How to make a cheap isolated dual-battery setup for $50

[dwh]

Since its NEC blabber time... Thats not correct.
For example using 10AWG to serve 20amp devices, (such as receptacles) Nevermind the conductor, its circuit is still protected at 20amps.

Okay you caught me. But that situation is going to take a lot longer wire run than you'll find on a truck, and we're dealing with trucks here.

 

[Stereo]

Verkstad, you were right. I just didn't try enough fuses. Here's the result of my probing: All the fuses are hot when the engine is off except for the taillights, headlights, and AC and the only time those fuses are energized is when those items are switched on.

An earlier post suggested the following:

I'll add a suggestion here, that you take one more step. You want to find a Ignition "on" hot wire, that's true. However, I would try and find one that that is "NOT" energized when the ignition is switched to the "Accessory" position. That way your batteries are still isolated when your engine is off and the ignition is in the accessory position (to run your radio or other accessories, etc.) If you use a circuit that is hot in both the "on" and "accessory", the relay/isolator will be engaged in both positions, tying the batteries together without isolation,(and eating up power) even when the engine is off and you're kick'in back with a beer at your favorite overlook listening to some tunes.:ylsmoke:

Don't my results suggest either I have to connect to an always-on circuit or, alternately, I'll have to turn on my AC in order to energize the isolator? (Tongue in cheek on the latter) If so, I guess I'll be missing out on some tunes and beer. Wait. If I'm connected to an always-on circuit, then that defeats the purpose of the isolator and I could suck power from my engine battery as my house battery drains. Right? So, where do I connect the isolator? I have another fuse box in the cab but I'd prefer to keep all the wiring under the hood.

 

[Stereo]

Firstly, Its called a "dohickey" not a whatchamacallit.

Rookie mistake. I'm still learning the lingo. Thanks for the lesson. :bowdown:

 

[Stereo]

Your dohickie thingofamabob it right here.

Thanks, but, I wasn't having problems finding the add-a-fuse device, just one that used 14 gauge wire, which your linked-to devices don't have either. BUT, given the advice I've just received, I'm confident I can use 16 gauge.

 

[Stereo]

BlueSea has some of the best wiring advice and tech on the internet. https://www.bluesea.com/resources/1437

The info on BlueSea was very helpful, but I'm missing two bits of info.

1. Is the battery-to-battery circuit critical or non-critical in regards to the chart? My circuit will probably be around 50'.
2. How many amps pass from the truck battery to the house battery while charging? I'm assuming not the full amperage of the battery. Is it however many amps the house battery is down?

 

[dwh]

1. Is the battery-to-battery circuit critical or non-critical in regards to the chart? My circuit will probably be around 50'.

For charging a lead-acid battery, non-critical.

Voltage drop is a variable based on load (amps flowing). As a lead-acid battery gets closer to full, its resistance increases and the amps flowing goes down - so the voltage drop decreases (and ultimately disappears).

(The opposite happens when feeding a load from a battery, voltage drop increases as the battery state of charge (SoC) decreases.)

2. How many amps pass from the truck battery to the house battery while charging? I'm assuming not the full amperage of the battery. Is it however many amps the house battery is down?

No amps pass from the cranking battery to the house battery during charging. The amps come from the alternator.

As to how many amps flow from the alternator to the house battery, that depends on battery SoC (resistance;variable), wire length/size (resistance;variable), alternator temp (resistance;variable) and the capacity of your alternator.

DiploStrat used to have a truck with dual 125a alternators, a huge battery bank and huge welding cable to the house batteries. And a microwave, electric stove and electric fridge to run the batteries down. IIRC he would see up to 150a going to the house batteries.

For a while. An hour maybe(?) before the amp flow started tapering off due to increasing battery resistance.

 

[DiploStrat]

...

As to how many amps flow from the alternator to the house battery, that depends on battery SoC (resistance), wire length/size (resistance), alternator temp (resistance) and the capacity of your alternator.

DiploStrat used to have a truck with dual 125a alternators, a huge battery bank and huge welding cable to the house batteries. And a microwave, electric stove and electric fridge to run the batteries down. IIRC he would see up to 150a going to the house batteries.

For a while. An hour maybe(?) before the amp flow started tapering off due to increasing battery resistance.

You forgot the air conditioner!

That is correct. Probably closer to only half an hour.

All other things being equal (see above), big wires make a great difference at the start of the charge cycle. The trick is that with lead acid batteries that initial amp flow starts raising the battery voltage and that slows the charging. (This is why the CTEK SmartPass actually works, even with 13.xv alternators - the boost kicks in when the amp flow drops to 20A.) Most of the time, the amp flow hovers around 50-75A. So, in the real world, 125Ah overnight drain takes at least three hours of driving and should have another couple hours of absorb charge. This is where solar is so handy.

So, if you want some rough rules of thumb:

-- Assuming alternators rated at more than 100A,

-- Size your wires for minimal voltage drop at 100A.

This will probably give you the most cost effective combo.

And remember, with lead acid batteries, it is that long absorb stage that counts. GO SOLAR! Your batteries will thank you.

 

[dwh]

I use my own method to choose wire size for battery charging:

"Size the wire to handle the max expected amp load. Ignore voltage drop."

So say a 100a alternator. That's the max expected, unless maybe there is a winch connected to the cranking battery...

Then winch max (say 250a) minus alternator (100a) equals 150a max expected (assuming you've drained the cranking battery already).


So say 100a max expected (according to the NEC chart, #3 w/90c rated insulation) and a 50' circuit length.

Let's plug those numbers into a handy-dandy online voltage drop calculator, here's one that has a mobi version so it looks okay on a phone:

http://www.calculator.net/voltage-drop-calculator.html

So..

copper
3 awg
12v (battery at 50% SoC)
DC
single pair of conductors
25' (one way distance)
100a

Calculate and OMGWTF!!??!?!???!

.99v drop!!
8.25%!!

Too much!!
Everyone says so!!!
Over 3% is bad!
Everyone says so!

Everyone, except that ************ dwh. That guy says for battery charging, ignore the voltage drop (what a tool).



Here's why:

That so-called 1v drop isn't really there.

The battery is at 12v. The alternator is running wide open trying to drive up the voltage. But it can't because the battery is limiting the voltage to 12v. So 12v is the voltage you get on that charging circuit. Wouldn't matter if you had a million amp alternator and 6" thick copper wire - 12v is what that charging circuit is operating at (goes up as battery voltage rises).

That voltage drop calculator says, "Voltage at the end: 11.01".

Really?

WHAT end? The battery end? The battery is already AT 12v! It sure as hell didn't magically drop to 11.01v.

The alternator end? That thing is running wide open with a 14v-15v potential. It is NOT at 11.01v.

So if the alternator is not at 11.01v, and the battery is not at 11.01v, then where the hell is this "Voltage at the end" they are talking about?

NOWHERE. NOT THERE. NOT HERE. NO FREAKING WHERE. NONEXISTENT.


Yes the wire has a resistance. Yes it has an effect. Ohm knew his ********.

The wire resistance doesn't drop the voltage (which is already being limited), but it will reduce the amps flowing.

A bit. For a while. Half an hour maybe.

Okay so you get a few less amps for half an hour and add a few minutes to the bulk charge cycle. Meh, so what? With lead-acid it's the absorb that takes hours.




Now, all of this applies to one special situation - battery charging. It does NOT apply anywhere else, such as feeding a load from a battery - where the voltage drop certainly IS real.

 

[dwh]

Now go back and change the numbers in the voltage drop calculator to some numbers that apply during that hours long slow absorbtion process - say 14v, 25a.

What's the (theoretical, not actually real) voltage drop now?

 

[dwh]

And as the battery approaches 100%, the voltage at 14 or more, the amps flowing down to 5 or less.

So now what's that calculator say?

 

[Stereo]

Thanks to all for the physics lessons and the practical application of that physics. I'm beginning to get a handle on how electricity works in conjunction with all the elements that impact its flow. As they say, you gotta hear/read/learn a new concept at least seven times, on average, before it's fully retained.

 

[dwh]

Thanks to all for the physics lessons and the practical application of that physics. I'm beginning to get a handle on how electricity works in conjunction with all the elements that impact its flow. As they say, you gotta hear/read/learn a new concept at least seven times, on average, before it's fully retained.

No worries. This site has over 100k members and way more lurkers than that. This thread has over 600k views.

So even if one guy whines about repetition, who gives a crap? I don't. He's no doubt outnumbered by people who are interested but don't speak up.

The repetition is for the benefit of the folks who are on the upslope of the learning curve and mostly lurking and learning.

 

[Stereo]

All the fuses are hot when the engine is off except for the taillights, headlights, and AC and the only time those fuses are energized is when those items are switched on.

So where do I connect the isolator? I can't connect it to an always-on circuit lest the isolator stays in a closed position when the truck is off, thus draining the truck battery into the house battery, right? And I don't want to have to turn on my headlights or AC to energize the isolator. (Do I have this right, or am I still a dunce?) Is my only solution to go into the cab's fuse box (or splice into one of the circuits off that fuse box, one that's off when the engine or aux is off)?

If I do need to find a suitable circuit outside the engine compartment, it might be more convenient for me to mount the isolator in the camper, instead of under the hood. It can be located anywhere between the two fuses on the cable between the batteries, right? As long as the wire from the power source (spliced wire) to the isolator is within recommendations?

 

[dwh]

You need a key-operated hot to power the isolator for automatic operation.

IGN hot is proper.

In a pinch, you could use an ACC hot, but it's not optimal if you tend to use the ACC a lot (like radio at camp). But if you never, or almost never use ACC, then it doesn't matter.

No one ever wires their relay to the headlights or the A/C or the horn...


And sure, you can locate it where convenient - the solenoid is just a switch. You can put a switch anywhere in the line.

 

[dwh]

Stereo,

What make/model/year is the truck?