Quote:
Originally Posted by cramermy
Thanks cgtech, BobBoyce, and DaveTM for your feedback. I wasn't aware that the stock 235 amp controller would allow the motor to draw more than 235 amps. I know the battery cells can handle more than 250A. I have to look back in some of my communications to find out what the max amps are that the cells can handle.
My understanding (please correct me) is that its the BMS that will need to be modified to allow for a peak amp rating of 500 amps for 1 min.
Is 100 amp continuous ok for normal cruising on mostly flat ground? i'm mostly using my cart at a campground and around the house which is mostly flat with some small hills.
|
The RXV is an AC drive cart. The controller takes the nominal 48V DC and inverts it into 48 Volts 3 phase AC. The current rating is the per phase current (X3) for a total of 3 times that rating. The factory programming limits motor current to 62% of that rating, which is where the 3 phase maximum of 437.1 Amps figure comes from. It's more complex than that, but it is simpler to keep in mind that the DC current can be 1/2 to 2/3 of the 3 phase AC value. There are many variable involved, such as current required to maintain rotor lock over the RPM range (allowed amount of slip to maintain torque demanded) that you are operating. The more torque demanded, the higher the current required to maintain that torque. So when climbing hills, max current is demanded. Speed is regulated by motor RPM, so when you reach speed, current required drops off and just enough rotor lock current is applied to maintain the speed regulation. The controller functions required are quite complicated, and there are a lot of calibration values stored in the controller to allow it to work with the AC motor. When you turn it off, the controller must write values to controller memory, which it reloads the next time it is started. Normally, the power is not cut until after this write is finished. If power drops suddenly, like when a BMS shuts down, the controller loses power in the middle of this memory write operation, which will corrupt the memory.
Your pack is likely similar to the LiFePO4 GBS cells I bought, except they are half of the capacity. I would have gone with the 200 A/H cells, but I was able to buy the 100 A/H cells for a much better price. I bought 48 of those cells for about $100 each. In order to size the BMS, I needed to provide the specs required by the vehicle. Originally, it was for 200 Amps max, using a 100 Amp shunt. After discussing that with the seller, he realized that I needed better than 500 Amps, with peaks of just over 1000 Amps. So he included a 500 Amp shunt with the BMS package before shipping it. I had to double up on the buss bars between cells as well in order to carry the incrased load.
Your next question is why I was able to get by with just doubling the buss bars... My continuous current requirements will be well under the peak demand, as it is for a road car to be driven on pavment. It has the peak capability for hard accell, as the Bandolero body I used is a race car, built for the track. But I intend to drive it on the street. So unless I'm climbing mountain roads, most of my power demands will be lowered. The controller I'm using is a Curtis RXV 350 Amp upgrade controller. While cruising, current should be well under 100 Amps most of the time. In your case, 50 to 60 Amps is going to be typical, once you are up to speed. I converted a 2008 RXV to lithium using a 7 Nissan Leaf module pack. Those modules are designed to deliver hundreds of peak amps, since they are in series and each module must deliver peak Amps for the car as it is driven upon the highways.
Your BMS will have to meet your needs, both normal use, and max peak demands. Make sure that the BMS you get can handle the usage. You may have to get them to add an external shunt to the design. Most of those chinese BMS boards have the shunt built-in. My RXV lithium conversion uses one of those, and I modified the way I use it to allow it to control the current path external to the BMS board itself. It controls the solenoid of the cart. Pack voltage is never turned off to the BMS, and the controllers logic (Controller Pin 1) is manually turned off when I am done riding.
I reprogram RXV controllers to deliver 100% of their rating. I know them well.
Bob