I joined a LiFePO4 group purchase that began (ahem) last summer (cough cough) which was originally going to be buying packs from LifeBATT. However various and sundry distractions and changes of plan occurred and the packs are now coming from Headway, and I finally received mine today. The pack is 48v 20ah and weighs 28 lbs according to my bathroom scale. It contains 16 pairs of 38120 format cylindrical cells, and the cells have screw terminals on each end. That makes the nominal voltage 51.2v (3.2v * 16 cells).
I don't know specs on discharge rate. There was a label which may have read 80A but I accidentally tore through it.
This is one cell I pulled out of the pack
Side view
Top view
A view of the end.. the BMS is mounted on a metal plate that's obviously a heatsink
The BMS
A view of the bottom along with the BMS
A closeup of the plates that connect the pack together. These plates are screwed to the cells and the BMS wires are soldered to the plates. Apparently a design change will be to have the BMS wires with a ring terminal connected to a screw in the middle of the plates.
An update...
One of the cell pairs read 0.2 volts when I received the pack. The other cell pairs read 3.34v (or so). A BMS wire connected to that cell pair was loose and broke loose while I was tugging lightly on it. I believe the BMS wire was not well installed and that the cell pair didn't get charged.
I tried charging the cell pair -- not having a single cell charger I jury rigged an old wall wart which had 5v 10w output, and that raised the voltage to 2.8v. That enabled the BMS to allow the charger to charge the pack. That raised the voltage of this cell pair to 3.04v (or so) but no higher. The rest of the cells were hovering around 3.7v. Then I took the charger off the pack to let it rest w/o load and overnight the voltage on that cell dropped to 0.8v.
That cell pair is a goner and I am trying to get a replacement cell pair. Bleah.
I'm also trying to find a box to put the pack in.
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
Too bad about the dead cell. Mind if I ask how much this cost?
It was a special deal nonrepeatable price. Don't expect to be able to ring up Headway and get this price yourself. It was on the order of $700 ...
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
You can now buy direct from Victoria at Headway for $16 per cell (plus ~$3 per cell for fed-x shipping). I suppose they will sell the packs and BMS as well. Her email address.... chcj55 [at] gmail.com
Thanks for the heads-up on Head-way David.
So, is the BMS balancing each group of four cells, rather than cell-by-cell?
And does it have LVC, or just overcharge protection?
It looks like a pretty good connection scheme with the plates, and easy to replace cells with the screw terminals.
Ross
It's per cell. There's one BMS wire connected to each plate.
There may be LVC protection. No instructions came with this pack and I haven't put any use to it due to the low voltage I mentioned on one cell pair. Am waiting for replacement cells.
Taking cells out of the pack is real simple, just undo the screws and slide them out of place. A bit over a year ago I'd been one of those fiddling with the innards of DeWalt packs, and got really frustrated with all the fiddly bits to get right to get a large pack built out of DeWalt packs. It really is much nicer this way.
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
Looks like each pair is 2 cells in parallel, with the "pairs" in series.
Thus, each "pair" (of 2 cells) is "protected" by the BMS.
With no specs or instructions, is the BMS something that one can trust, or even use properly?
Cheers, Gary
XM-5000Li, wired for cell voltage measuring and logging.