I felt my XM-3500Li scoot needed some sort of BMS for its liFepo battery pack after noticing the cell voltages wandering up and down and felt some cells were getting too far apart to use the supplied charger.
I had a couple of FMA Cellpro 10s chargers that I use in my electric RC planes that had alot of features that would help me get the cells in the pack balanced. These chargers are 10 amp and capable of charging 10s packs. The chargers support A123 chemistry, measure cell internal resistance, have a PC interface and are Active balancing (Charge each individual cell at up to 1 amp after 90% charge. The chargers are rated to charge up to a 60ah battery, granted it would take some time. Two chargers with power supplies and PC interface run approx $600. Which is pretty inexspensive insurance.
I usually use the supplied Xtreme charger to charge the 3500Li pack to 70 volts and then use the FMA Cellpro chargers to balance the pack. It takes approx 1 1/2 hours to charge and balance the pack to 3.65V per cell from 80% in accurate balance mode. Cell balance variation is usually within .014V on my pack.
I don't think that I need a discharge BMS at the moment as the cells stay pretty close to each other as the pack discharges.
This is a graph showing 34 minutes of balancing and the effects.
I chose to set this up outside the scooter first to get the harness work done, do some test runs and debug. This picture shows the set up with the pack, harness, battery chargers and power supplies. The harnessing and adding the balancing connectors for the charger took a full day of work.
Gordon
Hi Gordon,
Thanks for the shots.
I'm not that familiar with the RC stuff; it's been since the 70's since I used that stuff.
So, does the "10s" refer to ability to charge 10 cells at once, all with the monitoring?
It kinda looks that way with all the wiring.
Does this display the cell impedance somehow?
What's the deepest discharge you've done on your pack; either by dashboard, CA, mileage or other device/criteria?
I'm assuming the dashboard meter is just a voltmeter and thus means little for LiFePo?
I'm a little torn as to whether I should put in a BMS, maximize cell life and lose warranty, or keep warranty and have to go through the agony of replacing cells now and again over the next two years, then add a BMS. I think for now it's the latter.
Since Xtreme/mfg haven't supplied a BMS, it would be admirable if they published a list of available BMS which they'd approve for our use without voiding warranty; I mean, it would save them some warranty costs. I know of at least two or three commercially available BMS's on top of the plethora of BMS projects and individual research.
Ross
Ross,
The FMA CellPro 10s will charge up to 10 cells in series at one time with recording. There are 12 wires required for each 10 cell pack to use the balance connectors required by the charger. The 3500 pack is still wired as a 20 cell pack and is not split into two separate pack. So either the thundersky charger or the the CellPro charger can be used just by opening up the lower lock door and connecting either charger.
Internal resistance for each cell is measured and recorded while charging as long as the pack is 75% capacity or less. This gives you a good comparison of how well the cells are matched to each other. You see there is a pretty good spread between each of the 20 Thundersky cells. In my pack I had one cell that would take a full charge quickly and one that would take a charge very slowly and I suspect I would be replacing that cell very soon without the balancing.
The most I have drawn down my pack is 50%, as I am still limited to riding in my neighborhood while I still wait for the MCO paperwork from X-treme. The fuel gauge is really just a voltmeter, and mine needs to be recalibrated to 73V for the higher voltage pack.
Replacing cells in the 3500Li is not a task that I want to take on very often. Adding the wiring for BMS is also not that much fun. I can see why the scooter does not come with BMS due to cost and complexity. So far my track record with X-treme customer service has not been all that great, and I really don't want to have a scooter down while I wait for cells.
I guess the main point I am trying to make in using these chargers, is that there are alot of good developments in the electric RC arena that could be scaled and brought into the EV world. These chargers are not the typical analog style equipment we see in the EV world today.
Gordon