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Topic: Good reference for large-capacity, flexible-tab LiFePo4 battery module terminals (Read 1 time) previous topic - next topic

jwatte

When I went to school, re-chargeable batteries with energy density > 100 Wh/kg were but a dream, and packaging did not include foil wrapped flat cells with aluminum foil flexible terminals. Yay progress!
I'm hoping someone who has actually engineered with these things can give me a hint, link, pointer, or somesuch.

I have a medium-heavy load to supply for a longish time, so I'm building myself a big, re-chargeable battery.
I purchased four of these 3.2V 10Ah cells: http://www.all-battery.com/32v10ahlifepo4lithiumironphosphaterechargeablebattery.aspx
And a ready-made charge/discharge/protection controller: http://www.all-battery.com/protectioncircuitmodulefor3cellslifepo4batterypack-2.aspx

However, while the batteries come with "must read instructions before use," the link on the web site just links to a data sheet that talks about charge/discharge rates, but nothing about actual hook-up. What kind of connections would be best to use for these?
I don't particularly think I should solder stranded copper wire to aluminum foil tabs, but I suppose I could if that's what it takes.
Are there ready-made connectors/clamps that will work for these?
Also, to build the 12.8V pack, can I stack these zig-zag, and literally wrap the +/- terminals together for a wide contact area? If so, how do I secure that?

Also, the company that sells them is only a 15 mile drive away (although it took FedEx 3 days to deliver the package!) so worst case, maybe I can just ask them. Unless they only have sales here, and all the engineering speaks Chinese only...


MarkT

I'd be wary of applying heat to the tabs, heat and lithium cells are not good friends!

Aluminium won't solder without specialised high temperature aluminium solder anyway.  These tabs are designed to be clamped or perhaps spot-welded I think.  The nickel tabs I'm not sure if they will take solder, but again I'd think of a mechanical solution if possible (cells can then be replaced for instance).  Offhand I can't think of a nice simple solution.  Phosphor-bronze springs are used in some high current applications, perhaps there is some product in phosphor bronze available in an obscure catalog that would be perfect - perhaps!  I found this company that might know all there is to know on the subject, if you're lucky someone there might be able to help:  http://www.newcombspring.com/battery.html
[ I won't respond to messages, use the forum please ]

jwatte

Thanks! I'll actually want something like that.
The company did get back to me, saying that soldering stranded copper wire to the tabs is OK, though. Now, whether that is actually TRUE or not, ...
Note that it's LiFePO4, not LiPo, so it's less sensitive to heat (but not un-sensitive.)

MarkT

Well I don't know if aluminium solder can wet copper come to that - a little research would be needed.
[ I won't respond to messages, use the forum please ]

jwatte

Research says cover with oil, remove oxide, solder.
I may just end up using paper clips for this... don't laugh at the lo-tek solutions!

jwatte

This is what I actually ended up doing, and it seems to work fine:

1) stack the cells with mating tabs next to each other (every other flipped top down)
2) work on one pair of tabs at a time (the others are covered in tape)
3) wipe tabs clean with alcohol
4) spread flux over the tabs (I use the plain rosin stuff from Radio Shack)
5) solder the charger balancing wires to the tab through the flux -- it will stick
6) clean off all remaining flux with alcohol
7) fold tabs together and pinch with flat pliers (I did 2 folds, which is "one full turn" or 3x2 layers)

Time will tell how long these connections will last, but it really does seem fine for now. The contact area between the tabs after folded and pinched (and part soldered) is pretty big.

Now, the problem I have is that the picoPSU-120 I'm using to power a mini-ITX computer cuts off when the charge is over 14V. When fully charged, these cells go to 15.8 volts; when fully discharged, these cells go to 8.8 volts... Do I really need to build a boost-buck regulator, and try to get it to 95%+ efficiency, just to play to the whims of this device, that already does the same thing? Grr.

On the bright side, this is a 12.8V, 10 Ah, battery pack complete with charger/balancer/protection circuit, for under $150 including all parts!
As this computer seems to draw about 2A when using it "normally" (with peripherals,) that means a 5 hour run time, which is not bad!

kf2qd

Look up Anderson Powerpole connectors. PowerWerx.com has a nice selectiona dn some flexible wire for making cables. Put a cable set on each cell and then make a powerpole distribution panel.

If you hold the tab in a pair of pliers close to the cell it will act as a heat sink while you solder the outboard end of the tab.

Make sure your soldering iron is large enough to heat the wire, tab and solder quickly as that will allow you to make the solder joint quickly and put less heat into the cell.

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