I wanted to replace my SLAs on a Wilderness Energy kit with something more decent. I have managed to put 4 DeWalt packs in parallel (for range) using a single BMS module for both charging and low voltage and overcurrent cut-off.
The BMS has a 15A fuse, but it also has a switched output that the DeWalt tools use. The FET on that port is a 70A part, but there is a current sense resistor as well, and I don't know what the maximum current is before the cut-off is triggered. I measure 25A on the discharge, which is exactly the same I used to get on my 36V SLA system under full load, so I don't think there is any influence from the BMS. I wish I had a load that could take more than 750W even for short periods to test this, but I really have no clue what I could use.
I wired the 4 packs in parallel, connecting all balancing connectors together as well. (Thanks for the connectors Brian :) ) I also removed the DeWalt connector from the charger and used a standard circular connector so I can use a few feet of cable between the charger and the battery pack. When the charger is unplugged I slip in another circular connector that has a few wires and resistors to turn on the battery output. I found two links very useful:
post#14.
I have now run the bike for a couple of weeks. I have a short commute (just a couple of miles each way), but I have a climb of several hundred feet at the end (which is why I wanted an E-bike in the first place). The stock DeWalt charger (well, except for the cable) has no problem with the custom pack, and I am very happy that I don't have to worry every time if the charger will accept my pack since the discharge is controlled by the BMS. (It really is the BMS that makes the decision if the pack is bad, the charger just queries the BMS.)
I am happy to say that I avoided killing any of the BMS modules since I was very careful to disconnect the balancing connectors first, then the positive side, then the negative side. During re-connect the most important thing is to start with the negative side first. I always connected positive next and had no problems.
All in all I am really happy with the system! }:)
Hi Armin,
Sounds like your Paralleled Dewalt battery system is working out well for you. I am glad you were able to use those parallel BMS connectors I provided to you.
Can you post some pictures? I am especially interested in seeing how you worked your connectors and cable to connect into the battery charger and switch over to use.. . Are you carrying the charger with you?
Best Regards,
Brian
Hi Armin,
I am very happy with your report now. I come from Germany near Frankfurt and ordered today the first charge of 30 Dewalt packs and 5 chargers. My plans are to put 6 packs parallel and 5 of this in series that I can get ~ 150V at max. 100-120A peak for a 15kw asyncronous 3-phase Motor.
My vehicle is a CREE SAM http://www.cree.ch produced in Switzerland.
I read Brians explanations with a big interest.
Do you think it would be possible discharging via BMS Fet Outlet or only with bypassing The BMS discharging directly ?
Please excuse my english.
best regards
Klaus
Hi Klaus,
Your English is fine.
Very cool vehicle. I hope it gets over here to the States.
I think you have a choice - since each battery can deliver about 25A through the BMS with the correct external resistor network, you could have 6x25A = 150A of total delivered current.
You will be very happy with the Dewalt A123 batteries. They are currently the best high current safe Lithium battery with lots of advantages - Many 1000's of charging cycles, long shelf life of at least 5, maybe 10 years, Flat discharge curve, low operating temperature to -30 degrees C, and excellent power density. These batteries are definitely a good investment.
I think where things are not optimal yet is in the battery management and charging. Fortunately, this is where you will have options and flexibility and can improve in the future.
Charging 5 sets of 6 in parallel with 5 Dewalt chargers will work just fine and take around 6 hours. I have read some complaints at the forum endless-sphere.com about the Dewalt Charging and BMS: some claim that they don't do a good job of balancing. I have not had enough experience to make my own judgement. I am honestly not sure how well the balancing works when 6 Dewalt packs are wired in parallel. This is something you would want to monitor at first, by periodically testing the individual cell voltages.
One thing that will be important when you are operating your vehicle, is to be able to recognize when any of your 50 parallel sets of 6 cells falls below the 2.5 to 2.8 volt range. If you are using the built-in BMS module approach, the battery will take itself off-line. If you use the direct connect approach, like I currently plan to use, you will need a low-voltage detect circuit. A guy at endless-sphere.com forum named Goodrum has come up with a small circuit kit that he sells to do just that:
See the thread:
http://endless-sphere.com/forums/viewtopic.php?f=14&t=2498&st=0&sk=t&sd=a&start=60
His website and the board he sells is at:
http://www.tppacks.com/products.asp?cat=26
Of course, you will be monitoring 50 parallel sets of 6 cells and would need 5 of these boards.
Best Regards,
Brian
Hi Brian,
thanks for shipping your connectors to Germany. You have enough time its not very urgent.
Another question, now I remembered about the vehicles recuperation.
I didn't think first at it. The vehicle loads during braking up to 10 kw back to the batteries. Are Mos Fets switching bidirectionally ?
I don't know what will happen with BMS during recon ?
Maybe I get some problems and I need for this case a bypass for the BMS ?
More easy would be to discharge directly, but the better while saver way is via BMS.
regards
Klaus
MosFets are bidirectional, so you should have no problem even if you go through the BMS on re-charge.
I am not sure that this is the best in your situation, though. Since you are using 5 chargers I assume you will want to wire the batteries in parallel like Brian and I did, which would allow you only 25A (since a parallel set of batteries is connected to a single BMS board). This gives you only 25A*30V*5=3.75kW
You may be able to get a bit more than 25A out, I measured 25A on my bike, but that is all my controller can handle, and I have no better way of testing the battery.
You can use the BMS to monitor the battery during discharge and just use a simple (big) relay or external MosFet, or some killswitch input to your controller to shut down the motor if the BMS decides that a cell is too low. The BMS removes the negative voltage from the second spade connector once it detects an error. If your turn-off logic is hooked up to that output it should be very simple.
On the other hand if each Battery retains it's BMS you get full power on discharge and re-gen braking, but I don't know an easy way to charge them without a charger per pack.
Armin
Yes I could use them like they are without opening but you are right Armin. I would need 30 chargers and that ist to much for me at the moment.
Furthermore I would need a 230V/240V AC version of this chargers but is curious at three times higher price as the 110V version, so I will use first a transformer for AC 230V/115V.
Thanks for the tip with negative voltage of second spade connector. I think I will try out this way. Otherwise I must add a lot of batterie monitoring equipment.
regards
Klaus
how much are you paying for the a123 cells each ?
Are you going to use the packs whole, or bust them up to get at the cells ?
The LiFepo4 BattMan www.falconev.com
Hi,
I will bust them up because I don't want to use 30 chargers. But I will not touch the cells inside the case, only the balancer connectors.
Maybe I need to add some single cells for increasing the whole pack some Volts at the end.
I have a offer from States about 85$ each dewalt pack.
They shall be new but opened package. I hope not bad.
A part is on the way to me (I hope so) and I will check them first when arrived.
best regards
Klaus