> power draw for the device is 150mA nominal

Can you isolate the device which needs to finish what it's doing so
as to reduce current ? I've built circuits with a normally-open relay
which drops out if main power is lost, disconnecting the rest of the
circuit (including a power-hungry backlit LCD). This leaves just the
micro running on a small back-up battery. You could do something
like that with a small FET or bipolar

wbr
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