Update 2020-02-20
New
Lugs
I realized I had not defined lugs for connecting my 12AWG battery leads to my XT60 connector. Since I need to connect to the bolts on the N.E.S.E. pack, these need to handle an M5 stud size while handling higher current. After some research I settled on Wurth Elektronik lugs from Mouser.
These have now arrived and look great, I’ll get some pictures posted once I crimp.
Changed
Charge Connector
I was being very conservative on my initial criteria for selecting a charge connector. I wanted:
- A waterproof cover that could be firmly affixed to the body
- A port which required only a small hole in the enclosure
- A port which could handle 5 amps. (Even though I will be charging at slightly less than 4A)
This is how I arrived at the screw top DC barrel connector in my original BOM. However, after seeing this picture I was inspired to look back into offerings from CNLINKO again.
I don’t like the screw hole mounting option in the above picture. I’m doubtful the enclosure material will hold up when having holes in such close proximity. Luckily, CNLINKO offers a clamping style connector for mounting. Additionally they offer some smaller size ports (M16 and M12) with this style. The M12 port (while being smaller and capable of handling 5A) is made of plastic, but there is an M16 option which is made from metal, is still small, and can handle up to 10A. The one criteria it doesn’t fulfill is the strongly affixed waterproofing (since it’s a push cover), but I have relaxed on this requirement a bit. In addition, this port has a very short incursion into the enclosure (most of the structure will be outside) which should allow mounting of this port to a location similar to that in the picture (which is advantageous to my design).
Tail Lights
As I mention above, it appears that the Smartlights company is simply making their own mount for (2) standard Xlite100 bike lights. Instead of taking a risk here I just bought (2) of these lights from Amazon. I looked around for a mount that may already exist, but only found these two Thingiverse options for hummie decks:
In this case I’ll need to design and print my own mount for these two lights. I’m taking this as an opportunity to learn Fusion360 (although I still enjoy FreeCAD)
I have only tested these lights under limited circumstances, but I’m relatively impressed by their performance thus far.
BMS (maybe)
This is a tough one. Here’s been my food for thought:
As I mention in the initial post, the Neptune 15 will work as a charge only BMS with per P-Group monitoring. After reading the above two threads (and more) it seems like the other option would be the TinyBMS which would allow for both charge and discharge and (according to my dimensional drawing) barely fit into my enclosure. I’m not, however, totally convinced this is the right path. The statements of “I’d rather a P-Group go low than get thrown from the board” seem compelling. As an alternative, perhaps I can set an alarm with the Neptune app which would alert me when a group goes low. The TinyBMS also has an onboard audio alarm option, which could also be useful if I were to wire it as a charge only BMS. It also seems that the chance of a P-Group coming out of balance is low, especially if you are conservative about the low voltage cut-off setting. Even if a P-group were pulled down and damaged, it seems that the N.E.S.E. enclosures would allow P-Group replacement to be relatively simple. Lots to consider here.
Help Needed Re: Battery Cell Selection
Here’s where I could use some input from the community.
My background reading is from the following articles:
- The difference between “battery amps” and “motor amps” thread
- Battery Amps Thread
- 18650 Battery Cells Thread
- Help with vesc settings
What I’m trying to figure out is how to spec the battery pack cells (based on total current capacity) when using (2) eLofty motors. As per my original BOM, I was planning on using Sony/Murata VTC5D cells (2800mAh with 25A discharge) but it seems that the Samsung 30Q cells (3000mAh with 15A discharge) are the standard for most builds. I also wanted to take a look at Sony/Murata VTC6 cells, as I think these may be the sweet spot. Based on the content in the linked articles I’ve tried to gauge how much max current I can pour into these motors and therefore what the optimal cell should be for this build. I believe my math is sound, but I’m still not clear on what the reliable upper limit of these motors should be.
My one concern here is too much heat since these motors do not have a temperature sensor. I’m not sure how far these can be pushed before the magnets are affected or the wire insulation starts melting.
On the other hand, I want to maximize torque since I’m not using a transmission here and running everything 1:1. So while I’m not really concerned about top speed, I do want the acceleration and (more importantly) braking curves to be predictable and consistent with 6" pneumatics.
I could really use some input here on helping to clarify this subject.
Looking at the characteristics of the eLofty motors and pulling the equations out of the above threads:
Known Quantities
- eLofty motor Resistance: 0.19 Ohms
- eLofty motor rated power: 750W
- eLofty motor max power: 3000W
- eLofty Maximum Current: 50A
- Peak battery pack voltage for 12s4p: 50v
- Peak battery discharge current for 12s4p using Samsung 30Q Cells: 60A
- Peak battery discharge current for 12s4p using Sony/Murata VTC5D Cells: 100A
- Peak battery discharge current for 12s4p using Sony/Murata VTC6 Cells: 60A
- Peak battery charge current for 12s4p using Samsung 30Q Cells: 16A
- Peak battery charge current for 12s4p using Sony/Murata VTC5D Cells: 24A
- Peak battery charge current for 12s4p using Sony/Murata VTC6 Cells: 20A
Equations
- volts = amps * resistance
- volts * amps = watts
- % duty cycle = (battery amps / motor amps) x 100 (@ full throttle & below 200RPM)
For 60A max battery output:
- Substitute into second equation:
- volts * amps = watts
- 50v * 60A = 3000w
- per motor max battery draw = 1500W
For 100A max battery output:
- Substitute into second equation:
- volts * amps = watts
- 50v * 100A = 5000w
- per motor max battery draw = 2500W
So:
- Insert Resistance into the first equation:
- volts = amps * 0.19
- Substitute volts into second equation:
- 0.19 * amps * amps = watts
- 0.19 * amps^2 = watts
- amps^2 = watts / 0.19
- amps = sqrt(watts / 0.19)
For the motor’s 750W rated power:
- Isolating for motor amps:
- motor amps = sqrt(750 / 0.19)
- motor amps = sqrt(3947.4)
- motor amps = 62.82
- Plugging back in for voltage:
- volts = amps * resistance
- volts = 62.82 * 0.19
- volts = 11.9358
- VESC batt/motor/absolute: 15/63/63
- Duty cycle: 23.87%
For 1500W (per motor) max of 30Q / VTC6 cells:
- Isolating for motor amps:
- motor amps = sqrt(1500 / 0.19)
- motor amps = sqrt(7894.737)
- motor amps = 88.852
- Plugging back in for voltage:
- volts = amps * resistance
- volts = 88.852 * 0.19
- volts = 16.88v
- VESC batt/motor/absolute: 30/89/89
- Duty cycle: 33.76%
For 2500W (per motor) max of VTC5D cells:
- Isolating for motor amps:
- motor amps = sqrt(2500 / 0.19)
- motor amps = sqrt(13157.895)
- motor amps = 114.708
- Plugging back in for voltage:
- volts = amps * resistance
- volts = 114.708 * 0.19
- volts = 21.79v
- VESC batt/motor/absolute: 50/115/115
- Duty cycle: 43.58%
Equally (if not more) important is total breaking power. Looking at our (3) options:
- Peak battery charge current for 12s4p using Samsung 30Q Cells: 16A
- Peak battery charge current for 12s4p using Sony/Murata VTC5D Cells: 24A (taken from VTC5A datasheet)
- Peak battery charge current for 12s4p using Sony/Murata VTC6 Cells: 20A
Looking a bit deeper into the spec sheets gives some kudos to the VTC6 cells:
- A nominal cell capacity of 3120mAh
- Cells can handle discharge currents above 15A for short bursts
- Cells can handle charging currents up to 6 amps in pulses
So, based on the above, it seems like the VTC6 cells might be the best bet. Setting these to a somewhat more conservative setting of 50A battery discharge would result in:
- Substitute into second equation:
- volts * amps = watts
- 50v * 50A = 2500w
- per motor max battery draw = 1250W
- Isolating for motor amps:
- motor amps = sqrt(1250 / 0.19)
- motor amps = sqrt(6578.947)
- motor amps = 81.11
- Plugging back in for voltage:
- volts = amps * resistance
- volts = 88.852 * 0.19
- volts = 15.41v
- VESC batt/motor/absolute: 25/81/81
- Duty cycle: 30.864%
For braking I could then set:
- -81 breaking (seems like this should be as high as possible with the pneumatics)
- -8 max regen (since the spec sheet lists the high temperature recommended charging current as 2A per cell and the high temperature recommended max current as 5A per cell)
- I’ll need to figure out how to enable smart reverse below a certain RPM to help the 1:1 on 6" pneumatics stop.
With these somewhat conservative settings, it seems like the VTC6 cells should have enough cushion in them that battery degradation should be minimized. Even in the case where one P-group loses a cell, this cushion should be enough to avoid damage to the remainder of the cells in the group until the offending cell is replaced.
I would love some feedback on the above. I’m slowly purchasing items off my BOM but the next decision really comes down to the battery cells. Thanks in advance. 