Simple Dual Battery Isolation?

[kjp1969]

Conventional wisdom says that when adding a dual battery, you should incorporate a heavy duty relay (isolator) that will charge the second battery while the engine is running, and disconnect the starting battery when the engine is off. That way, both batteries get charged while you drive, but only one gets discharged while you're stopped and running accessories. (Forget about winches and other high-amp draws while the engine is running). This type of relay (Warn, National Luna, etc.) tends to be complicated and expensive.

So I was brushing my teeth this morning when it occurred to me that vehicles with tow wiring harnesses may already have this circuit built in. A 7-pin trailer plug has a charge circuit which charges up a trailer battery while driving, but disconnects while stopped. If I wanted to create a second electrical system for lights, radio, etc., is there any reason why I shouldn't simply tap into that trailer charging circuit for the "house" battery?

Then I could take positive power from the house battery, run it to the accessories, and leave the starting battery for starting. I know it wouldn't have the extra features of a true isolator, like being able to join the two batteries for starting, but for essentially a "free" dual battery setup, is there anything wrong with this idea?

 

[DaveInDenver]

A simple isolator is just a diode and there is nothing wrong with that in principle. Traditionally the main downside is the voltage drop across the device, which with the most simple type device (a pair of diodes and a heat sink) will still be non-zero (roughly 0.3 to 0.7 volts). But with better device (HEXFETs and other modern MOSFET components) the designed drop across the circuit can be very low indeed and in fact will be very much like an electromechanical device in operation.

 

[Martyn]

Two things come to mind with utilizing the trailer 12 volt output to charge a second battery.

First, standard wiring back to the 12 volt socket is 10 gauge, so the charging of the second battery will be slow. 4 or 6 gauge would speed things up.

Second, the 12 volt supply back to the trailer socket may be coming off the primary battery, not direct from the alternator. The trailer sockets I have worked on have had live 12 volt even when the vehicle is turned off (it's limited as we usually work on the trailer end of things). That makes me think that most vehicle have the supply wired through the primary battery.

 

[kjp1969]

Two things come to mind with utilizing the trailer 12 volt output to charge a second battery.

First, standard wiring back to the 12 volt socket is 10 gauge, so the charging of the second battery will be slow. 4 or 6 gauge would speed things up.

Second, the 12 volt supply back to the trailer socket may be coming off the primary battery, not direct from the alternator. The trailer sockets I have worked on have had live 12 volt even when the vehicle is turned off (it's limited as we usually work on the trailer end of things). That makes me think that most vehicle have the supply wired through the primary battery.

I have a Sequoia and a Bronco, and both have switched charge circuits, measured at the socket by the hitch. It makes sense, if you're using your trailer electrics while connected to the tow vehicle (engine off), you don't want to kill your starting battery.

I hear what you're saying about slow charge, but I think I'll give it a shot. As far as the Bronco charging system knows, its just charging a trailer battery.

 

[Martyn]

I have a Sequoia and a Bronco, and both have switched charge circuits, measured at the socket by the hitch. It makes sense, if you're using your trailer electrics while connected to the tow vehicle (engine off), you don't want to kill your starting battery.

I hear what you're saying about slow charge, but I think I'll give it a shot. As far as the Bronco charging system knows, its just charging a trailer battery.

Yes, I agree it makes sense to have the 12 volt back to the trailer controlled by the ignition switch, but that is not always the case. We recommend disconnecting the plug from the tow vehicle to make sure discharging doesn't occur.

 

[eugene]

I know most of the new gm's the trailer power is connected to the truck battery, no isolaton or relay.
In any modern vehicle the voltage drop of an isolator. is overcome by the voltage regulator so the drop doesn't really matter. Real downside is the voltage regulator reference comes from the main battery then so your second doesn't get charged all that well.

 

[ntsqd]

My '91 Sub has OE trailer charge wiring, no disconnect - just a breaker on the firewall added by whomever installed the trailer light socket. Cole-Hersee makes a continuous duty 85 amp solenoid that used to be commonly used for this. Waytech shows them for $20-$27 depending.

A more expensive, but potentially better solution is an Automatic Charge Relay. They've been discussed here and is what I have for the FJ60's eventual dual battery system. I will be combining it with a marine battery bank switch. Normal operation will see the No. 2 battery turned off at the switch, but connected/disconnected by the ACR according to it's programming.

 

[pete.wilson]

Hey

I must be missing something here. Why the big deal about running an isolator when having two batteries connected in parrallel? The two batteries will still charge to the same level on both batteries and it does allow a great deal more cranking power; the only problem I see would be if they both got drained to a large degree, may be more than what a normal alternator could handle and then take longer to charge. But diesels pickups and suburbans have had this simple setup for years, if your worried about using one for a house battery and keeping it isolated from the starting battery, then I understand. But you could also buy a cheap battery indicator/voltmeter and monitor your useage and cut it off after you have reached your minimum voltage level. You can buy the volt-meters that go into a cigarette lighter from Wal-mart for about $15 and work quite well.

Pete Wilson

P.S. With the Marine Switch/isolator you can select battery 1, battery 2, both or off. Which is nice if you to work on something without disconnecting the actual terminals on the battery. They also carry a rather high current rating also. Worth looking into. Good way to slow down a thief too boot.

 

[kjp1969]

if your worried about using one for a house battery and keeping it isolated from the starting battery, then I understand. But you could also buy a cheap battery indicator/voltmeter and monitor your useage and cut it off after you have reached your minimum voltage level. You can buy the volt-meters that go into a cigarette lighter from Wal-mart for about $15 and work quite well.

Pete Wilson

P.S. With the Marine Switch/isolator you can select battery 1, battery 2, both or off. Which is nice if you to work on something without disconnecting the actual terminals on the battery. They also carry a rather high current rating also. Worth looking into. Good way to slow down a thief too boot.

That's it- I want a cheap and idiot resistant way to let me use power at "camp" and still have a fresh starting battery. All I have to do is install the battery and an aux fuse block, then run the CB, lights, and stereo to the aux fuse block. It won't keep me from leaving the headlights on, but in that case I could still jumpstart myself with plain old jumper cables which I always carry anyway.

I was originally thinking of a Perko switch like you mentioned, but then I have to switch the switch, and I'd rather have an automatic setup.

 

[Brian McVickers]

The upcoming Winter issue of Overland Journal will have a review of dual battery system options.

Regards,
Brian

 

[akfj40]

heres some options
http://www.wranglernw.com/ShowCategory.aspx?categoryid=790

http://www.painlessperformance.com/webcatalog/largeview.php?SearchField=40102

 

[ntsqd]

Hey

I must be missing something here. Why the big deal about running an isolator when having two batteries connected in parrallel? The two batteries will still charge to the same level on both batteries and it does allow a great deal more cranking power; the only problem I see would be if they both got drained to a large degree, may be more than what a normal alternator could handle and then take longer to charge. But diesels pickups and suburbans have had this simple setup for years, if your worried about using one for a house battery and keeping it isolated from the starting battery, then I understand. But you could also buy a cheap battery indicator/voltmeter and monitor your useage and cut it off after you have reached your minimum voltage level. You can buy the volt-meters that go into a cigarette lighter from Wal-mart for about $15 and work quite well.

Pete Wilson

P.S. With the Marine Switch/isolator you can select battery 1, battery 2, both or off. Which is nice if you to work on something without disconnecting the actual terminals on the battery. They also carry a rather high current rating also. Worth looking into. Good way to slow down a thief too boot.

I've had that OE system on two different Suburbans. It sucked in both cases. The batteries do not always charge exactly the same due to different circuit paths (specifically they do not ground to the same point). With age this is enough resistance to start a destructive discharge see-saw between the batteries. That seemingly small thing has cost me a set of batteries for each Suburban. On the '91 I finally gave this grounding idea enough value to move the grounds to be on the same bolt and that appears to have eliminated the problem.

I've had the marine switch on both Sub's and it is simple and robust except for the brain-fade. Which is why I plan to use this ACR in combination with the switch. From my reading of the specs, the ACR that I have charges the second battery only after the first battery has reached a near full charge.
The ACR was about $70 and the marine switch was something under $30. So my whole battery combining/charging system comes in less than most of the kits, and to me, offers more robustness and redundancy with fewer gee-gaws.

 

[kjp1969]

Every kit I've seen looks like at least $150 plus the cost of the battery and mounting hardware. With the installation I'm thinking of, it will cost around $20 in a mounting tray plus a few feet of wire. I can't think of a good reason not to try it.