[ausev] MOSFETs for contactors?

[MLAB]

Maybe someone can reality check this idea for me. 

I need a battery wire contactor for my EV project.  It will need to 
switch up to 55V and up to about 55A.  The coil voltage will be only 12V 
(I have no choice, since a BMU will be switching the contactor, while I 
switch the BMU).  I have thrashed around and finally found one, but it 
is a big hulkster with only 15 ohms resistance on the coil, which seems 
like a silly waste of power. 

Here in my Mouser catalog is a Fairchild FQA65N06 power MOSFET in a 
TO-3P thru-hole package measuring a couple of cubic centimeters, rated 
for 60V drain-source voltage and 72A drain current.  (And I can get 
bigger ones.)  I'd need an appropriate resistor between the gate and the 
switch, and a big resistor, like 100K between the gate and ground (100K 
is better than 15 for coil resistance waste, IMO), and likely a 
capacitor to ground, and a little heatsink, all rigged up to some 
terminals for my wires and cables.  It seems like it can be done for a 
few bucks, and could even be done with higher voltage and amp capacity 
MOSFETs.  The big contactor I found cost around $50.

But if this is as easy, cheap, small, light, and efficient as it looks, 
why aren't more MOSFET based contactors being sold for this kind of 
application?  What am I missing?

-- 
Mike Librik, LCI #929
Easy Street Recumbents
(512) 453-0438
45th and Red River Streets, thereabouts
Central Austin
info at easystreetrecumbents.com
www.easystreetrecumbents.com
www.urbancycling.com