Or maybe a different way of doing it?
Problem: Electrical circuit (12vDC) on my boat is supplied from the battery but on engine start there is a brief voltage drop. This doesnt affect any equipment other than the chartplotter (marine satnav) which shuts down. When re-started it takes a minute to boot up and find itself which can be a touch nervewracking at times!
Some boats deal with this by having the instrumentation powered by a separate battery to the one that is used exclusively for starting the battery, but that isn't really economically viable to retrofit
A friend has suggested that a capacitor and diode will provide adequate temporary power to maintain the voltage until the engine start is finished. The plotter draws maximum 13 watts at minimum operating volatge of 10.8 V.
Is this viable - and if so, what size Cap?
I am not too worried about load dump as there is little else to shed!
Alternatively (it has been suggested) a circuit with a small lead acid gel 12v battery that will cut in when the voltage drops and otherwise charge up when the main circuit voltage is at normal 13+V with engine running etc.