New to Big Li-Ion

Hey all!

I've been working on an electric motorcycle for a few years now (Baja bv250 frame, Perm132 72v motor).
I bough six 55ah AGM SLA batteries probably earlier in the project than I aught to have and while I think they'll still be good for bench testing with some desulfurization I don't think they'll be worth continuing with as my final power source so I wanted to explore alternatives.

Digging around online I've been seeing that high discharge Li-Ion cells have come down a fair bit in price since I embarked on this project but while I've used Li-ion for small scale projects I've never done anything that even holds a candle to this.

My thinking was a 20s20p pack to approximate my current bulky lead acid rig in amp hours, voltage, and discharge capability but I'm not sure where it would be best to start on such an endeavor.

I just found this forum and I already see a couple of threads that might be what I'm looking for but while I start sifting through those I wanted to put the question to the community.

I guess boiled down here are the general sticking points that I can see (I'm sure there are others though):

• How best to source good cells in this quantity
• How to handle a BMS on this scale
• How best to layout the pack for extensibility (possibly adding additional parallel cells later to increase range if needed)
• Pitfalls to look out for on a pack of this size

If possible I would like to build the BMS from scratch since making all the electronics for the bike myself was one of the challenges I set myself going into this project.
Are there any build logs of people doing this before?

Any help would be greatly appreciated.
In the mean time I'm going to start sifting through the treasure trove of information you all have complied here.

Thanks!
CP

Caractacus Potts said:

My thinking was a 20s20p pack to approximate my current bulky lead acid rig in amp hours, voltage, and discharge capability

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Using what cells? There are many kinds, sizes, shapes, capacities, discharge rates, etc.

Typically one that has higher discharge rates has less capacity.

There are flat cells, cylindrical cells, box cells, etc.

Some bolt together, some must be welded, or soldered, etc.

but I'm not sure where it would be best to start on such an endeavor.

Click to expand...

The first place is anywhere *except* the battery. That would be the very last thing I would build for a project, once the entire rest of it is completed enough to be able to determine several of the battery characteristics that it will need.

That said, the first place to start is determine what the peak and continuous currents will be when the system as a whole is setup and used on the road.

Then you check how much space (and weight allocation) you have for a battery pack.

Then you can begin looking for cells that can handle the load you'll put on them without much (any, preferably) voltage sag, and have the capacity required to give you the range you need, but that are small enough and the right shape to fit within the space and weight limititations above.

Then you can begin figuring out what layout and connection methods to use.

Side note that the chemistry of the cells may also matter to you, because some have higher cycle life than others (meaning they'll last a longer time before you have to replace the pack). Some also have certain characteristics that make them more or less desirable for various uses.

If possible I would like to build the BMS from scratch since making all the electronics for the bike myself was one of the challenges I set myself going into this project.
Are there any build logs of people doing this before?

Click to expand...

There are several self-designed BMS systems documented here on ES, but there are no simple and quick ways to locate them. The only way I know is to search in titles of threads for "BMS", and display by topic, then look thru the list generated. You might have to look at the first posts of some threads to determine which are what you're after.

Fechter and Ggoodrum and Hecker (IIRC) have threads for development of four different versions of their BMS. Methods developed at least one, but I don't think it's all on the forums. I can't remember who else has done so and published it all.