Alaska90

Hello,

I'd like to install a 2nd battery in my 1990 Pickup. I'm going to use a relay to connect positive terminal of the aux battery to charging system.

Ideally, I want the relay to only be energized when the ignition switch is in the RUN position. (Although I understand it might be necessary for the relay to be energized when the ignition switch is in the ACC position.)

I'm planning to use one of the "Add a Circuit" products to create a power lead for the relay.

Does anyone know which circuit, preferably located in the engine compartment fuse box would be best to piggyback to?

Thanks so much.

AK

habanero

Can't answer your question so bump....

I used a Blue Sea ACR #7622 for my dual battery set-up
https://www.bluesea.com/products/762..._-_12V_DC_500A
(I did not pay that for it though, by a lot)

From when I was doing my own research I know there's a ton of different ways to do it and the info's out there, just kinda hard to find sometimes.

scope103

What you're heading toward might work.

I assume your aux battery is for, I don't know, the refrigerator. You don't want it to run down the main battery, so with key-off you want the two batteries disconnected.

Now, with your-almost dead aux battery, if it connects to the charging circuit with key-on, it will immediately (as in POW!) pull the main battery down, BEFORE the truck starts.

I would want a circuit that would not connect the aux battery until the truck is RUNNING and producing power from the alternator. That signal is from the alt-light, but I don't know of any off-the-shelf product that will do that for you. You'd have to build it up from 1-2 FETs and a relay.

But I'm still not sure it's wise to hook up an almost-dead battery to a good battery, even with the alternator running. The almost-dead battery will pull an enormous number of amps. You might need a fusible link (which of course will keep you from charging when you need it the most.)

wyoming9

iTrader: (1)

Just why are you adding the second battery ??

That helps figure out how to wire it.

Look into a dual battery isolator .

If your handy you can build your own It keeps the flat battery from pulling down the charged battery.

you pretty much want to keep the two systems separate . If your using it to power aux power only.

If your running the stock charging system it is no way going to be able to charge a flat battery.

Connecting a flat battery to the charged battery results in the the batteries equalizing which might be enough to cause the ecu to shut down to protect itself.

For your second battery are you going a AGM ??

Alaska90

Thanks for the replies.

The AUX battery will be used while at campgounds—lights, TV, web access.

Here is a drawing of the setup I'm looking at:
http://www.expeditionportal.com/foru...y-setup-for-50

I understand that the "new" technology for these setups uses isolators that sense which battery is low and shunts charge to it as needed. But, I believe the diagram shown above is the "traditional" method used for years on campers, etc. before the development of the new electronics.

grumpin

I've used this set up several times for campers. Used it on my old Ford trucks that didn't have a tow package to keep the light switch from overheating. As stated, when the aux. battery is low, it will need to charge longer, as in the long dive home. If I were doing this, I would use a toggle switch, so if the battery is really dead, you could get home and charge it with a battery charger. You would have control of it.

scope103

Did the fact that the writer kept suggesting you "get a beer" before starting give you any clue about how sophisticated his plan was?

As soon as you turn the key to on (not even start), you're shorting your carefully-preserved starting battery to your now almost-dead "camping" battery. If you're lucky, one of those 80amp fuses will let go in time to keep from running the starting battery all the way down. But then you won't be charging the aux battery.

Why don't you try something dead-simple like this: ? Supposedly, it uses a voltage-sensitive relay to hold back your low battery until the voltage comes up far enough to indicate that the alternator is charging. It will still put a sudden load on your electrical, but as long as it's not TOO dead your alternator can probably catch up. (Note that your alternator can only put out 60amps flat out.)

grumpin

That's why I would use a toggle switch to control it. I had low batteries and never had a problem with this set up. It has been used for years successfully. Yeah, you could skip the beer part.

Alaska90

"For your second battery are you going a AGM ??"

Correct, the 2nd battery is a yellow Optima.

RJR

There seems to be a (mis) conception that connecting an almost dead battery to the system is akin to connecting a dead short to it. Not true. Dead/discharged batteries actually have a fair amount of resistance, and will typically only draw 10-20 amps at a charging voltage of 14 volts, which is an easy load for the alternator or the good battery. In fact, if a battery is completely dead it will often take 20-30 seconds of applied voltage before it will start accepting any significant current at all. That's why jumping a car with a dead battery from another vehicle works without destroying the battery or charging system of the assisting vehicle. It's also why the alternator of the dead vehicle doesn't blow a fuse or worse once the engine is started and the dead battery is now receiving full charge from the alternator.

(You've probably seen or heard of high capacity "quick" battery chargers that can push 50-75 amps into the battery. They do that by raising the voltage to 20+ volts. As long as the charging voltage stays down around 14 volts the currents won't get anywhere near that high. Also keep in mind I'm talking about discharged batteries, not defective batteries here. A battery with a shorted cell can definitely create more of a load. Thus the reason for the 80 amp fuse between the batteries.)

There are two important reasons why you need an isolator.
- You don't want the starting battery to run down and leave you stranded while you've been camping for three days running lights and fridge off of the aux battery.
- If you don't have an isolator, when you try starting the vehicle, your aux battery will dutifully try to help, and unless you have 200 amp wiring between the batteries, you'll probably burn up a wire or a fusible link. For certain an 80 amp fuse won't survive.

The circuit described in the link will work, but you want to make sure it's wired such that the relay only closes after the engine is started, not when the key goes to "on" or "start". That's tricky to do, but if you can pull that off it should work fine. A circuit that taps into the COR using a FET to buffer the signal or something similar could work and I think scope has suggested such a thing before. The other way is to sense the bus voltage with a precision circuit and only close the relay when the voltage is above about 13 volts.

Alaska90

Thanks RJR for the excellent explanation.

After digging around some more I think this might be a good alternative:


Unlike the diode isolators there is no voltage drop during charging. It senses voltage in the system and only connects the 2nd battery to the circuit when voltage reaches 13.3V and even then after a 10 second delay. When the engine is off it disconnects the 2nd battery once it senses a voltage drop to 12.9V, so the starting battery stays at 12.9V and since the 2nd battery is now isolated it can by used without worry regarding starting the vehicle.

Essentially its the same as the previous URL I posted—the beer guy—but, instead of the relay being energized via the position of the ignition switch its energized by system voltage.

grumpin

Those continuous duty solenoids work good and have been around for years. I'm pretty sure the beer guys "on" means after the engine is running. Thanks RJR.

scope103

There is no wire in the fuse box (or, really, anywhere else) that is only 12v with the engine running. So the beer guys are talking about any wire that is 12v with key-on, which means BEFORE the engine is started.

That's why the commercial packages referred to above use a "voltage sensitive" relay to determine that the alternator is running only once the "bus" voltage exceeds 13.5v or so.

grumpin

Scope, good point, I didn't read all the way through the article. He did state hot. Again, why I liked the toggle switch. However the ones I used for camper batteries, were on a truck camper, and it was off the truck most of the time.