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Old 05-24-09, 05:47 AM   #1
Kingofgreens
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battery building questions

I'm thinking of endeavoring into building my own packs.
Does anyone here have any experience they'd like to share?

I'd like to start compiling a list of websites, suppliers & of course tools I'll need to get this off the ground.
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Old 05-24-09, 09:17 AM   #2
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check out endless sphere
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Old 05-25-09, 06:56 AM   #3
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Lithium chemistry, I assume? Just be careful with balance issues and low voltage cutoff. The only way to properly do it is to monitor each cell and pack voltage during charge and discharge.

If everything is balanced to start with, you don't have to do it every cycle use but I do check each cell every 5-10 charge cycles with a dummy load. If I see a weak cell compared to the rest I employ a small 3.3V wall wart charger to pump a little extra charge current into the low cell. It's guesswork but so far I've avoided balancing problems and once everything settled down, it's been very balanced to date.

A good rule for battery pack building is to start with double the AH capacity you think you need. I also monitor a volt meter on the vehicle to know when the battery is exhausted. It would be best to rig a simple low voltage cutoff but I just haven't gotten around to that yet.

I used A123 System developer kits and simply soldered the tabs for my series/parallel strings. One of those battery tab welders would be awesome but they tend to be expensive and I've drawn over 100 amps from my packs with no issues regarding solder tab connections.

For bulk charging I use 8 cell LifePo charger from BatterySpace dot com.
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Old 05-25-09, 08:11 AM   #4
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I have a 48v 10ah 3c LiFePo4.
I'm learning more about these cells every day.
I have yet to cut the shrink wrap from my cell to examine it's internals but from what I can tell , I may have to soon so that I can test each cell for balance unless there is another option. It has an output pigtail & another for charging.

It's still new. Only about 20 or so charge/discharge cycles on it as of now but from what I've learned, I will have to balance it eventually.

Is there a way to balance it without taking it apart?
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Old 05-25-09, 02:08 PM   #5
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Quote:
Originally Posted by BroadwayJoe View Post
I used A123 System developer kits and simply soldered the tabs for my series/parallel strings.
Is there anything in the documentation with those kits that states a limit on how many of their batteries can be put in series? ISTR reading on their site that they don't recommend more than 10s, but can't find anything on that now.
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Old 05-25-09, 09:10 PM   #6
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Problem with a123 is their low capacity cells - only 2.3 ah so you need to parallel several of those. I'm in the process of utilizing BMI LiFePO4 cells. They're in 10 ah cells and have m6 posts so you just connect them with nuts and copper bars. I typically make my batteries in 6 cell groups becasue my charger can charge and balance 6 cells at a time. It's a duo charger so it can really charge 12 cells at a time and balance all 12 but I need to separate them into 6 cells due to voltage reasons. If you're using a small motor, the typical boxy LiFePO4 cells might be fine, but with bigger motors, you'll really need a battery that can supply decent amperages. I'm running the 5303 motor and I routinely pull 80+ amps during acceleration so I need batteries that have low internal resistance. Presently, I'm actually using a 74 volt LiPoly cells which need to be balance charged, monitored, etc. (they blow up literally if not careful) but due to their dangers, I'm going to BMI LiFePO4 cells - they're heavier but safer. They both have 10C continuous discharge rating.
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Old 05-26-09, 09:28 PM   #7
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Interesting topic. I too would like to know how to build my own pack.
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Old 05-29-09, 08:18 AM   #8
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resistance your enemy

Every 1/1000 of a volt counts and when you are putting lots of series cells in there, those 1/1000's add up. It's tough some times to use large gauge wire because of the diameter. So copper straps are in order to connect the cells up.
Silver would be nice but cost is way up there.
Use tin copper solder where ever you can. If your using crimp connectors solder the crimps. No connectors are the best solder everything that gets a connection. Only use connectors for charging.
And the shortest possible electrical connections everywhere.
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