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Friday, January 15, 2010

Lithium Traction Pack From Scratch--Update

Not much progress here yet. Still trying to find enough time to test the cells, sort them, and then find a good way to assemble the packs.

A new problem with doing it is that the Sorenson I had been using to charge the batteries on CrazyBike2 before, and was going to use to charge up the NiMH packs (from deardancer) and test and charge the Li cells with, just sparked and smoked when I turned it on last night, even though it wasn't connected to any load yet. It's a linear supply, and does not require an external load to operate, so something must've died in it during power-on-surge. A transistor burned and a resistor smoked, and a couple of traces on the board's cardedge connector vaporised, too.

I've got three others (only 40V models) that are not working for various reasons, so I will need to check their boards for the values of parts, and then see if I can trace out enough of the circuit to ensure replacing the blown parts is all that will be needed, so I don't smoke more stuff when I turn it back on again. :roll:

Maybe I'll get the others fixed while I'm at it. :)

I did have another idea for charging the NiMH, though: I already had two little NiMH chargers for AA batteries, and I found another for a couple bucks at a thrift store, so now I could charge up to twelve cells at a time. All three chargers will charge either two or four cells at once. They are all microcontrollers or dedicated chip chargers, so they should be safe to use with any NiMH cells.

I am considering putting charging taps on every cell in the NiMH packs, in connectors of four cells, and one of two cells. Then put a mating connector wired to the contacts on each little charger so I could not only plug in to these cells, I could also still use them for the AA and AAA cells I already use them for (for my flashlights/headlights/taillights, etc. on the non-ebikes).

Then I would be able to charge three sets of cells in the packs at a time. On the 36V pack, that's 30 cells, so two charge cycles for 6 of the 7.5 sets in it, and one more charge cycle for the other set plus 2 cells. On the 24V pack that's 20 cells, so only two charge cycles total (one for 3 of the sets, and one for the 2 full and 1 half set).

The whole pack will not get that warm this way, and though I will have to manually plug and unplug the charging connectors after each cycle, it will let me safely charge the packs.

Well, assuming I don't burn out the chargers doing it; they only put out less than 500mA charge current. Well, one of them can put out almost an amp charging current if it's only charging two cells. I'll try it out with the cheap thrift store one first. It's the model down from one I already had, and is an Olympus BC-100.

I'm also considering taking a bunch of old celphones I have that are broken in various ways, and wiring them up to charge cells. They're all isolated from each other, so I could theoretically charge the pack "in place", with charging taps coming out of it for each cell, and a charging connector. It would never be a fast charge, but it would be possible, though a lot of work.

I would need to take the little BMS board off each celphone battery, and lay them all out on something flat. Then solder wires from the cell connection tabs to the interconnect that would go to my pack cells. More wires from each board's input connection tabs to the celphones themselves.

If I have enough celphones that actually give me a display (many have broken screens or even broken-off flip tops) then I can even monitor the charging process via the little battery meter on the displays. Not much of a meter, but better than nothing. :)

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