Ya I’ve done it both ways too but a field engineer once told me to change a DMC battery with the power on (obviously) and change a Tac 20 battery with the power off.

When I worked at Thyssenkrupp, we told the mechanics to change the TAC20 battery with the power off.

Can someone explain why you'd change the tac20 battery with the power off? I always change DMC batteries with power on.

My thought is the battery is preserving some function of memory and to remove it with power off would wipe that out (similar to a DMC). But if you can change it with power off, then what is it's function? Why is it needed at all if the controller is fine with no main power simultaneously with no battery power? What is it preserving?

@Indirtwetrust. hello
I can't send you pm. if possible can you give your e mail adress. thank you very much

It all depends on the type of memory used. DMCs are older technology that used volatile SRAM to store field adjustable parameters. Volatile means that the memory is lost when power is removed, hence the battery to keep power to the SRAM when the CPU has no power.
Tac20 is a bit newer and has FLASH memory, non-volatile RAM, & volatile RAM. The parameters are not stored in volatile RAM. The battery on a TAC20 is mostly used to keep power to the processor and clock chips to shut down safely on power loss or shutdown, & for loading software. Tac20s will work just fine without the battery installed, you’ll just have a constant 1078 fault and things that are supposed to “latch” on power loss like fire service status will not. Same goes for fault conditions that require a manual reset to clear like an SFR on a Tac32.
In my experience, the problem with replacing the battery on a Tac20 with power on is the CPU locks up and drops the WD. This is when, I suspect, parameters are lost in some cases.

Thank you very much. Makes sense now