Basic wiring question
- Thread starter Wiley
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Michael
Adventurer
My philosophy is a bit different when it comes to fuse size selection. I have a set up very similar to yours (aux battery for non-OEM loads and Blue Seas ACR) in my Tacoma. Photos with wiring diagrams of the system are here and here.
For my fusing, I calculated what my maximum load currents should be, not what the wiring was capable of carrying, and then chose a fuse size that gives me some room for starting surges (and wrong guesses) but blows readily if something is out of the norm. For instance, I use 80 amp fuses at both ends of my main #4 ga. buss line between my ACR in the engine compartment and the aux battery in my camper shell (wiring diagram shows 60 amp fuse, but I increased it when I added more equipment). 80 amps is far below the amperage that a 4 ga wire is capable of carrying (135a). My charging current and loads don't come anywhere near that and it has never blown in the 3 years I've used the system. The experts may jump in and say it is just a matter of time, but my thought is that if something grounds out, then I want the fuse to blow as soon as possible - especially if the buss itself gets pinched or worn and grounds to the frame. In my thinking, 'down-fusing' gives me more protection against abnormal conditions. I'm more interested in avoiding a major short or fire than in protecting the cable from melting. My system allows me to switch the batteries, or put them in parallel for a last-ditch starting effort, and I can start my engine from the aux battery alone without blowing the 80 amp fuse. If you're ever going to draw max current from the aux battery however, then fusing it for max capability makes sense. From reading all the threads about aux battery systems, it's clear that there are a thousand ways to skin this cat! Contrary opinions welcome!
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dwh
Tail-End Charlie
Which charging system?
Yea, the truck's alternator/voltage regulator charging system would be on the engine side. If you added another charging system, such as a battery charger that plugs into shore power or a generator, then it wouldn't matter which side you connected it to - that particular Blue Sea device will tie the batteries when it detects charging current on either side.
And that actually, could turn out to be a bad idea...
If you connected a 3-stage charger to the aux battery, and the aux was low, when you fire up the 3-stage the ACR will tie the batteries and you could overcharge the engine battery.
That wouldn't be a problem if you used either a constant voltage charger (like a regular old car battery charger), or a 2-stage charger like an Iota without the IQ/4 module or a Samlex with the dip switch set to 2-stage. It would only be an issue if you used a 3-stage that has an absorb stage.
dwh
Tail-End Charlie
Not exactly.
#4 is rated up to 135a when used as "chassis wiring". "Chassis wiring" does not mean "chassis of a vehicle", it means "inside of a box or machine". The wiring inside a Mr. Coffee is an example of chassis wiring. The wiring inside of a window unit air conditioner or a drill press is chassis wiring.
Battery to inverter, or fuse block or whatever is power transmission.
#4 is rated for a max of 60a when used for power transmission. That's what you are using it for.
The purpose of a fuse or breaker is to protect the wire. The wire has to be large enough to supply the load. The fuse has to also be large enough to supply the load, but small enough to melt before the wire does in case of a short.
You are correct that one could choose to use a smaller fuse, which will certainly melt before the wire, as long as it is large enough to supply the load.
Michael
Adventurer
I appreciate the feedback! I'm still comfortable however, using the "chassis" amp rating for #4 gauge wire in my application. I was unclear about the difference between "chassis wiring" and "power transmission wiring" when I worked on that project and did some research regarding the two applications. Both my research and discussions with several electrical engineers schooled me that the difference was all about dissipation of the heat generated by the internal resistance of the conductor. Amps are amps in any cable, the issue is the insulation. More amps = more heat. The purpose of limiting the current carried by power transmission lines is to avoid failure of the insulation. The reduced capacity for power transmission cables is based on the fact that power cables are often run bundled with other power lines, and usually in a vault or conduit that limits cooling. Chassis conductors, whatever their length, are typically run in open air and benefit from convection and radiation cooling. The cables available at your local auto or marine store are usually inexpensive, and the insulation likely isn't the best available, so your point is still a good one in applications where heat dissipation is limited. Although wire and cable runs in Expo-type projects are usually very short, conservative estimates are always safer if you're not a professional. The only penalty in choosing a larger diameter wire is spending a bit more for it and wrestling with the stiffer cable -- or a few extra ounces if you're pushing GVWR
Thanks for all the good info in this thread!
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LandCruiserPhil
Expedition Leader
There can be a lot of argument on wiring fusing and in the end its all about being safe. I break it down into categories 1 being loaded and the other being a dead short. I have #4 between my batteries and fused at 150A. For my application I don't ever see a 150A load being used for no more then 10 sec. in a self jumping situation and #4 fused at 150A is fine. The other side in the event of a dead short I found 150A to blow in under a second. I see my system safe under the perimeters I use it in. That being said I would not dump a 100A load for any given time on #4. The biggest problem with dual battery set up I see is the owner not understanding how his/hers system works. Keep it simple
4RunAmok
Explorer
With regards to mixing battery types, here's where the problem is.. AGM batteries recover/charge faster than Flooded Cell, and mixing the two in a system that pairs them for charging will confuse the voltage regulator, which will continue to deliver voltage at the rate needed by the battery measuring as low, the Flooded Cell, and the voltage regulator will continue to charge both batteries, in essence overcharging the AGM, weakening it, if not kill it!
Borrego60
Rendezvous Conspiracy
dwh
Tail-End Charlie
Nah.
It's impossible to confuse a voltage regulator. It's just a dumb switch. When the voltage drops to X, it turns on, when the voltage rises to Y, it turns off.
Since both Flooded and AGM batteries are considered fully charged at approximately the same voltage (14.4v while charging or 12.8v resting) it doesn't matter if they are connected together when charging with a dumb charger.
The problem described in the quote happens when batteries of different types (or sizes or ages) are connected to a "smart charger" or "3-stage" charger. I.e., a charger which has an "absorb stage". With that type of charger, then yes, in the absorb stage, what is described in the quote can happen. The charger can get hung up holding an elevated voltage for many hours to get the FLA fully topped off, and it ends up being far too many hours for the AGM. (Or with different size batteries, the smaller gets full first, and ends up overcharged by the time the larger finally gets full.)
But for a dumb single-stage charger (i.e., cheapo benchtop automotive charger, or automotive alternator/voltage regulator setup), or a "two-stage charger" (such as Iota without IQ/4 or a Progressive Dynamics), it's a non-issue. For instance with a 2-stage, since there is no absorb stage, the charger will run a bulk stage until both batteries reach a 14.4v "surface charge" and then the charger will drop down to float voltage and it doesn't matter if the AGM absorbs and reaches full in 12 hours and it takes the FLA 18 hours to get there. Either way, the float voltage isn't going to overcharge anything.
For a dumb benchtop charger, it's likely just taking the batteries to 12.8v and then holding there, and again, it doesn't matter how long it takes each individual battery to finally reach 100% - 12.8v isn't going to overcharge either of them.
You DON'T mix batteries of different types, sizes or ages in a PERMANENT battery bank. You DON'T mix them if charging with a charger that has an absorb stage.
Tying different batteries together WHILE CHARGING with a charger that does not have an absorb stage is fine.
LandCruiserPhil
Expedition Leader
keezer37
Explorer
Am I missing something? The cables cannot be removed from the posts? I have a AGM (sec) and flooded cell (pri), had this set up for five years. I disconnect and run a three stage charge every fall and spring. Flooded takes about 18 hours, Odyssey takes about 3 hours. Same thing every year. Same results every year (almost).
You know, you can gnat's *** this electron flow thing from now until hell freezes over, you will always hear comments about "best practices". The thing to consider is, to what extent is this detrimental? Is this really a concern?
Someone once told me (3 years ago) about the see-saw effect because I have my ground running through the frame. I'm still waiting for that to destroy my batteries.
Horsepucky.
dwh
Tail-End Charlie
Different levels? As in one more drained than the other?
If they are the same size, age and type, then it won't make much difference if one is lower than the other, they'll both end up at the bulk stage surface charge voltage and then require roughly the same time on absorb to reach 100%. So a 3-stage would be okay there.