[Noriyasu]

I think you need to confirm the battery voltage again. pls measure it with circuit tester.

Have you not start engine of the '92SVX for more than a few months? If so, the battery may not come back even if you recharge it. Once the battery is fully discharged, it cannot be recovered.

If you can't think of what caused your car to break down, I think that it seems battery problem. Two cars are unlikely to break down equally at the same time.

1) First, with the key off, it must be above 12V.If not, you need charge again.

2) Next, with the key ON it should be above 11V. If not, the battery may be dead. This car consumes more than 4A of current even if the engine is not started. Try replacing it with another live car battery. If the battery is the reason, it should work.

3) If voltage is almost same as 1) when you turn on key (not start). Power may not be supplied due to fuse, immobilizer or wiring disconnection. The battery voltage should be down when there are some load. If there is no change, the cause is probably not connected somewhere.