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Posted by RamRodOz Sword of Baal on April 8, 2008, 6:17 pm
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> Slightly off topic - sorry.
> I want to install a 12 volt back-up sump pump in my basement sump
> hole, in tandem with the existing 110 v. submersible.
> I'm thinking - submersible bilge pump with marine deep cycle battery
> - but I'm not sure about a battery charger.
> I would like trickle charge plus re-charge capability.
> Would any old sump pump float switch work for the bilge pump ?
> Any ideas or suggestions appreciated.
> John T.
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By 12 volts I expect that you are getting your 12 volt supply from a 12 volt
battery, meaning a DC (Direct Current) supply.
DC is much harder to switch than AC (Alternating Current), so make sure that
your float switch is capable of carrying the full starting current of your
pump and it is rated for DC power.
Alternatively use the float switch to turn on a 12 volt DC relay that can
handle the pump starting current.
This means that the float switch only need to carry the power that the relay
coil needs, and the relay contacts carry the pump current.
The relay contacts would need to be rated for the full power (amps) used by
the pump. I would oversize it somewhat to allow for the starting current,
which is much higher than the running current.
I do not know how much higher the starting current is on DC motors as
opposed to AC motors, but as a rule of thumb, AC motors starting current can
be 5 to 7 times higher than running current
You should be able to pick up an automotive or marine 12 volt DC relay to
do the job. Make sure the relay is rated for continuous operation, IE not
say a horn relay that is only rated for short operation.
I would put a fuse the wire going to the relay for both the coil and motor
in case of a fault. People tend to forget just how much power can come out
of a battery when there is a bad fault.
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