Main Contactor

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The main contactor is a high-capacity relay-operated switch separating the battery's positive side from the bike's propulsion systems.

It mutually protects the battery and the bike's systems from damaging each other, particularly for rush currents or overcurrent conditions.
Contactors for the 2013+ years are GIGAVAC GX-series contactors.
The 2013 year contactor is not identified, and some reported an occasional failure mode where "contactor welded shut" could occur.
2014+ years feature the GX11, which appears to fail safe open by design (SPST-NO) and has had no significant reported issues.
The 2013+ power pack contactor is housed within the "dog house", an enclosed area extending from the main power pack enclosure top face at the rear.
For the Power Tank accessory and bricks and long bricks, each enclosure has one contactor operated by the BMS within.
See Charging Control Unit and Contactor Limits.
Theory (ref)
A contactor is a large relay: it allows a fairly small voltage/current signal to switch a much larger voltage/current.
It has two sides:
  1. The drive/coil side (control).
  2. The load (contact) side.
On the drive/coil side, there's a "kickback" diode which handles the inductive spike caused when the drive to the coil ceases.
Without that diode, the energy represented in the magnetic field inside of the coil would have no place to go, so it would cause a voltage spike which can be damaging to components.
The kickback diode gives that energy somewhere to go, allowing it to ramp down fairly slowly without a big voltage spike.
Depending on how the coil side of the contactor is wound, it has a certain current requirement to pull the contactor shut, say 12V @ 100mA, which will be provided by the driving circuitry.
There's not necessarily any relationship between the coil rating and contact rating, except that bigger (higher-current rated) contacts might be heavier and require higher coil current to get the armature to move.
The kickback diode only needs to carry this amount of current, regardless of the contact side current.
If a 3A kickback diode is specified for that contactor, it will be adequate for any contact side current.
The make/break current is the rating for the contact side of the contactor.
You won't actually want to make or break the contactor under the limiting conditions (you'll always want to both make and break the contactor at exactly 0 amps), but in an emergency, it's rated to do so without the contacts welding together -- and an emergency is what the contactor is there for.
In automotive use, you often see the coil set up for 12V, drawing maybe 100mA (the current can vary widely though).
That small current pulls the contacts together, which switches on the high-current side.

Contactor Limits

Zero's effective contactor limit for the charging rate is 95% of 1C rate.

The BMS will open the contactor for the following conditions:

If the motorcycle is keyed off and not otherwise set in charging mode, the contactors will open after a few seconds.
If the motorcycle is keyed on and not otherwise set in charging mode, and then 30 minutes pass without control input.
Twisting the throttle is sufficient to reset this timer; otherwise restart charging once it happens.
Voltage is too low or too high outside its safe operating range.
If voltage is too high (117V):
  1. Confirm with the Zero mobile application
  2. Attempt to bring it back to normal range by discharging the battery:
    • operating the motor (fastest but requires a closed contactor, and the Sevcon won't operate above 118V) or:
    • turning on all available electrical loads.
If voltage is too low, charge by any means necessary:
Prefer the onboard charger or an equivalent rate source through the charging port.
In an emergency, the regen feature can be used to charge the battery through the motor.
Drive the motor by spinning the rear axle with a suitable power source, with the bike in Custom Mode with regen set to 100%.
Incoming power exceeds 1C rate continuously for a minute. A 1C rate charges the battery fully in 1 hour.
For a battery with 13 kWh max capacity, this value will be 11.4 kWh nominal / 102V = 110A (say).
If the charge rate is too high for the battery, downgrade the charge rate manually and restart.
Field testing indicates that the short time setting for the contactor opening is 30 seconds, and that a 20 second period under that limit will reset it.
This may vary due to environmental conditions like ambient temperature.
If the battery temperature (not ambient) is outside safe operating bounds:
BMS Behavior over Temperature Ranges
Cold Battery Normal Hot Battery
Range operation prevented charging prevented charging tapered operation prevented damage likely
(F)  … -22F -22F … 32F 32F … 110F 110F … 122F 122F … 140F 140F … 
(C)  … -30C -30C … 0C 0C … 43C 43C … 50C 50C … 60C 60C … 
The Zero battery will emit heat while its charging rate exceeds 4 kW, and it will cool if under that threshold.
If temperature is out of safe range, do whatever is appropriate to bring the temperature back in range.
Calculating Contactor Limit Per Bike

Per Zero's specs pages:

Maximum capacity (kWh) = COUNT(cells) × (cell capacity rating (Ah)) × MAX(cell rating (V))
A 2015 SR w/ Power Tank has 15.3kWh max, 13.5 kWh nominal, with a 102V nominal voltage, so is nominally rated at 133 Ah.
1C rate means that it can charge at up to 133A.
At 10% actual SOC (3.4 volts per cell), 1C = 3.4 volts per cell × 28 cells in series × 133 A = 12.7 kW.
At 95% SOC (4.1 volts per cell), 1C = 4.1 volts per cell × 28 cells in series × 133 A = 15.3 kW.
These are DC power ratings; AC input power will read 10 to 20% higher (due to conversion losses).
Contactor Limit Per Power Tank or module
The current capacity will be proportional to the number of bricks vs 4 providing a scaling factor.
Or, more directly, the Ah rating on the label indicates the 1C rate.
Charging Limits
Capacity (kWh) Voltage Capacity Contactor Limit
Year Cell # Bricks Max Nominal Max Nominal (Ah or 1C amps) A (95% assumed)
2013 25Ah 4 11.4 10.0 116.4 102 98 93
2015 27Ah 4 12.5 11.0 108 103
2016 29Ah 4 13.0 11.4 112 107
2018 32Ah 4 14.4 12.6 125? 119?