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Old 02-15-2010, 03:02 PM   #1
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continuous duty solenoid vs. isolator vs. separator

I was looking over this great site:

http://cheaprvliving.com/howtohaveelectricity.html

and noticed the section on charging the house battery.

To summarize (I pasted the section below), they recommend a continuous duty solenoid over a separator. Pretty strongly.

Does anyone have more info on this option? Seems all I hear about is an isolator or a separator. The solenoid seems more reliable and more robust (and cheaper).

I'm not an electrical type so I'd appreciate any info. or opinion.

Thanks

Tom




* There are three solutions to
this problem.


1. The first and simplest is a battery selector switch. You attach the cable
from the starting battery to this switch and run a cable from the switch
to the house battery. When you are going to be using the house battery,
you turn the switch to off. This isolates the house battery from the
starting battery so it can't draw it down. The next day, when you are
done camping and about to drive off, turn the switch to on and now the
two batteries are connected again and the house battery is being
recharged. Simple and easy. But what if you forget to turn the switch
off? You run the risk of running your starting battery down and being
stranded. One way around this is replace your starting battery with a
marine starting battery which will not be damaged if you forget to turn
the switch off. And carry a jumper battery as discussed earlier.
2. The next solution is a little more complex but solves all of our problems.
It is using a continuous duty solenoid between the two batteries. A good
auto store will sell these or you can google continuous duty solenoid and
order one over the net. Don't buy an intermittent duty solenoid, it won't
last. It must say continuous duty. Once you have it, you mount it to the
firewall of your vehicle. A cable runs from the starting battery positive
post to one of the large studs on the solenoid. A cable leaves the other
large post and runs back to the positive post of your house battery. On
some solenoids there will be two smaller posts. One is for a ground to the
frame, the other is to a hot wire in the vehicle wiring harness. Some
solenoids are self-grounding, so they only have one of the small posts.
The screws that secure it to the firewall act as the ground. If that is
what you have, take extra care to have a good clean connection for the
ground. If there is paint on the fender or firewall where you are
attaching the solenoid, you must scrap the paint off so that you have a
clean metal-to-metal connection. That thin layer of paint will prevent a
good ground. Whether you have one or two smaller posts, you must run a
wire to the vehicle wiring harness. Wherever you splice in, it must be
after the ignition, so that when you turn the key off, power is turned off
to the solenoid as well. A good easy place for this is the power to the
radio since we know that when you turn off the key, the radio losses
power and turns off. What happens is that when you turn on the key,
power goes to the solenoid which activates a magnet inside it. The magnet
lifts a bar which makes a connection between the two large posts,
allowing charging current to flow from the starting battery to the house
battery. When you turn the key off, current no longer flows to the
solenoid, the magnet turns off, the bar falls and there is no longer a
connection between house and starting battery. That means that if the
engine is running, the house battery is being charged, and if it is off, the
house battery can not run down the starting battery. The best of both
worlds! Absolutely no drawbacks.
3. Another solution, and by far the most common, is using a battery isolator.
These are commonly available at any auto parts or RV store. Because
these have a half volt drop between the house and starting battery, I
don't recommend them. Therefore, I am not going to cover them very
much. Just follow the instructions that come with them if you want to use
one.
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Old 02-15-2010, 03:17 PM   #2
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Re: continuous duty solenoid vs. isolator vs. separator

Quote:
What happens is that when you turn on the key,
power goes to the solenoid which activates a magnet inside it. The magnet
lifts a bar which makes a connection between the two large posts,
allowing charging current to flow from the starting battery to the house
battery.
It's just a separator without the brains. Personally I don't think that just making a big battery block of all your batteries every time you turn the key is a good idea, although I do plenty of things (esp. to my batteries) that aren't a good idea. I don't even like the standard separator setup where there is no easy way to disable it (usually there is a force-on option but not a force-off).

Anyway, I'd make it a bit more complex, adding a relay so the batteries are NOT joined in the Start position, but only in Run, and a switch to override this (manual jumping from your own batteries). It might cost you more batteries in the future (someone more knowledgeable can weigh in...) which might not make it the cheaper option after all.
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Old 02-15-2010, 04:07 PM   #3
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Location: Washington - Ridgefield
Posts: 4,725
Re: continuous duty solenoid vs. isolator vs. separator

The best part of a separator is the fact that it is automatic. And a close second is the bi-directional part.

My solar panels charge the house batteries. When they are charged, the separator automatically switches to charging the starting batteries. For me that is as important as charging the house batteries off the alternator.

Jage,

My separator does NOT connect the batteries during start. That is an option, but not connected on my van. You could connect 12V to the Start Assist terminal (through a switch) to force the batteries together upon demand.

See the Sure Power brochure:

http://www.surepower.com/pdf/separatorinterconnect.pdf

Mike
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