What's the big difference in a battery isolator, versus starter solenoid?

[Perfect Switch]

Solenoids are cheap, easy to install, and because it has a moving parts, it will arc every time it opens and closes. They transfer current efficiently when new and degrade over time. Solid state battery isolators have no moving parts and the efficiency of current transfer will vary depending on the type of electronic components internally. Most common are silicon diode isolators which are relatively cheap and are built into large aluminum heatsinks. With a .5 to 1 volt drop as current passes through the diode, they will get quite hot and heat cycling is what will eventually kill it. More recently MOSFET based isolators came into the marketplace. They are much more efficient than older-style silicon or Schotky-based isolators, but not all MOSFET-based isolators are created equal. There are only a handful of manufacturers that provide MOSFET-based isolators. Devices that don't require heat fins are more efficient than those built onto aluminum heat sinks. In order to compare devices, you need to know the voltage drop at a specific current. All isolator devices are brilliantly efficient at 5 amps. What separate the men from the boys is performance at elevated current levels.... where most alternators will periodically create healthy output...say 150 to 250 amps.

[Rando]

I agree with DWH - no need to be up-sold to a National Luna or a MOSFET switch. If you will just be using the isolator for charging a second battery from your stock alternator (no winching or welding etc) you can get away with a 12V 70A relay for < $7: http://www.newark.com/panasonic-ew/c...vdc/dp/21M9059 These are rated for at least 100,000 cycles, so they will easily outlive the life of your car.

[dwh]

Solenoids are cheap, easy to install, and because it has a moving parts, it will arc every time it opens and closes. They transfer current efficiently when new and degrade over time.

True.

But, solenoids used as battery isolators - unlike solenoids used for say, winching or golf carts - aren't generally switching very much current, so the contacts do last a very long time in normal use. A couple of decades of daily use without a problem is not uncommon.

What separate the men from the boys is performance at elevated current levels.... where most alternators will periodically create healthy output...say 150 to 250 amps.

Very true regarding diode-type isolators.

With solenoid-type isolators though, the solenoid is usually switched on before there is a heavy load applied, so it's not like a solenoid isolator is going to do a lot of switching (and contact arcing) of 150 or 250 amp loads.

It's also pretty rare to see an alternator putting out that much amperage. It takes a heavy load on the bus to draw that much - a welder (as mentioned by Rando), a winch or BigAss(TM) inverter could do it...but again, the solenoid wouldn't be switching those loads, just (possibly) carrying them.

Battery charging won't draw anything near that. 5a-10a usually, 30a sometimes. Depends on the battery and whatever other loads happen to be running (lights and radios, etc.).

[Erik N]

Will the isolator allow trickle charging of both batteries at once, say from of a small solar battery maintainer while in storage?

[dwh]

Will the isolator allow trickle charging of both batteries at once, say from of a small solar battery maintainer while in storage?

A diode-type probably would - if the solar put out enough juice to overcome the diode. But a solenoid type wouldn't unless it was a smart IBS type. But for that, the solar would have to put out enough to power up the solenoid (an amp or so) as well as maintain the batteries.

For that situation, I'd go with a Morningstar Sunsaver Duo charge controller, which is designed for RV use and will get, and keep, both batteries properly charged independently - but again, that's IF the solar panel is big enough.

[Erik N]

I guess if I run a second battery (my Suburban has a factory second battery tray) I will just buy another solar maintainer. I have been using one for several years with success, and the truck runs only 1-2x a year. I disconnect the battery cable during storage.

[Perfect Switch]

A conventional silicon diode isolator's voltage drop may not work well with a trickle charger.

A MOSFET-based isolator will work fine with a trickle charger because the voltage drop should be negligible.

A typical solenoid will DRAW more power to close the contactor than a trickle charger will output and is NOT a good solution for storage. If you run the trickle charger output directly to the battery(s), they you'd be good to go.

Will the isolator allow trickle charging of both batteries at once, say from of a small solar battery maintainer while in storage?

[rb70383]

Little off topics, but at a junk yard hunt I got a 95 amp isolator by sure power.

Like said the voltage drop to forward bias a diode prevents charging a the battery fully on internally regulated alts.

Now if the halfish volt drop is a problem what about jumping the main alt feed to the bat 1 connection but leave the 2nd battery through the isolator? That way only the 2nd battery is affected by the volt drop.


2009-06-27_002237_diode_isolator.gif

[dwh]

It would work. You'd just be bypassing the batt 1 diode.

Might not be needed though for that setup though.

If the voltage regulator is reading the voltage at batt 1, and batt 1 is fed through the diode and is .5v low, then the voltage regulator is going to keep the alt on for longer until batt 1 reaches proper voltage.

So, IF the voltage regulator reads the voltage at the battery, AND both batteries are being fed through diodes, then you wouldn't have the voltage drop problem (on either battery) because the voltage regulator would cause the alternator to keep running until the input (center) of the isolator was .5v (or whatever the exact drop is with that particular unit) high and the battery was where it's supposed to be.

[rb70383]

Ok so it will work at least to prevent the ve drop on the service battery. My Ramcharger has the vr inside the pcm. ('89) If it goes, going to run an old style external vr and hook the sense wire up to the fuse block. I think I will run it and see if the isolator ve drop affects the battery any.