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deep cycle marine battery

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Posted by hubops on April 9, 2008, 12:14 am
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.

Posted by Vaughn Simon on April 8, 2008, 9:58 pm

   You need a charger that is advertised as a "float" charger.  With anything
else you run the danger of slowly overcharging your expensive battery.    An
"automatic" charger is not necessarily the same thing!  I use the products from
this company: http://www.batterytender.com/  but there are also others on the


Posted by Jim on April 9, 2008, 8:24 pm
 "Battery Tender" is used by almost all motorcyclists, and it is
cheap and it works.  Hook it up and forget it.  I use one on my
motorcycles and another on my Honda Generator.  You can
buy them in the $5 dollar range, it looks like a "wall wart".
It comes with a quick disconnect plug,  and this connects
directly to the battery terminals..

On Tue, 08 Apr 2008 21:58:01 GMT, "Vaughn Simon"

Posted by hubops on April 10, 2008, 1:32 am

  Thanks, Vaughn.
The price seems OK also  ...
  John T.

Posted by RamRodOz Sword of Baal on April 8, 2008, 10:17 pm

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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