A few misconceptions on your side. The battery will deliver considerably more than 20A into a short circuit, and thus you need to plan accordingly. The PTC device does not fully interrupt current, it limits it to a holding current, which will probably be enough to hold the thyristor on until the battery is completely discharged and the PTC is cooked. Better is to use a thermal breaker or another mechanical circuit breaker, or a replaceable fuse element.
As to the thyristor, it has to be rated for a single cycle surge current larger than the current the battery can supply into the impedance of the circuit. You need to consider the internal resistance of the battery when fully charged and the resistance of the wiring and the current breaker as part of this resistance, and select the device to be able to handle more than this. you may need to add extra resistance ( a low value wirewound resistor capable of handling this current) to get it low enough, or select a massively overrated device so as to have the single cycle surge capacity.