Electrical Questions for My Adventure Wagon!

[Starballs]

Hey Guys. I have a Tradesman shell on my truck that I'm converting to a camper. I want to put an auxiliary battery back there for a stereo and some lighting. My question is how to hook it up to the truck's charging system. My concern is that I'll drain the cranking battery. I'm not sure if I need a charge controller or an isolator or what. I'd like to charge the house battery off the alternator, then have it isolated while i'm camping, so that if I have too many beers and leave music blasting all night long, I can still get the old girl started and get on my way in the morning.

-Thanks!
Max

[dwh]

The simplest setup is a "split charge relay". Basically a heavy duty solenoid relay that is wired so it's activated when the key is on. All it does is tie the two batteries together and they both get charged. When the key is off, the batteries are separate (isolated).

At the bottom of this page is a wiring diagram:

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

The only difference between Powerstream's setup and a regular ol' split-charge relay is that the Powerstream has an extra little black box added that simply doesn't energize the solenoid until the engine battery has come up to 13.6v and stays there for 15 seconds.

Diode type isolators (the ones with heat sinks that you see at any auto parts store) aren't great because the diode drops the voltage a bit and so the aux battery never really gets a full charge.


I have a regular old split-charge relay in my old camper van and it works fine. Here's a lowbuck solenoid - buy one that is rated for continuous duty and which has a higher amperage rating than your alternator:

[ame="http://www.amazon.com/Continuous-Duty-Solenoid-80AMP-12V/dp/B0028OO05W"]Amazon.com: Continuous Duty Solenoid 80AMP 12V: Automotive[/ame]


EDIT: Also, use FAT wire to the aux battery. Skinny wire will drop the voltage and the aux battery won't get fully charged.

[Starballs]

Awesome! Thanks for the info. One more question: when I run the aux battery down, then start the truck and the solenoid closes, will the aux battery "suck" power from the starting battery? Thanks again!

[ldivinag]

Awesome! Thanks for the info. One more question: when I run the aux battery down, then start the truck and the solenoid closes, will the aux battery "suck" power from the starting battery? Thanks again!

yes.

you can install a manual switch to make sure the connection is broken until the vehicle is started.

also, you can take a hint/tip from the NLL dual battery controller in which it waits 5 minutes before the aux batt is connected to be charged. they say they do this to make sure the main batt is fully charged before "connecting" aux to the charging system.

user TREMORS has a cool diagram. note the switch positions where the aux batt is disconnected from the charging system altogether:
http://www.expeditionportal.com/foru...7&postcount=41

[dwh]

Awesome! Thanks for the info. One more question: when I run the aux battery down, then start the truck and the solenoid closes, will the aux battery "suck" power from the starting battery? Thanks again!

Not enough to matter.

I've run my aux battery down to damned near completely dead a bunch of times in the two years I've had this truck and fully dead several times (forgot and left something turned on inside when I parked it) and have never had any problem starting the truck (Ford 460c.i.) except when I also killed the engine battery by doing something stupid like leaving the headlights on.

There really isn't any big surge from one battery to the other. If you want the full explanation and the math about why that's true, it's here:

http://www.smartgauge.co.uk/nosurge2.html


If you leave the two batteries tied together then the current will flow from one to the other and after some hours they'll level out - the one will come up in voltage and the other down until they are the same. But that doesn't happen very quickly.

[rdonahe]

Awesome! Thanks for the info. One more question: when I run the aux battery down, then start the truck and the solenoid closes, will the aux battery "suck" power from the starting battery? Thanks again!

If I understand correctly buying the isolator from Powerstream (or something functionally equivalent) wouldn't allow this right? It will only bridge the batteries together if the voltage is higher than 13.5V on the main battery side. Am I getting that right?

[Starballs]

Yeah, that's what I'm getting from it. I'm concerned because my truck has a diesel which requires the ignition to be turned on for ~10 seconds before cranking to heat the glow plugs, so something with a time/voltage delay would be nice.

[dwh]

If the current flowing from one battery to the other was 150 amps, that would be 150 amps per hour, which would work out to 150 / 60 = 2.5 amp*hours per minute. 2.5 / 6 = .41 amp*hours in 10 seconds.

Even if the current was 600 amps, then it would be 1.64 amp*hours in 10 seconds.

Like I said...not much.

But yea, the Powerstream or other such setup would be a step up over a plain old relay. Not really needed, but nice to have if you've got it. The other view would be that's an additional potential point of failure.

[dwh]

If I understand correctly buying the isolator from Powerstream (or something functionally equivalent) wouldn't allow this right? It will only bridge the batteries together if the voltage is higher than 13.5V on the main battery side. Am I getting that right?

Sorry, overlooked this before.

Yes, you're right. The little black box on the Powerstream senses the main battery voltage to decide when to switch on the solenoid.