|Dec 07, 2012, 09:03 AM|
My 5s3p 24ah LiFepo4 field battery.
to plan for next year, I am now building a field battery from Headway 38120SP 8Ah LiFePo4 cells.
I got these from the german dealer www.Lipopower.de :
For the rest of the world, there is Ebay.
There are cells with the same size but 10Ah. But these are not rated for high currents (20C) like the 8Ah version.
What I like about these cells is that they are prepared to be easily assembled to packs and connected.
So I will actually have a little fun during the process, instead of getting a soldering nightmare.
I liked the method of bolting the cells to a PCB though, so I keep this for the new pack.
There are, however, connection brackets available that would make things even easier.
I tested each cell for capacity, and assembled the cells in 5 groups with about the same combined capacity. After some initial discharge tests, I swapped individual cells between the strongest and weakest groups, until I reached a difference of less than 0.05V throughout the whole discharge process.
I cut the PCB material to shape with a dremel and a disc. I have a vacuum with a water filter, but if you don't, this work must be done with a dust mask, and outside.
To get the "traces" or better, the separations between the individual copper surfaces, I grinded the blunt side of a workshop knive to have a single saw tooth, and planed away the copper in thin layers about 0.5mm wide, until I reached the base material.
If you look carefully, you can see two of these strips with the copper removed in the last image, parallel to the silver connection bracket.
I used lugs to connect the power and balancer cables with the individual copper areas.
The CAD for the case is ready, and I will soon make it at a friend's who happens to own a CNC router.
The case is now finished, and everything fits and works perfectly! The flying season can start!
At the front panel, one can see both the MPX output connectors, and the balancing connector. During operation, I plug in a programmable LiPo warner (~$3 on eBay) programmed at 2.8V, which will immedialtely alert me, if the pack is flat.
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|Dec 07, 2012, 09:42 AM|
I'd be interested to understand how you wired the balancing cables too. My simple mind just can seem to grasp it as I'm just beginning to learn about parallel set-ups.
I'm thinking about a 4s2p configuration for my charger and would like to be able to balance charge them when on ac power at home.
Also, do you recommend the high discharge of 20C for anyone looking to do this or is that just the bulletproof, ready for armageddon, German engineering mind? I thought a 10c continuous rating would be enough since there isn't a burst moment when charging, correct?
|Dec 07, 2012, 09:57 AM|
here is a start for balancing information:
I will attach a more detailled connection plan soon.
Yes, I chose the 20C version because when I do something, I do it right. These cells have a lower inner resistance, and keep the voltage under load better.
As energy is not only based on capacity, but also on voltage, the lesser capacity might be compensated by the better voltage.
Also, these cells can stand a higher charge current, and I might charge with 30A if I'm in a hurry.
|Dec 07, 2012, 04:15 PM|
I don't know about any US suppliers, but there must be some. EV folks and Ebike enthusiasts use them.
I know of one plane powered by these cells:
A large piper powered by a 16s pack.
I have also updated the initial posting with a wiring schematic.
|Dec 07, 2012, 05:16 PM|
Joined Feb 2007
I made up a 24V pack of these Headway cells to power my wife's disabled shopping buggy about 18 months ago although I used the 12Ah version.
They have done the job perfectly. Previous efforts to replace the standard lead acid batteries with LiFeP04 packs from HK were disastrous as they were rubbish.
I also used a battery management system which keeps them in perfect balance although I use a simple non balancing CC/CV charger. It also isolates the cells via series Fets in the case of excessively high charging voltage or overdischarge.
After 18 months use I checked the pack last week and all the cell voltages were balanced to better than 10mV.
I don't know whether there is a 5 cell version, but the 8 cell is at http://www.eclipsebikes.com/cell-30a60a-p-1040.html
Thoroughly recommend both the cells and management system
|Dec 07, 2012, 05:37 PM|
thanks for your interesting information. It's good to hear that the Headway cells perform well!
On my field packs, I only balance charge, and on the field, I use a warning beeper on the balance contacts, that alerts me as soon as a single cell group falls below a certain threshold.
This prevents both overcharge and deep discharge.
For my electric bike, I prefer a BMS though, as 16s are another dimension.
Here, a BMS system is essential as you write.
|Jan 06, 2013, 04:20 PM|
Brasil, São Paulo, São Carlos
Joined Sep 2004
Here is my field battery (LiFePO4 4s 10Ah) from Headway 38120S 10Ah cells. I´m using to charge A123 2s-4s 1100mAh packs with 5A.
|Feb 06, 2013, 11:07 AM|
Wonderful! Very leightweight, ideal for recharging on the slope from the backpack!
Meanwhile, I have finished my pack. I updated the forst posting.
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