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My first thought is to add an Uninterruptible Power Supply. Once power is cut to the UPS, then the UPS can signal W7 to shutdown gracefully via a USB connection and the UPS can provide the power for that shutdown. However, UPS systems are not designed to handle repeated discharges and they take a while to charge. A few short trips and you would not have any charge left in the UPS. It would require about 4 hours to recharge.
You have an auto battery:
With enough tinkering, you might be able to get the UPS to use the auto's battery instead of its own battery. You would just need to prevent the UPS from attempting to charge the auto's battery. Diodes can probably do this, but that might put the UPS into a fault mode. It might panic if it is not able to send current into "its" battery.
Striking a middle ground:
To keep the UPS system in tact (to allow it to send current into "its" battery) you might be able to just add the auto's battery in parallel with the existing UPS battery. You would still need to prevent current from flowing from the UPS and into the auto's battery - so diodes are still in order. And UPS batteries are not going to like the heat that can build up inside a vehicle. So you might need to buy a high temp model.
There are UPS systems with connectors that are designed for external batteries and those UPS systems won't try to charge the external battery... but finding one UPS with the desired hardware (connectors) and firmware (signal computer to shut down) might be hard.
There are UPS systems that accept an auto's DC as its power source, but they might not have firmware to signal shutting down a computer. Using a UPS system that accepts an auto's DC as its power source would save you a DC to AC conversion. Such conversions waste power and generate heat.
Hopefully this Point of Sale terminal was designed to be installed in an auto and thus has provisions for being powered via DC. If not, then you will have a DC to AC conversion to put up with inside the UPS. If the Point of Sale terminal can use DC for a power source, then you will want to look for a UPS that has a DC output.
I know that it sounds silly to think that there are UPS systems that use DC in and put out DC... but
they do exist.
If you do opt to use a UPS with an AC output - be aware that there are two basic types:
ones that use a square wave output
ones that get closer to a sine wave
PFC Sinewave Series UPS Systems: The most affordable, feature-rich Pure Sine Wave UPS
You might want to see if someone in sales or support at a UPS company has a better suggestion for you.
Desired feature set:
DC in (or AC in)
DC out (or AC out)
USB to signal computer to shutdown
turns itself off
turns itself on
high temp battery
(or does not freak out when there is only an external battery connected)
external battery connection with charge isolation
Below is just me thinking out loud (so to speak).
Once the computer has shut down, the UPS should turn itself off, thus preventing further power drain on the auto's battery. The UPS should turn itself back on once its power source has returned.
Point of Sale terminal must use AC input:
Worst case is a UPS with AC in and AC out:
When the ignition is on:
Converter1 = Auto's DC is converted to AC (AC1)
(We will call the DC in the line above DC1. It is switched off when the ignition is off.)
(Converter1 is something extra that you would have to buy.)
Converter2 = AC1 is converted to DC2 to charge UPS battery
AC1 is passed thru the UPS to power the computer
When the ignition is off:
DC1 is no longer supplied to converter1
Converter2 stops charging the UPS battery
Converter3 = UPS converting DC3 to AC2 to power the computer while it is shutting down
(DC3 is from UPS battery and is supplemented with auto's battery [DC4].)
(DC4 is a connection to the auto's battery that is not switched via the ignition.)
Point of Sale terminal must use AC input:
Better case is a UPS with DC in and AC out:
When the ignition is on:
Auto's DC (via ignition) is DC1
DC1 feeds UPS
Regulator1 = DC1 is regulated to DC2 to charge UPS battery
Convertor1 = DC1 is also used to create AC1
AC1 from the UPS powers the computer
When the ignition is off:
DC1 is no longer supplied to the UPS
Regulator1 stops charging the UPS battery
AC1 from the UPS continues to power the computer while the computer shuts down.
The UPS is using DC3 from the UPS battery and is supplemented with auto's battery [DC4].
(DC4 is a connection to the auto's battery that is not switched via the ignition.)
Point of Sale terminal can use DC input:
Best case is a UPS with DC in and DC out:
When the ignition is on:
Auto's DC (via ignition) is DC1
DC1 feeds UPS
Regulator1 = DC1 is regulated to DC2 to charge UPS battery
DC1 is passed thru the UPS to power the computer
When the ignition is off:
Regulator1 stops charging the UPS battery
DC3 (UPS battery) and DC4 (auto's battery) powers the computer while the computer shuts down.
(DC4 is a connection to the auto's battery that is not switched via the ignition.)
