I don't see how it is, but maybe you're referring to something else. I think you're talking about the CPS, or more famously the CPS2, which had a battery power its anti-piracy measures. You only had to send it in if the battery dipped below a certain voltage, after which it would destroy the security key (in memory), so the ROMs would remain encrypted. You could turn off the machine just fine. Arcade machines were regularly powered off. This coupled with the fact that the CPS2 batteries especially are prone to leak over critical components dubbed them the suicide batteries. These security measures have since been defeated in a few ways, all while removing the troublesome battery. You can replaced the encrypted ROMs on your board with Phoenix ROMs, which are decypted. You can solder on a PCB called the Infinikey, which loads the decryption key with power from the board and not the battery. Maybe there are others, too.

I think the HNG64 just had obscure/undumped/not understood hardware.