View Full Version : continuous duty solenoid vs. isolator vs. separator

02-15-2010, 05:05 PM
I was looking over this great site:


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.



* 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

02-15-2010, 05:45 PM
The solenoid connects the house and starting banks (batteries) for emergency starts. That's it.
As for charging, I recommend, have installed and have been using a Voltage Sensitive Relay like those from BEP in lieu of an isolator. In addition to the voltage drop caused by an isolator, these also generate heat, further inhibiting charging.

A VSR senses voltage and sends charge to whatever bank needs it, keeps both banks isolated, and requires no action on the skippers part after install. Its wired like a three-way battery switch ( 1-2-Both). A simple on off switch is installed also. Works like a charm--no heat, no voltage drop, no switches to throw to direct charge. You can install the solenoid wired to a momentary switch for emergency starting as insurance--I did--but a VSR is very reliable.

When sizing your house bank, estimate the amp-hours at 50-85-percent anticipated usage and then go up 15-percent. ( Getting a battery to accept its last 15-percent of charge takes a lot longer than the bringing it up to 85-percent.) That way you should have the juice you need when you need it. http://www.bepmarine.com/


John P Lebens
02-15-2010, 06:37 PM
My electric guy insisted we do a Balmar Digital Duo Charge. It replaced an auto solenoid.

02-15-2010, 11:35 PM
I put a Battery Link ACR in my system. The alternator is always connected to the starter battery. When the engine is started it senses the higher voltage, waits for one minute then switches in the house battery to start charging it. Once the engine is shut off it waits one minute before disconnecting them. I've had this on board for several years now and there have been no problems.
Also there are two master switches - one each for the house and starting.

02-16-2010, 12:24 AM
Also there are two master switches - one each for the house and starting.
With the VSR, are the master switches just ON/OFF switches?

Also, would a dual-sense VSR handle a situation where you have 2 battery banks and two ways to charge the batteries, like below?

[110V Charger]==>House===VSR===Starter<==[Outboard Charger]

02-16-2010, 02:02 PM
Yes. Even if you had three or more ways (alternator, charger, generator, solar panel). Wire your charging device to any one battery and the VSR will do the rest.In fact, with a VSR you can charge multiple banks/batteries with a single output charger.

02-17-2010, 07:29 PM
Thanks everyone.

02-17-2010, 07:44 PM
With the VSR, are the master switches just ON/OFF switches?

Also, would a dual-sense VSR handle a situation where you have 2 battery banks and two ways to charge the batteries, like below?

[110V Charger]==>House===VSR===Starter<==[Outboard Charger]

Mine are both simple on/off switches. If I find the starter battery flat I can unhook the cables and switch the batteries - which I did once.