Brian wrote:

I really can't see any drawback to the make before break switch other
than cost and availablity. As long as you have appropiate fuses or
circut breakers in the system fire should not be an issue and 99.99% of
the time you will only be switching from a low battery to a good
battery. and the two batterys are only connected for a few milliseconds
which may not be such a bad thing as it will slightly slow the voltage
transition to your instruments.

Based on my experiments with two 18 amphour batteries, one freshly
charged and the other discharged below 10 volts, the "surge" current
will be less than 10 amps. For the more typical case (two 7 amphour
batteries, one fresh and one discharged to 11 volts or so), the surge
current will be only a few amps. This is well within the capacity of
switches, wires, and fuses commonly used, especially given the short
duration.

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Eric Greenwell
Washington State
USA