I can imagine many different uses for a small GPU in a hangar, so, being an engineering type, I bought the most versatile of them all – a 0-30 V 0-10 A bench power supply for about 60 EUR. It has been serving me very well.
Posts moved to existing thread.
Further back you can see some options discussed.
I use 2 × 12v car batteries in a plastic tool chest on wheels, on my 28V aircraft. I can then use the avionics for whatever purpose (software changes, training etc) for over an hour.
Modern digital systems are voltage tolerant and don’t require 28V. My main GPS/NAV/COM units operate in the 11V to 33V range.
The low voltage warning is a specific feature which tends to be adjustable. For example I have the KLN94 set to 25V, and that will indicate alternator failure.
The actual box works over a much wider range.