What size wire? Big 3? IBS-DBS Second Battery Install ’14 Taco

[I_F]

So I’ve already purchased the IBS-DBS dual battery system w/microprocessor (http://www.sierraexpeditions.com/index.php?l=product_detail&p=2074) as well as a blue top Optima deep cycle battery. My plan is to run the OEM battery under the hood until it dies and put my blue top aux battery in the bed of my truck.
My dilemma is that I don’t know what size wire I need to run to the second battery in my bed (I have a cap) and whether or not I absolutely have to do “the big 3” for this.
As far as the wire size goes, I’m reading some people saying I need to total the length of the power AND ground wire both coming from the bed. I don’t have exact measurements, but I’m sure it’s at least a 12-15’ run, meaning I’m looking at 24-30’ total. According to some of the websites I’ve consulted regarding length of runs and current, I need to go as big as 2/0 wire. I had only anticipated maybe 2awg, and there is a significant price difference between them as you probably know. But I may not be reading these charts right either… I total the length of the run, and then I must determine the number of amps running through it at any given time, correct? That said, where do I get the amp number? From the alternator?

In terms of the big 3, f from the decent amount of research I did before I initially purchased the IBS-DBS (though that was 3 months ago… yeah, this thing has been collecting dust on my desk for 3 months :/) I was under the impression that it wasn’t necessary. However, after poking around a bit I’m seeing some people say it is imperative because when the vehicle is on the alternator will have to charge both. Based on some videos I’ve watched, wouldn’t the IBS-DBS be considered one-directional without installing a winch? Meaning that it will charge the main battery and when that is full then charge the aux battery? Thus not overloading the alternator?

Also, on a side note regarding gases emitted from batteries… I’ve read conflicting information on the optima blue cap batteries releasing gases. I will be sleeping in the bed of my truck and also have dogs that ride in there, so if these gases are being released I definitely need to rethink my placement or get a new battery.
Thanks in advance all!

 

[Adventurous]

Why would the ground wire from the battery in the bed need to be that long? Looking at that diagram I don't see any reason it needs to be any longer than a few feet to go down to a suitable ground point on the frame.

The Optima blue tops are AGM batteries. Under normal operating conditions there should be no release of harmful gasses.

 

[dwh]

WTH is "big 3"?

Regarding the wire size - like everything, "it depends". If all you are doing is charging the aux battery (chassis->aux) then you could get away with #10 wire. It would take longer to charge, but it would work (apparently, most factory 7-pin trailer plugs have #10 for the "hot").

If you want to charge faster, or need to supply a heavy load on the aux side while the engine is running (like an inverter running a microwave), then you'd want larger wire.

Voltage drop due to wire size is pretty much irrelevant in regards to battery charging. It doesn't prevent the battery from reaching a full charge, it just makes it take longer to get there. But even that isn't as bad as it sounds, since extra time is added only when the amps flowing is high - but when battery charging with an alternator, amp flow isn't going to be high most of the time.

If the battery is say 50% down, you'll see a lot of amps flowing (depending on battery resistance and wire size, could even be up to the limit of the alternator), for a little while. But as the battery voltage rises, and the resistance rises, the amps flowing will become less and less. Half the time, you'll be lucky if even 40 or 50 amps flow. Most of time, it'll be less than 30.

Big fat wire can get the battery up to 80-85% full a lot quicker - but that last 15-20% is still going to take a long time as the battery slowly absorbs whatever it can at whatever its resistance is at the moment.

So if all you are doing is charging that battery, than you can use #2. You could even use the #4 that comes with the kit if it was long enough.


You can ground to the frame. It'll work but it's not optimal, especially if you're a radio operator. The reason is that power conductors run together in pairs generate less electrical noise due to inductance. Running the positive through a wire, and then the negative through some other path can add noise. Matched pairs of wires is best practice but not required.


Lead-acid batteries only vent gases while being charged. Sealed lead-acid (VRLA) such as AGMs (Optimas are AGMs) only vent gases when they are over-charged and the safety valve opens to release extra pressure. You don't have to worry about sleeping in the same space with that battery as long as you don't overcharge it while you are sleeping.

 

[I_F]

Why would the ground wire from the battery in the bed need to be that long?

My understanding was that a direct run from negative terminal on the aux to the negative terminal on the main battery would prevent noise in my radio like dwh said. I am already experiencing a tad bit of ground noise because I wanted to keep my stock head unit, so I had to add a BitTen sound processor. I can handle the ground noise there now, but anything more will drive me insane. So based on that, is totaling the distance of BOTH wires the right method?

WTH is "big 3"?

Regarding the wire size - like everything, "it depends". If all you are doing is charging the aux battery (chassis->aux) then you could get away with #10 wire. It would take longer to charge, but it would work (apparently, most factory 7-pin trailer plugs have #10 for the "hot").

If you want to charge faster, or need to supply a heavy load on the aux side while the engine is running (like an inverter running a microwave), then you'd want larger wire.

Voltage drop due to wire size is pretty much irrelevant in regards to battery charging. It doesn't prevent the battery from reaching a full charge, it just makes it take longer to get there. But even that isn't as bad as it sounds, since extra time is added only when the amps flowing is high - but when battery charging with an alternator, amp flow isn't going to be high most of the time.

If the battery is say 50% down, you'll see a lot of amps flowing (depending on battery resistance and wire size, could even be up to the limit of the alternator), for a little while. But as the battery voltage rises, and the resistance rises, the amps flowing will become less and less. Half the time, you'll be lucky if even 40 or 50 amps flow. Most of time, it'll be less than 30.

Big fat wire can get the battery up to 80-85% full a lot quicker - but that last 15-20% is still going to take a long time as the battery slowly absorbs whatever it can at whatever its resistance is at the moment.

So if all you are doing is charging that battery, than you can use #2. You could even use the #4 that comes with the kit if it was long enough.


You can ground to the frame. It'll work but it's not optimal, especially if you're a radio operator. The reason is that power conductors run together in pairs generate less electrical noise due to inductance. Running the positive through a wire, and then the negative through some other path can add noise. Matched pairs of wires is best practice but not required.


Lead-acid batteries only vent gases while being charged. Sealed lead-acid (VRLA) such as AGMs (Optimas are AGMs) only vent gases when they are over-charged and the safety valve opens to release extra pressure. You don't have to worry about sleeping in the same space with that battery as long as you don't overcharge it while you are sleeping.

95% of the time I'll only be using the aux while the truck is off for things like fans or lights in my camper shell, but I would like it setup to allow for a winch if possible in case I add that in the future. My understanding is that you'll need the power of both batteries and the alternator for that? Based on this and me not wanting to destroy my battery by preventing it from fully charging, what size wire would I need? Certainly larger than 4, right? 2 wouldn't even do it, right?

I did find a chart and calculator about size, but do I base the amps on the chart to my alternator? The battery? How would I factor the winch into this? Also, the calculator I'm looking at shows how much of a voltage drop there will be based on what I put in. What is an acceptable/ideal voltage drop? <3%?

 

[dwh]

So based on that, is totaling the distance of BOTH wires the right method?

If you are using an online voltage drop calculator, then it depends. Some require you to input the total loop length (round trip), but some require you to input the one-way distance and the calculator doubles it to get the total loop length.

If you are doing the math yourself, then yes, you have to figure the total wire length of the loop (circuit).

95% of the time I'll only be using the aux while the truck is off for things like fans or lights in my camper shell, but I would like it setup to allow for a winch if possible in case I add that in the future. My understanding is that you'll need the power of both batteries and the alternator for that?

Again - it depends. If you don't tie the aux in while winching, then the full load will be supplied from alternator + engine battery. But how much will that be? Figure the winch might draw 250a FLR (full locked rotor), and your alternator is say a 130a unit. Then IF you ever tried to run your winch under so much load that it was stalling, then yes, the other 120a would have to come from the battery. But in that situation, you should use a snatch block, which would cut the winch load in half - and also cut the amp draw in half. So now the winch is drawing 125a and the alternator can supply 130a - so nothing comes from the battery at all.

Wouldn't matter if you had the aux tied in or not - nothing is being drawn out of the batteries anyway.

But even if you did have the aux tied in and the winch was drawing say 200a. The alternator would supply 130a of that, and the other 70a would come from the batteries. But most of that would be coming from the quick-discharge design cranking battery, and less would flow from the slow discharge design deep cycle battery. So again, you might only be seeing 30a flowing from the aux battery - no matter what size the wire is.

Based on this and me not wanting to destroy my battery by preventing it from fully charging, what size wire would I need? Certainly larger than 4, right? 2 wouldn't even do it, right?

Wouldn't do what? The wire size is not the main factor determining whether or not the battery reaches full charge. TIME is the main factor. Once the battery is "bulked up" to say 85% - it could take as much as 8 hours of driving to get it up that last 15% to "full".

You gonna be driving that much on a regular basis? Probably not.

It doesn't much matter how fast you get that first 85% done. Bigger wire can get it done faster. A bit faster - maybe the full charge cycle only takes 11 hours instead of 12. But that's not what will determine if the battery ever reaches full charge. Time spent in "absorb" will determine that - and that will be at relatively low amp flow rates because when the battery is that close to full, it has a very high internal resistance and simply won't allow a lot of amps to flow through it.

I did find a chart and calculator about size, but do I base the amps on the chart to my alternator? The battery? How would I factor the winch into this? Also, the calculator I'm looking at shows how much of a voltage drop there will be based on what I put in. What is an acceptable/ideal voltage drop? <3%?

You base the wire size on how many amps you need the wire to carry. So if your alternator can produce 130a, then you need wire that can carry that. If you need to feed a winch that can draw 250a, then you need wire sized to carry that. You size the fuse to protect the wire.

The battery is irrelevant - it's just a storage device like an air tank. What matters is not the size of the air tank, but the pipes that feed air in to and out of the air tank.

Again, when charging batteries, the voltage drop doesn't really matter. In fact, it doesn't actually exist. It's just theoretical.

If you size the wire appropriately to carry the expected amp load, the voltage drop will be somewhere around 1-2%. (At least, it will be over a short distance like <20'. Over longer distances...that's another discussion.)

Forget the voltage drop - it's a red herring in this situation. Focus on the amp loads the wire has to carry.

 

[I_F]

I just realized I didn't mention the "Big 3" - I meant upgrading the existing stock wiring (which I believe in my vehicle is maybe 6awg) going from the battery ground to chassis, chassis to engine block and alternator positive to battery positive to a larger gauge wire.

To clarify on your latest post:

Forget about voltage drops, worry about the load I'll have running through the wire. Which means: In terms of a winch (I use this because it's the biggest thing I'd ever run, of course), assuming 250A FLR like you did, If I have an alternator pumping out 130A, the absolute most that would be drawn from my batteries would be 120A, so the wire from my main battery to alternator would only need to support 120A, right? The aux battery would then only be sending a max of maybe 50, so wire from the aux battery in bed to main battery according to that, right? I will always overkill at least slightly to be safe, but in theory that's what it is. Then, per the diagram in my OP link, I would need to make sure the wire going from the alternator to the winch supports the full 250A (again, I'll overkill a bit to be safe)?

I think I'm on the right track so far, but now I want to clarify one other thing. Still assuming the above scenario at 250A FLR, I then would have to make sure the wire from negative terminal to chassis and chassis to engine block would handle 250A, right?


Now I know you brought up a snatch block, but I want to do a price comparison on wire based on 250A FLR and actual practical load at any given time. If it's a difference of $50, I might as well get the bigger wire to support 250A. If we're talking a difference of $200, then I'll wire for less amps. Odds are I'll never max out a 250A winch. Granted this may be stupid anyway, because my truck probably weighs a max of 4,500lbs and a winch at 250A is probably rated for something like 12,000lbs lol.

 

[I_F]

Thank you VERY much for all of your help by the way. You're a plethora of knowledge!

 

[dwh]

I just realized I didn't mention the "Big 3" - I meant upgrading the existing stock wiring (which I believe in my vehicle is maybe 6awg) going from the battery ground to chassis, chassis to engine block and alternator positive to battery positive to a larger gauge wire.

Ahh. Never heard it put that way before.

In terms of a winch (I use this because it's the biggest thing I'd ever run, of course), assuming 250A FLR like you did, If I have an alternator pumping out 130A, the absolute most that would be drawn from my batteries would be 120A, so the wire from my main battery to alternator would only need to support 120A, right?

No, the wire from alternator to the battery needs to be big enough to carry whatever the max that the alternator can deliver. The winch would be pulling 130a though the alternator to battery wire, then 250a through the battery to winch wire. So the alternator to battery wire would need to be able to carry 130a.

The aux battery would then only be sending a max of maybe 50, so wire from the aux battery in bed to main battery according to that, right?

You could, but I think you would limit the max charging going the other way by sizing the wire to only carry 50a. If you went with #2, you're good for 100a either direction. I doubt you'll see that much when battery charging, and it's unlikely that you'll draw that much the other way when winching, so you probably won't blow a 100a fuse.

But that's trying to save money on copper. Personally, I'd size it up to 1/0 (one ought or O) with a 150a fuse - that will probably exceed the alternator's output so no worries about charging, and it's unlikely you'll ever draw more than 150a from the aux when winching.

Then, per the diagram in my OP link, I would need to make sure the wire going from the alternator to the winch supports the full 250A (again, I'll overkill a bit to be safe)?

That diagram doesn't actually show a wire from the alternator to the winch. It's not an actual wiring schematic - it's a block diagram that shows the general arrangement. In reality, you'll have a wire from alternator to main battery, and a wire from main battery to winch. So no, the wire from alternator to main battery only has to carry the alternator's output (130a or whatever you alternator's actual rating is).

Only the short run of wire from the main battery to the winch has to be able to carry the 250a (or whatever).

I think I'm on the right track so far, but now I want to clarify one other thing. Still assuming the above scenario at 250A FLR, I then would have to make sure the wire from negative terminal to chassis and chassis to engine block would handle 250A, right?

Only if you didn't run a dedicated ground from the winch to the main battery. I would just run a matched set of positive/negative from main battery to winch. In that case, the chassis grounds never have to carry the 250a load.

Now I know you brought up a snatch block, but I want to do a price comparison on wire based on 250A FLR and actual practical load at any given time. If it's a difference of $50, I might as well get the bigger wire to support 250A. If we're talking a difference of $200, then I'll wire for less amps. Odds are I'll never max out a 250A winch. Granted this may be stupid anyway, because my truck probably weighs a max of 4,500lbs and a winch at 250A is probably rated for something like 12,000lbs lol.

Actually, I think I got that 250a number from remembering the FLR number on a Warn 8,000. And it'll only draw that much as it stalls - and you'll let go the switch at that point anyway. If you keep holding down the switch while the winch is stalled...well...you deserve whatever happens.

But yea, if you size the wire for the expected max amp load, you'll be good to go no matter how loaded up the winch is, and you'll also end up with a "theoretical voltage drop" of far less than 3%.

 

[dwh]

Thank you VERY much for all of your help by the way. You're a plethora of knowledge!

No worries. Old electrician, trained a bunch of apprentices. Passing on the lore is a habit.

Also old musician - same deal there.

 

[I_F]

Do you think I could squeeze by with 1/0 even if I did CCA? Or should I bite the bullet and pay the extra $60 for OFC?

 

[dwh]

Do you think I could squeeze by with 1/0 even if I did CCA? Or should I bite the bullet and pay the extra $60 for OFC?

You lost me. What does cold cranking amps have to do with anything? And what is OFC?

 

[I_F]

You lost me. What does cold cranking amps have to do with anything? And what is OFC?

CCA as in copper clad aluminum and OFC oxygen free copper.

 

[I_F]

I was able to answer my own question though with some sizing charts and rereading this post. Thanks again for all of your help man!

 

[dwh]

Ah. I wouldn't use aluminum.

They used to use it for branch circuits in houses in some areas. Had a few fires. The aluminum expands/contracts more when it heats/cools and the connections eventually get loose and become high resistance and end up as hot spots.

Nowadays, it's pretty much only used for feeders to breaker panel bus bars, where it can be stuffed under a set screw and torqued to spec so it doesn't loosen up over time and turn into a hot spot.

I wouldn't even consider it for a vehicle.