[Tony Bogs]

100 Ohm, 10W is a good starting point, but not a full spec.
Add the rest of the specs (overload rating in particular) and the rest of the precharge components (~96V is not a problematic rating for a PTC thermistor).
Value/ratings might need adjustment for higher voltages (140V is max for the Curtis?) when the overload rating is 5x nominal for 5 seconds.

 

[Tony Bogs]

If the precharge resistor does not have an overload rating of at least 5X,
the resistor may fail after a number of precharge sessions.
Examples have been posted on this forum.
A PTC in series is needed as an overload protection if precharging takes longer than
the overload spec of the resistor, for instance 5X nominal for 5 seconds at 25 deg C.
It's basic electrical engineering.
BTW, I am a BSEE: analog, digital and power electronics, datacommunication, computer sciences, systems design and schochastic analysis.

 

[Tony Bogs]

Last time I looked inside my SOL1 precharge was a 150. 10 watt ceramic ( I think).

Many high power ceramic wire wound resistors have the required overload spec, usually 5X nominal for 5 seconds.

I am using a 47 Ohm wire wound ceramic resistor with the 5 seconds overload rating in the input stage of my charger design.
The PTC (auto reset fuse) in series has prevented the resistor from melting down in all cases where precharging took longer than than a few seconds.

It looks like the Tesla resistor is rated for continuous pre-charging.
Would also be my choice when designing a US$100.000+ car.

 

[Tony Bogs]

The PTC is indeed a safety feature in case of a failure.
Just like any other fuse. If it is left out, the ceramic resistor often acts as a fuse.
That's also fine ( If carefully designed ); in the 80's there were ceramic resistors with a solder joint for controlled blowout (leaf spring actuated).

The PTC has the advantage of auto-reset.

 

[Tony Bogs]

A resistor-LED circuit in parallel shows the tripped PTC.
But in many cases not needed: the smell of a tripped (very hot) PTC is very distinctive.

In some cases a non-resettable fuse is the better option.
I'm using a "standard" fuse for the auxiliary power supply in my charger design. It is even soldered in, no holder.
The power supply has about all imaginable safety features built-in. When that fuse blows, there must be a non-recoverable cause.