Fusible Resistor for DClink precharging >20W

[Amper]

Hi!
Im working on a small battery system at 48V DC which works well but needs precharging before the contactor closes. Id like it to be semiconductor free, so no buck or similar. Using a Resistor works, however at 300mA and full system Voltage ill have 15W of losses continuously. Either i use 15W Resistors or i make sure to cut the power in some passive way. There is Fusible Resistors up to 5W but at DC its risky business.

Im just asking for some brainstorming if there is good alternatives besides binding a thermal Fuse to a Resistor manually

[Slh]

You could use a positive temperature coefficient (PTC) resistor like https://www.tdk-electronics.tdk.com/en/530034/products/product-catalog/protection-devices/current-protection/ptc-inrush-current-limiters . The resistance increases with temperature so the continuous power will drop as they heat up.

They only way to completely cut power is to use another relay or a switch to switch the pre charge resistor in and out.

[Vovk_Z]

I would suggest large size low resistanse NTC too (like 1-10R 15-20D).

[Amper]

I thought about PTC of cause but they have the issue of being very temperature dependent (duh). The operation range for the battery is -20 to 55 °C so a resistor that will precharge the system nicely at 55°C will have almost a short at -20 (or even at 0). This will result in welded relay contacts at some point.

[floobydust]

You can consider going the other way. NTC minimum resistance is when they are hot. Pleanty of app notes etc. here:
https://www.ametherm.com/inrush-current/thermistor-protection-precharge-circuit-lithium-ion-batteries
I note Vishay bought Ametherm June 2024.

Otherwise, you need a big resistor and thermal fuse.

[Amper]

NTCs are for inrush current but that is not my issue here. Lets say i have a Charge current of 300mA max (relay contacts at DC). An NTC that draws enough current at nominal conditions and room temperature (or even -20) will have way too low curretnt to heat up and have its desired runaway condition. Furthermore what im concerned with here is thermal protection in a fault case. Using an NTC and having a fault (Device powered by the battery shorted or low impedance) the NTC will not limit anything, it will increase current until it explodes in an exponential runaway, definitely not better than a normal resistor getting to hot.

Big resistor and thermal Fuse yes, thats what i said in the opening post but im looking for alternatives, preferably one that is not bodgewiring a thermal fuse to a resistor by hand.