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

[Carl2500]

Ok my goal is to have my camper battery charge off the vehicle alternator, and not have it kill my starter batteries when camping.

I picked up a ford starter solenoid for 13 bucks; I priced an isolator for about $140 bucks.

The idea is to connect grounds to camper battery and starter battery together, solenoid will connect positive lead of camper battery to starter battery only when ignition is on.

If this sounds like it'll work for what I'm looking to do as much as I think it does, what more does a battery isolator really do for the huge price difference?

 

[dwh]

The big difference is "continuous duty" rating.

You don't need a 140 dollar solenoid. Here's one rated 90a continuous, 150a surge for under 20 bucks:

http://www.amazon.com/SOLENOID-GOLF...Z2NE/ref=sr_1_1?ie=UTF8&qid=1333667105&sr=8-1


There are smart battery isolators which simply add a little brain to a dumb solenoid to wait until the cranking battery is somewhat recharged before tying in the aux battery, or some will tie the batteries if they detect a voltage rise (charging) on either engine or aux side.

Don't really need all that though. A dumb solenoid works just fine.

 

[dwh]

And BTW, the sort of rigging you are describing is referred to as a "split charge relay".

Here's a split charge relay with a little brain added to the solenoid, and a decent wiring diagram at the bottom of the page:

http://www.powerstream.com/battery-isolator.htm

 

[Erik N]

Couldn't you use one of those big red switches that gives the choice of "Battery A, Battery B, Both, or None"? It would involve opening the hood to change, but so what?

 

[tombodad]

FYI we use this isolator in our wildland trucks and the Sheriff's patrol cars, and it works great.

http://www.amazon.com/gp/product/B001HC6UJ0/ref=ox_sc_act_title_1?ie=UTF8&m=A3J1PF183V710G

I have yet to install one on my truck (same purpose as you, charge the camper while driving, then not have the camper suck power from the truck when parked), but it's on my list.

 

[Kelly 4x4]

I have used the solenoid for about 20 years in my 89 chevy suburban with no problems and you can jump the post off the solenoid to jump the dead main batt.with the 2ed batt. if you need to..

Kelly

 

[1stDeuce]

I think nobody answered Carl's question... A battery isolator generally takes the charge and splits it (wisely) between two batteries. At the same time, it does not connect the batteries together at all, so you can run either battery down without killing the other. It will not allow you to ever pull power from both batteries at the same time.

A solenoid is a electrically driven high amperage switch that simply connects the two batteries together when it is energized. They will both charge, and you will pull off both batteries so long as the solenoid stays engaged. I do this for my camper charge wire by using a 50A relay mounted under the hood. The relay is only hot in "run", not in accy. This way the camper battery charges when the truck is running, but once I shut the truck off, it's on it's own.

As for your Ford starter solenoid, it may work for a while, but what your really wanted is a slightly more expensive "Continuous duty" rated solenoid. The starter solenoid is not made to be energized for long periods of time, and the windings may get really hot, which will kill it. If you're only looking to charge, and not to draw large amounts of power, a simple relay is really all you need. I would go heavier than the 30A relays that are commonly found, but I am charging my camper through a 30 or 40A fuse right now, and I haven't blown it yet... Knock knock knock...

Good luck!!
Chris

 

[TheAlmightySam]

Typically as well, with an isolator, there's a small amount of voltage drop as part of the isolation process. This isn't a big deal if you've got an external voltage regulator, but if your alt is internally-regulated, as most are these days, you'll wind up with your batteries slightly discharged.

The amount of voltage drop varies, but it sounds to me like 0.5 to 0.7V is fairly typical. That doesn't sound like tons, but depending on the battery, half a volt can be a significant chunk of the capacity of the battery. With an external VR, you can just hook the sense wire to one of the batteries, and it'll compensate by running the alt a little harder. With an internally-regulated alt, however, you're kinda stuck.

The other thing to consider with an isolator is that you've got no self-jump capability aside from popping the hood and running cables. If you want to just throw a switch and have the truck fire up, you'll still need a solenoid of some flavor.

When I scrape together the coins to put together an aux battery system on my rig, I'm considering something like either the Blue Sea Systems ACR, or National Luna's Intelligent Solenoid. Both offer isolation capabilities as well as allowing self-jumping, without the heavy voltage drop penalty of an isolator.

 

[damienperu]

In my opinion a few extra bucks for an isolator is well worth it if you do a lot of camping and don't have the memory of an elephant. While remembering to flick the solenoid switch is less risky than manually monitoring voltage with nothing at all, I'd rather not have to remember to do anything at all after a long days drive or a few drinks at camp... I get distracted easily and needless to say, I love my isolator. It's also nice to be able to use what you can from your starter and increase the amp hours of your entire system without jeopardising its starting capacity, instead of just having the auxiliary battery to get you through the night.

Some isolators do come with override switches which allow you to jump start as well, though I wouldn't make my decision based on that, it shouldn't happen very often unless you have a problem your battery. The case for a smart isolator is even bigger if you have expensive batteries you want to preserve, especially if they are distinct in size and type to your starter. There is a wealth of information on the battery type-charging-longevity issue can of worms.

 

[dwh]

Technically, a solenoid wired as a split-charge relay IS an isolator. It's a solenoid-type isolator, as opposed to a diode-type isolator. They both isolate and are both called isolators.

And yes, I did answer the OP's question. He wasn't asking about the difference between a starter solenoid and a diode-type isolator. He was asking about the difference between a starter solenoid and a solenoid-type isolator. The difference, as I said, is continuous duty rating.

A typical dumb solenoid wired to be energized when the key is turned to ignition is as simple and bulletproof - and fool proof - as it gets. There is nothing to remember.

Sure, some companies do add a switch to give some options, but it's not like you have to remember to flip the switch every time you drive. I suspect most people just leave it switched to automatic and leave it there.

And a solenoid doesn't have the voltage drop issue that diodes have.

I run a dumb solenoid wired to come on with the ignition (not a fancy IBS system - which is just a dumb solenoid with a little brain to decide when to energize the solenoid). Done deal.

 

[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/cb1ahf-12v/automotive-relay-spst-no-12vdc/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.