Is very simple to develop a poe of redundancy in power,
you need to have two DC
and two diodes for DC, its function in this application is to determine of the flow direction tension.
you will put an diode on each power injector, the output voltage or the positive.
the connection is as follows:
Power 1 dc (diode) power direction conect this two powers
----------vcc(+) ->| -------- vcc(+) ------------
Power 2 dc | ------------- vcc redundancy Routerboard
----------vcc(+) ->| --------- vcc(+) ------------
(anodes) (cathodes)
conect Power 1 and Power 2 ground(-) ====================== ground redundancy Routerboard
Notes: its two power injectors has to be the same voltage and current (example: 12V 2A)
note2:tension will not go the opposite direction in the circuit so the source will not burn
note3: the current is the sum of the two sources while your two are enabled but the current consumption will be just what the Routerboard require, if one source fails, the other is already on.
note4: this image has explicate the simbol (->|)
note5: diodes are several amps, look for a 50% higher than its implementation,
example is the source of its 2A diode should be 3A
But if what you seek is redundancy batteries using the tip above is very good.