge and discharge cycle or two, logging the results, then grouping the closest matched cells. You can do all this manually with some sort of load, a charger, and a volt meter but if you have very many to do it can become very time consuming and difficult to monitor. There are devices made just to do this that connect to a computer or phone for the logging part so you simply start the process then at the end write down the results (usually on the cells themselves) and you are ready to make your groups.

For how to use the new pack on the bike the simplest is to just have the same connector on both packs so when one is discharged you just unplug that and connect the charged pack and you are back on the road. There are ways to connect both packs at the same time but are more complex with more factors that need to be considered.

[AZguy]

Using different cells in a large multi-cell battery is always a [very] bad idea even if the only difference is their age... sort of like the chain being only as good as the weakest link your battery will only be good as the weakest cell and that's the best case scenario... worst case is fires/explosions/sparks and other stuff more appropriate for 4 July

A good large multi-cell battery will have all the same new cells, hopefully from the same manufacturing lot (really)

If building just one, doing it for economic purposes is pretty much almost always a loss.. by the time you've invested in the proper tools and supplies even before you amortize your time into it you are likely spending more than just buying one and even if not spending more, once you look at the time spent vs. any cost saved you'll be envious of minimum wage.. and that's if everything goes really well

Building batteries on the small scale is typically an avocational pursuit and likely only makes economic sense when building more than just a couple...

[Diggs Ut]

I've been down the road of building ebike battery packs with random reclaimed computer LiIo cells. It can work but I've never gotten more than a season or two out of them as the weak links mentioned above give out quickly. I have made packs from reclaimed medical equipment cells (think portable oxygen enrichment) that have held up pretty well but for the most part those cells lived their lives on standby. Best cells out there right now are the surplus Tesla Model S cells that test around 3600 mA. Yes, they cost a couple of bucks each but are more than worth it. If you are going to build your own packs then pick up an inexpensive 18650 capacity tester. LiitoKala and Opus are the two most popular.

You can do some reading up on battery pack details at Second Life Storage. They mostly focus on Powerwalls but have good discussions on pack design and building -

https://secondlifestorage.com/index.php

[shezza]

Thanks for all the replies. I have invested in the nickel and welder, but still think I will give this one a pass. I am more after functionality than saving money. Cool if I could have both, but I am going to look at buying a new one. I will probably build one for a home solar project or something like that, but not for my bike. Do Teslas actually have a bunch of 18650's? Not custom built from the ground up? I have actually started looking for a battery, though I am running into a wall. I shall start a new post though as it is really a new topic.

[lectrik al]

Hi. My 2016 Chevy Spark ev has 600 lbs of 18650's in it with an 80 mi range. The tesla's have about 2000 lbs of em for about 300 mi capacity.

The Spark can take 480v @ 118 amps DC on fast charge. That's some electron flow going into a bunch of little batteries!

[AZguy]

Thanks for all the replies. I have invested in the nickel and welder, but still think I will give this one a pass. I am more after functionality than saving money. Cool if I could have both, but I am going to look at buying a new one. I will probably build one for a home solar project or something like that, but not for my bike. Do Teslas actually have a bunch of 18650's? Not custom built from the ground up? I have actually started looking for a battery, though I am running into a wall. I shall start a new post though as it is really a new topic.

Teslas use cylindrical cells as do pretty much all EV's - they used 18650's in the past but have been transitioning to 21700 since they were introduced in 2015 by Samsung and LG... tesla began manufacturing their own 21700's somewhere around 2017 but they aren't substantially different than those produced by the large asian suppliers

[stts]

waste of time