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Old 12-08-08, 06:50 PM   #1
ElJamoquio
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Making your own battery pack: teach me battery pack 101

I want to put together 12 Lithium batteries. I'd prefer Lithium-Ions, but the size I'd like (42120) only appears to be available as LiFePo's. There is no pre-made battery box that fits the shape requirements I would like to contrain myself to.

I am reasonably technically savvy, but I know very little about battery-pack fundamentals.

In any case, assume, for the moment, that I have convinced you that building my own battery pack is a worthwhile idea.

How do I put these together? I don't have a hub, but right now I was thinking about running a 36V hub, and straight-foward-ly stringing the batteries together in series... but I assume, for recharge and optimal life, I should somehow balance these, right? So I should solder them together, but also run leads to each connection, so that I can monitor the voltage, correct?

Is there an off-the-shelf discharge/recharge controller? An off-the-shelf monitoring system for the bike (fuel gauge)? Am I missing something else that's obvious?

Thanks...
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Old 12-08-08, 08:24 PM   #2
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It's not a trivial project, though it can be quite manageable if you are proficient.

http://www.rcgroups.com/forums/showthread.php?t=599316

That's a nice tutorial with good basics on how you connect the things together. 12 cells is really manageable, and though you probably could get away with doing 12 in series if they are quality off the shelf batteries (like A123) I very strongly recommend against it. If one cell is somehow defective the nominal voltage of the entire pack could still be within range, so you run that cell until its dead. Or worse, the charger can blindly continue charging thinking the pack is not fully charged while actually overheating one of the cells until it explodes and destroys the whole thing.

Batteryspace is unmatched in customer service and product quality, in my opinion. They've got a large selection, I just picked these out and I think they'd generally work for your purposes.

PCB: http://www.batteryspace.com/index.as...OD&ProdID=3558
Fuel Gauge: http://www.batteryspace.com/index.as...OD&ProdID=4617
Chargers: http://www.batteryspace.com/index.as...S&Category=705

The idea of the PCB is that you wire each cell individually to the PCB, then the PCB has circuitry on it and does all the complex regulating and keeps you safe. This one also has a fuel gauge (Second link). Though they also sell a fuel gauge type thing that just functions based on the voltage, which I actually prefer because the fuel gauge thing is not designed to go on a handlebar.

Last edited by NorskeDivision; 12-08-08 at 08:27 PM.
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Old 12-08-08, 09:14 PM   #3
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check out endless sphere for more technical info on battery technology. There isn't enough time to explain.
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Old 12-08-08, 10:15 PM   #4
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Thanks Norske!

Quote:
Originally Posted by NorskeDivision View Post
It's not a trivial project, though it can be quite manageable if you are proficient.

http://www.rcgroups.com/forums/showthread.php?t=599316

That's pretty much what I envisioned, yes. Thanks for the link.



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Originally Posted by NorskeDivision View Post
That's a nice tutorial with good basics on how you connect the things together. 12 cells is really manageable, and though you probably could get away with doing 12 in series if they are quality off the shelf batteries (like A123) I very strongly recommend against it. If one cell is somehow defective the nominal voltage of the entire pack could still be within range, so you run that cell until its dead.
I was hoping I could take care of this with the individual leads - in other words, charge each cell individually. I was hoping I wouldn't have to monitor on discharge, but who knows.


Thanks again. Are you aware of a similar product for LiFePo's?
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Old 12-08-08, 11:02 PM   #5
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Quote:
Originally Posted by ElJamoquio View Post
Thanks Norske!
That's pretty much what I envisioned, yes. Thanks for the link.
I was hoping I could take care of this with the individual leads - in other words, charge each cell individually. I was hoping I wouldn't have to monitor on discharge, but who knows.
Ah, yeah that would work well enough. You could use some sort of auto-switching circuit or a charger that could attach to a whole bunch of separate leads. Just to be clear though, you can't just split off 12 separate leads and charge all 12 cells at the same time with a charger designed for one cell.

You'd extend the life of your batteries if you used a PCB with auto-balancing, because some cells will run down ahead of others during discharge.

Quote:
Originally Posted by ElJamoquio View Post
Thanks again. Are you aware of a similar product for LiFePo's?
http://www.batteryspace.com/index.as...&Category=1301

That's tough, I just have experience making traditional Lithium ion packs from "discarded" laptop battery cells. 36 volt PCBs must exist somewhere for LiFePo4, but the above link has the best selection I know of. I wouldn't know where to source them. Happy to help though, keep us updated!
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Old 12-09-08, 12:41 PM   #6
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NorskeDivision -

I don't think PCB is the word you're looking for.
PCM = Protection Circuit Module, which is essentially the same as
BMS = Battery Management System
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Old 12-09-08, 01:31 PM   #7
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PCB = Printed Circuit Board

BMS also correct, and more specific.
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