Ok, I believe the rule came about in the early stages of the program, when there were karts with different motors and controller's and as a method to level the playing field,the program administrators selected the 10kW number. Now the vast majority of the schools / organizations are using the same motor and controller supplied in the kit so its not quite as important and should be reviewed in the future possibly.

The HOW:
In your Alltrax Controller box came a USB A to B cable. Remove the rubber plug that covers the data programming port. Insert the B end (HP PRINTER Style) into the controller box.
Power up your laptop, make sure you have the latest version of the Alltrax software installed and running. Plug the USB end into your laptop, shortly after doing that the software will communicate with the controller. The Kart does not need to be energized to perform the programming. 

On the initial screen all you would need to do is adjust the Maximum Battery Amp number from 300 to 208 point(wherever you feel comfortable) See Below.
Your Alltrax SPM48300 has peak Amps of 300. Your kart is a 48 volt system.  

So as an example, to reduce the power output to under 10kW you would divide 10,000 by 48,  or the output power would be 208.33333 amps. At 208 you would be at 9,984 kW. At 208.3 you would be at 9,998.4 kW

Once you have your PEM (Power Energy Monitor) as supplied from the Administrators you can play with your calculations. 

You must remember to save your changes back to the controller and remember to replace the rubber plug on the Programming port.

If you have questions please reach out to us.
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That is all correct.   Collegiate evGrand Prix has various controllers and hence each controller must be programmed down to a power limit set by the rules.   If everyone has the same controller theoretically it's already an even playing field, however the 300A of the SPM48300 * 48v battery pack would allow for 14.4kW.   That is too much power/current draw from these lead-acid battery packs.  Also, a lower limit of 10kW was deemed more appropriate for the HS series. 

In terms of the 208A limit that is recommended.  The math there is correct IF your battery pack is at 48v.  However teams must realize that the 48v number is the NOMINAL VOLTAGE.  The battery pack will actually range from [about]  10.8v - 13.2v per cell.   Multiply by the 4 "cells" in the pack (lead acid batteries), your battery pack will actually vary between 43v - 53v depending on it's state of charge (SOC).  

What this means is the current you are allowed to pull to stay under the power limit increases as your battery pack's SOC decreases (battery is depleted). 

At the beginning of the race (fully charged battery at 53v) you can only pull 10,000kW / 53v = 188 amps.

At the end of the race (battery pack more near 44v) you can pull nearly10,000kW / 43v = 232 amps.

The issue is that Alltrax controllers must be programmed to one current rating and cannot be changed during the race (unless you want to pit for this purpose..)  Ideally, a motor controller will be able to program a power limit instead of a current limit.  However only a couple motor controller brands can do this. Therefore, you must figure out what current limit you want to program your controller for. 

Also note the important concept of voltage sag.  This is the voltage drop of the battery pack as you pull current from it.  Meaning the pack could have 53v (fully charged) resting voltage, but when you pull 188 amps from that pack then it's voltage will drop to something around 47-50v while those amps are being pulled.   This is due to internal resistance of the battery cells.   

The implications of voltage sag come into play as your battery pack gets to low SOC (low charge).  You theoretically may be able to pull 232 amps out of a 44v battery pack to stay under the 10kW limit, however this pack will have a very significant voltage drop if you try to pull 232 amps.  The pack could likely drop to <35v when attempting to pull that kind of current.  This will likely cause an under voltage error with your controller and shut your kart off. 

The above implication is what will happen when your kart is low on charge.  You will try and throttle the kart, it may move a bit, but then it will shut off because of the voltage sag drop. 

If you have any questions about this, respond below.

Ej Williams

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