There are several options

You have several options that you can choose if and when you go with a battery system for the locomotives.  You can still put current on the rails for the purposes of signal detection. You'll have to install resistive wheelsets or resistors on rolling stock and locomotives that you want to detect.

Detection systems using a current transformer, like many of the systems sold for use with DCC, may not work because there may not be enough current traveling through the resistors to trip the circuit.  Experimentation will be necessary. Twin-T type detectors should be OK.

If you want totally electrically dead track, you can choose optical detectors.  There are two types, interruptive and reflective.  Optical detectors have the advantage of being independent of the control system.  They'll work with DC, DCC and battery.  But they have the disadvantage that they only detect at a single point and not for the entire block.

I working on an optical system for my railroad to gain the advantage of independence of the track system and I'm planning ways of mitigating the single-point detection limitation.

TT

With current battery technology, battery will probably be backup

I don't see strictly battery power being viable for ho and smaller scales in the foreseeable future.  The range of the current batteries is probably too limited.  What I see right now is the trains being powered through the rails with the operating signals sent wirelessly via rc, and a small battery back up to enable the locomotive to traverse any dead track without interruption.  The back up battery is then kept charged by the power coming through the rails.  I think that sort of system would give the best of both worlds.  In that sort of system, you could continue to power signals in the same way as conventional systems using the rails for power.  Just my thoughts on where technology is headed.