Tuning your RXV for a Lithium Battery

[TheCartBoy]

We are only discussing tuning parameters related to the battery pack voltage levels, not how to make your cart faster. This guide is only for RXVs with a Curtis controller.

We're also going to skip tuning the controller to prevent exceeding the BMS current limits, this is more involved, and a very nice feature about the Epoch kit is the included display gauge will turn red and show an early warning that you are exceeding current limits and you should adjust your driving.

The relevant parameters are:
User Overvoltage
User Undervoltage
Braking Resistor Turn On Voltage.

We are not going to adjust the Nominal Voltage parameters, as this will affect the voltage applied to the main contactor coil and EM brake.

To determine the set points, you will need to look at the datasheet/manual for your battery or cells/BMS settings if you have a DIY pack. For this example, we are using an EPOCH GC2 48V pack.


User Undervoltage:
Undervoltage protection is applicable to drive (discharge) current, the motor controller will reduce the drive current (torque) to prevent the pack voltage reducing further. You will feel the cart slow down. Ideally you want to use a voltage level slightly above the voltage level the BMS will use to shut down. That way the cart will stop driving but will have enough residual energy in the pack for you to use tow mode to unlock the EM brake. Epoch calls this Discharge Cut-off Voltage, 44.8V. A value of 45V - 46V will work well.

Calculate this as a percentage of the nominal voltage: 45V / 48V = 0.9375. Enter value 94%.


User Overvoltage:
Overvoltage protection is applicable to regen (charging) current, the motor controller will reduce the regen current (braking) to prevent the pack voltage increasing further. You will feel the cart will coast more when you lift off the throttle. Ideally you want to use a voltage level slightly below the voltage level the BMS will use to shut down. Epoch does not publish this value, but using the published upper charging voltage level is a good starting point. 57.6V.

After additional testing, it was determined that 57.6V is when the BMS will disconnect the cells from B+/B-. This was confirmed using the Bluetooth app, as this type of fault is not displayed on the gauge. I must assume this was intentional, as their supplied charger will regularly charge the cells to this level, the BMS disconnect, and the charger will keep the bus bars at 58-59V. No way a consumer wants to see a fault message every time their battery reaches 100% charge. Now, this behavior might be unique to the Epoch kit, you will need to understand your system to select the best overvoltage level.
A value of 55V – 56V works well, as it is above the normal floating voltage when fully charged, and below the BMS shutdown point.

Calculate this as a percentage of the nominal voltage: 55V / 48V = 1.1458. Enter value 115%.


Braking Resistor Turn On Voltage:
The braking resistor is unique to the RXV. It is used as an excess energy dumpsite when your battery is fully charged and cannot absorb any more energy due to regenerative braking. The scenario is this; the battery is charged to 100%, you live on the top of a mountain and as you make your descent, the AC motor is generating current, and this needs to go somewhere. Since the battery cannot absorb it, the voltage in the system will increase. Without something else able to absorb it, you will start to lose your braking (due to Overvoltage protection) and if the voltage still increases, you risk the BMS shutting down due to its overvoltage protection, and the parking brake may lock up. So let’s solve that.

Again, we need to know the upper voltage level the pack will shut down at, this needs to be above the maximum standard charging voltage, as we don’t want the resistor to turn on while we are charging the cart normally. If you migrated the charger interlock signal which lets the cart know when the charger is connected to the new lithium charger, then the cart will never turn on the resistor when the charger is connected. But chances are, you simply bypassed it by permanently connecting it to B+.

Unfortunately with the Epoch kit, the BMS shutdown point is less than the maximum charging voltage their charger will output. So we would end up with the resistor activating once the battery reaches 100% (if you left the cart on). This is where the ideal solution is to preserve your charger interlock signal, if you did, you can set the turn on voltage level to 56V – 57V. This will ensure during a regen braking event, the resistor will activate and absorb the excess energy before the BMS decides to shut down.

If you bypassed the charger interlock wire, I highly recommend to restore it, otherwise you will need to set a resistor turn on voltage higher than the charger output. A value of 60V will work. The risk here is, during the regen braking event the BMS will disconnect at 57.6V and the voltage in the system will rapidly increase, which could cause damage to the controller if the resistor does not activate and reduce it soon enough.

[kohlschamber]

Wow okay this is possibly going to save me some headache from all my customers who live on the top of a hill with a RXV fleet with lipo pack conversion.
Can this programming be achieved with the factory Curtis Handheld?

[TheCartBoy]

Yes, these are OEM access parameters.
You'll probably want to advise those customers to only charge to 80% if they live on the top of a hill, the braking resistor smell is concerning for those not aware of it's purpose.

[kohlschamber]

Okay so if I am reading this correctly as well you must keep the oem light blue interlock wire connected to the factory charging port? Instead of wiring directly to B+?

[TheCartBoy]

Originally Posted by kohlschamber

Okay so if I am reading this correctly as well you must keep the oem light blue interlock wire connected to the factory charging port? Instead of wiring directly to B+?

Highly recommend you preserve the interlock signal, but only if the lithium charger is going to toggle it. Otherwise it's still considered bypassed.

If your lithium kits use an external charger, install a 3 pin ezgo plug end on it, make sure to get polarity right, and your done.

If you are installing onboard chargers, then using a relay to pull the interlock signal to B- when the 120VAC is connected will work.

Some chargers like deltaq quiq (which can be reprogrammed for lithium) have an interlock signal already available, you'd have to check their wiring instructions. It's essentially an internal relay to switch the interlock pin to B+ or B- when AC is connected.

[kohlschamber]

Okay so if I preserve the factory charge port, with interlock connected and connect the aftermarket lithium two wire charger (external for lithium) to rxv plug then it will be fine? Sorry I wish I could be as technically advanced as you. Thanks again for your generous help I really appreciate it.

[TheCartBoy]

To make your new lithium charger compatible with the existing 3 pin receptacle and preserve the charger interlock signal: Buy an new charger plug end, these are pretty cheap. Connect the positive from the charger to the B+ pin (bottom left) and the Signal pin (top). Connect the negative from the charger to the B- pin (bottom right).

Here's a picture of the receptacle looking at the pins.

If you wire the plug correctly, you'll see the LED on the receptacle turn on when you plug in the charger and the cart will remain undrivable while its plugged in.

[AlexBen]

Can you clarify this one more time?

While plugged in and charging, we want the interlock (blue on the cart side) to be grounded? So that the controller sees ground on that wire and won't run?

When we unplug the charging plug, that blue wire needs to be (+) correct for the cart to run... right?

It seems to me then, that if one wanted to do this, there are a couple of choices:

Modify the charging plug to be shorting the interlock lead to ground.... that takes care of locking the cart when its plugged in.

and

1) a switch on the dashboard that feeds 48V from the B+ on the battery to the interlock (blue) lead on the cart side... this would signal that its ok to drive, and it could be an ignition switch more or less that the driver just has to "turn on" for the car to work. (without knowing how the key switch is wired, I don't know if its possible to just tap into the back of that to feed the blue interlock wire)

or...something "fancy"

2) relay, resister, some circuit that allows the blue lead to return to (+) when the AC driven charging plug is removed.

[AlexBen]

Originally Posted by TheCartBoy

To make your new lithium charger compatible with the existing 3 pin receptacle and preserve the charger interlock signal: Buy an new charger plug end, these are pretty cheap. Connect the positive from the charger to the B+ pin (bottom left) and the Signal pin (top). Connect the negative from the charger to the B- pin (bottom right).

When you say above, "new charge plug end" do you mean the plug, or by saying end, do you mean the receptacle?

I thought from earlier comments that to run the cart, you need the blue wire to B(+) when you want it to run.... so you wouldn't want the plug itself to have blue tied to the (+)..... you'd want the plug end of the charger to have the blue tied to (-).. right?

[kgsc]

If you are using the original charge port when you unplug the charger cord the internal circuit will provide power to the blue wire. When wiring the new end you only need the main + and - wires.