DC-Converter Destroying USB Device?

Any help or suggestions would be greatly appreciated - and if I have posted this in the wrong forum let me know.

I have a basic setup of a laptop and a receipt printer powered by a 24v lead acid battery (2 x 12v cells in series) - crude sketch attached

View attachment BasicSetup.pdf.

The printer is 24Vdc and so is powered directly from the battery, the laptop is 19v and so is powered from the battery via a dc-dc voltage converter - in this case it is a model from Maplin (https://www.maplin.co.uk/p/12v-auto...ter-with-1a-usb-socket-12-charging-tips-a83la). The laptop is then connected to the printer using a USB-Parallel cable/adapter.

The problem that I am experiencing is whenever the dc converter is connected/disconnected from the battery it seems to blows the chip in the USB-Parallel cable, destroying it. Having gone through quite a few cables testing out various setups I have found that the data lead wont blow if:

1. Only connected to the laptop and not printer (open end).
2. Connected to the printer and laptop but the printer is powered from a separate battery.
3. The dc-dc converter and printer are connected to the same battery but the dc converter is powering a different laptop to the one that the data lead is plugged into.

Strangely the lead will blow if the printer and laptop are connected to the same battery, but the printer is switched off. So, lead will only seem to blow when the dc-dc converter is connected/disconnected if - the printer and computer are powered from the same battery and the data lead is connected to both devices.

Just to clarify there is no bang, or smoke or melting of cables, it just seems that the pcb/chip within the parallel end of the data cable is damaged so that it is no longer a recognized USB device when plugged back into the laptop.

As a layman I would guess that connecting/disconnecting the dc-converter is creating some sort of surge or spike that is damaging the USB devices, but if anyone has any suggestions on what is causing this USB devices to blow and what the solution might be I would be very grateful.

that is the nature of cheap commercial grade products, almost no protection against even small surges or spikes, rather than a DC/DC converter of unknown quality a linear dropper from 24 -> 19V would be better, no spikes at turn on / off...

Recall guidelines that say a voltage should not be applied to an input of a device unless it is powered. I don't know that that is causing the problem. Or it might be that the ground path has no supply return path.

What BradtheRad indicated is the reason for the damage caused in the parallel cable.

This was a major problem when parallel interface was in use (called as centronics interface).

One should not connect the printer interface cable(parallel type) to a PC , unless the printer is powered.