Battery isolator or solenoid?

[AC]

I've been searching the archives for info on how to isolate my truck batteries from my camper. Many seem to suggest that a solenoid would do the trick quite a bit cheaper than an isolator. I was looking at a unit on line from an outfit called hellroaring.com It looks pretty spiffy but at $140 seems like overkill. Does anyone know what the advantage of an isolator over a solenoid is? All I'm really looking to do is keep my truck batteries separated from the camper when the ignition is off. How is that done with a solenoid?



TIA AC

 

[Casey Balvert]

An isolator uses diodes to prevent discharging of the chassis battery by your camper. The problem with diodes is the small voltage drop that occurs at the diode and prevents full charging of your camper battery. A solenoid on the other hand has very little voltage drop therefore insuring that your camper battery is fully charged. Then when the truck is not running, the camper battery side is disconnected therefore protecting the chassis battery from being discharged by the camper. However, few solenoids are designed for continuous duty and can overheat evetually. The best solution is the battery combiner that uses a continuous duty solenoid combined with a sensor circuit that only allows the camper battery to be connected to the charging system when there is sufficient voltage. Marine supply places do carry these. It sounds like the Hellroaring unit is such a device. These units eventually pay for themselves by reducing battery replacements. FWIW.



Casey

 

[John - K5AWO]

I agree with Casey completely.



There are ways to apply full voltage to a solenoid to make it close and then reduce the voltage to 50%-75% to reduce coil heating. But the cost isn't worth it and is non-standard. Not good for the non-technical person to have.

 

[Joe G.]

I used a 70 amp relay. It's closed in run only so no feed back during start and no battery drain when the engine is off.

 

[LJones]

I am not advocating either method, but let me ask a question here. In my truck I have two deep cycle batteries under the bed of the truck for boon-docking. These are charged off an isolator which splits the power from the alternator and the feed back to that alternator is on the cathode side of the diode. This is a closed loop system, meaning that the diode does cause a . 7v drop but the alternator pumps a little higher to the anode. ALL systems on the truck and boon-docking batteries is on the cathode side of the diodes. I also have Volt meters on each leg of the isolator and always see 14. 1 to 14. 4 volts. Which is perfect to charge batteries. In the five years I have had this setup I have never had a problem. That being said when you guys are using the isolator, are you picking up the feed back so the diode drop is minimized? Or leaving the feed back on the anode side? so you see the diode drop.



BTW: on my last truck I used Ford starter solenoids and they are more trouble than they are worth. I know there are better solenoids on the market today. Also I have cooked one isolator, not sure what I did, about two years ago.



Happy Camp'n

 

[BV]

Solenoid works fine for me. Never had a problem with Tekonsha brand which they call a battery switch .

 

[John - K5AWO]

If the alternator happens to be sensing the voltage at its' own output, then the batteries would be under charged.



In your system, it is done properly, with the alternator sensing one of the batteries and the other battery just goes along for the ride.



Obviously, when the optional battery reaches full charge, it will quit accepting current and be quite content.



The isolator properly installed is the route to follow.



I have seen it used with some GM alternators that did their voltage sensing internally.

 

[AC]

Now I'm more confused. Casey says that an isolator allows the camper batteries to be fully charged but John says that the batteries could be undercharged. And you seem to be agreeing with one another

I thought a solenoid was just an electromagnet that would slap shut allowing voltage from the alternator to the camper batteries when voltage from the ignition was present.

You guys are talking over my head. Can you dumb it down a little? AC

 

[John - K5AWO]

I didn't make myself clear on that one. Sorry,



Once again.



IF, as LJones has done, the voltage sense point is AFTER the diodes, or on the vehicle battery, the alternator will make up the voltage drop and regulate the battery voltage correctly.





On the other hand, IF the voltage sense point is on the alternator side of the diodes (anode), the alternator will still regulate at ~14. 4 volts, but the batteries will never see more than 14. 4 minus 0. 7 diode drop, or 13. 7 volts.



That is a good floating voltage for standby batteries, but no good for a vehicle starting battery.





A vehicle alternator, without a proper load can easily reach voltages of 150 volts. This is how the portable welders get their power. The regulator is set for 48-60 volts for welding. I don't advise this for a standard auto alternator of the 60 amp variety though.



From the alternator's stand point, it could care less whether it is set for 12 volts or 20 volts. It will faithfully try to do what the voltage regulator tells it to do. [within the current capability of the alternator]



Hopefully this is less muddy?

 

[AC]

What I'm getting is that the alternator can send either too much or too little voltage to the batteries without an isolator. I'm guessing that would be because the alternator is set up to mind the batteries in the truck and doesn't have any way to keep tabs on the needs of the camper batteries without some sort of a regulator. An isolator would send voltage to the camper batteries on an as needed basis. Do I have that right?



I already have a charge wizard hooked up to an Inteli-power converter in the camper. Is this unit only minding the store when I'm hooked to 110?



The clerk at the local RV store told me I would need the 120 amp isolator and that the unit is " huge". He said they mount them below the drivers seat under the truck because it's the only place they fit.



Any other suggestions? AC

 

[LJones]

AC, not sure what the RV store is telling you have to mount his under your seat. I have an 160 amp isolator, alternator is a stock 140amp unit, mounted on the outside of the passenger side battery facing the alternator on a grounded . 5" of aluminium. Works just fine.



Also checkout earthroamer's site, he used a 300 amp unit that he mounted down on the frame, not sure that is need. Hopefull I havn't conffused anybody.



Happy RV'n

 

[Joe G.]

I'm a little confused at some of the explanations. If the alternator is sensing the battery side of the diodes how does it only sense one battery when both batteries are in parallel joined by a heavy cable and ground? That should appear to the alternator as one battery. The trailer battery is different because the wire to it is a lot smaller.

 

[Deezul1]

The solinoid/relay/battery switch mentioned above in the Tekonsha link has been in use on my other truck for 23 years with no problems at all. It is made for continous duty unlike the Ford type starter relay that is made for intermittent use only. Usually those are available at any RV parts store and should be quite a bit cheaper than an isolater. All It does is pull the extra battery out of the charging circuit when the key is off which protects the truck battery from discharging.

 

[AC]

That's what I was thinking when I started down this road. A simple solenoid that would open up and disconnect the truck batteries from the camper when I shut down the truck. The more I read about these isolators the more confused I become. But then again batteries aren't cheap. I don't want to be replacing them every couple years if I don't have to. Are people going to an isolator because the solenoid failed or is it just another way to skin a cat? AC

 

[GerryDrake]

I have two batteries of similar size to the truck's originals, in my toolbox. Under the hood, I have a continuous duty solenoid, similar, I presume, to the Tekonsha one mentioned above. I got it at an air conditioning repair shop. Looks very similar to a Ford starter solenoid, but for CONTINUOUS duty. When the solenoid is activated, it joins a battery cable from the positive side of the truck battery closest to the alternator with a heavy (#4-0 ?) that runs to the positive terminals of the ones in my toolbox, and goes on to a quick plug at the rear bumper. The solenoid's activator terminal is wired to a switch on my dash. That switch is 3 position... The normal position is UP, which activates the solenoid when the ignition is on, so I routinely run on and charge all four batteries. The CENTER position is off, and the batteries are in no way connected to the other batteries. They are electrically equivalent to sitting on the tailgate, unconnected. The DOWN position activates the solenoid, regardless of the ignition.



With this setup, the spare batteries are always charged, and at the same voltage as the truck's batteries, and are (with the switch in the CENTER position, available for jumping another vehicle without endangering any of my truck's electronics, or, plugging in a cable I made that goes to the 5'er and activates a relay to disconnect the trailer's batteries and connect the ones in my toolbox to the triler's system instead. I don't often do that, but it IS available.



Of course, in my toolbox, I have appropriaate ventilation holes such that the hydrogen generated doesn't gather. . Some holes high in the back and some in the bottom. .



Some may not like this setup as if I use the ones in the toolbox and run them down too far, hooking them to the truck's batteries while recharging might give uneven charging... I am not too sure on this...

 

[John - K5AWO]

Originally posted by AC

[B An isolator would send voltage to the camper batteries on an as needed basis. Do I have that right?

[/B]

YOU HAVE IT RIGHT...

 

[John - K5AWO]

The reason the RV places harp on using the isolators is it prevents the RV / TT/ Motorhome appliances from running down the main vehcile battery.



Gerry Drakes' use of the relay is just fine. I would use it myself for the sake of ecomony. But, if the solenoid were left in the ON position, it is conceivable to run down the vehicle battery right along with the travel trailer appliances' battery.



Most of us with CTDs though are smart enough to avoid that episode.



The solenoid has another advantage if you will... IF the vehicle battery ran down overnight because the lights were left ON (I have never done that in my life ) you could just turn on the solenoid, wait a few minutes for the vehicle battery to charge from the RV battery and crank it off. Assuming the cables were big enough and most likely they will be.

 

[Deezul1]

Usually the solinoid is wired so it can only access the vehicle battery/charging system when the key is in the on position. This would eliminate running down the vehicle battery unless the ign key was left in the on position with the engine off.

 

[DKlimas]

AC,



I am running a Hellroaring isolator/combiner on my '03/camper combo and it works like a champ. I wanted a good reliable source of DC to my camper to run some ham radio gear. My setup starts with a run of #6 battery cable fused at 100 amps connected directly to the alterator side battery. The isolator is mounted in the camper battery compartment. I use a 70 amp fuse from the isolator to the batteries (two Optima #31's). With this setup the truck alternator will charge the truck batteries and the camper batteries quite efficiently.

I also have a Prosine 2. 0 inverter/charger mounted in the camper to charge and maintain the batteries while 115VAC is available. I can also use the inverter to provide 115VAC to the fridge while driving down the road. The Hellroaring isolator can be wired to automatically connect the truck batteries to the camper batteries when the alternator is running or optionally a switch can be added that gives you the option of 1) automatic operation, or 2) combining both truck and camper batteries, or 3) not combining the batteries at all. The Hellroaring website explains these options.

(The Hellroaring isolators do not use diodes - they use FETS with next to zero voltage drop. )



Dave