Isolated/Auxilary Battery Help

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derektice

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I am in need of some wiring/circuit advice. I've done plenty of research and believe I am on the correct path but would like to get a bit of a back check on my learning/research so I don't start my van on fire during my remodel.

I have a 2003 Ford E250 with the 5.4L engine which has a 130A alternator. The previous owner did a build-out that utilized 4AWG wire directly off the starter battery to the back "camper" circuit with a 150A circuit breaker and small fuse panel. However, as I am remodeling my van I plan on adding one isolated auxiliary battery on the chassis rail so I don't drain my starter battery on longer trips. The loads of the auxiliary battery will be about 30 amps fully loaded. I plan on buying the Keyline 140A Isolator and getting a deep cycle battery that will fit in my chassis battery mount (Group 65, I believe).

So, my biggest concern is the size of the wire I should be using from my starter battery through the isolator to my auxiliary battery (about 13 feet of cable length from battery to battery). Attached is a circuit diagram of what I would like to do. 
I plan on using the existing 4AWG to connect the starter to the auxiliary battery. I would like to leave the 150A breaker before the isolator but since the isolator is rated to 140A I believe I should install a smaller rated breaker/fuse. and adding another breaker or fuse before the auxiliary battery.

My questions to the generous soul that wants to help are:
Is 4AWG adequate to charge the auxiliary battery?
What size of fuse or circuit breaker should I use after the starter battery and before the auxiliary battery?

Thanks in advance
 

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    van circuit diagram.jpg
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According to the wikipedia awg wire chart
https://en.wikipedia.org/wiki/American_wire_gauge
AWG 4 wire is good for 70 to 95 amps continuously depending on the insulation temperature rating.  The 10 second rating is 946 amps.  If the wire chafes against a sharp edge cuttiing the insulation and there is a short circuit AWG number 4 will hold up long enough to blow a fuse.  The resistance inside the battery and the resistance of the poor connection at the point of the short circuit will limit the current to the 946 amp neighborhood until the fuse blows.  In my opinion 130 amps is a bit too big. 

The fuse or breaker should be located as close as possible to the energy source, the battery.  The wire between the battery and the fuse is not protected.  If it chafes against a sharp edge and shorts a fuse located farther down the line will not provide protection.  If the wire is too short to be able to reach a good ground point it is less likely to be a problem.

When a fuse fails that means it does not open the circuit with a high current.  It is my opinion that fuses are less likely to fail than circuit breakers as they have no moving parts and the tripping mechanism is quite simple.  If you search the internet regarding Federal Pacific Electric you will find that the company is gone because they sold a bunch of breakers that failed.  Replacing an FPE breaker panel in a home is typically more than $1000. 

Breakers typically have a higher voltage drop than similarly sized fuses.

4 AWG is a good size for 13 feet but no size is really big enough unless you drive a long time every day.  Lead acid batteries are slow to charge.  The vehicle is set up to recharge the starter battery and not set up to fully recharge deeply discharged batteries.

Consider using a cheap battery as your "learning battery" to save tuition cost.  In the USA a flooded battery will be cheapest and it will allow the state of charge to be measured with a $10 hydrometer.
 
I’m not good enough with electricity to help you but I want to welcome you to the form anyhow.
 
Trebor English said:
Welcome to the CRVL forum.

Thanks for the welcoming and the lengthy response! 

Looking into specific isolators a little further, it looks like the Battery Doctor 140-150A Isolator calls for 4AWG wire for rear/trailer mounted batteries and one 150A fuse. I'm not keen on the single 150A suggestion so I plan on [font=Tahoma, Verdana, Arial, sans-serif]leaving the 150A breaker in the circuit as a rear battery cutoff switch for future maintenance and adding two 100A fuses at each battery for protection. [/font]I'll utilize the existing 4 gauge wire along with a "learning battery" to see exactly how the circuit works without the worry of extra cost.

I appreciate the input and amazing information. Thanks!
 
I do not recommend isolators. either a solenoid or a battery to battery charger.

Also those car audio components with the screw pinch terminal are junk IMO.

Also some 12v breakers only pass power in one direction.

highdesertranger
 
I searched on the intertubes for "Battery Doctor 140-150A Isolator" and came away with few hard facts.  It says voltage drop is "none" so it appears that it probably is not a two diode isolator.  It says the the current draw is 500 mA, a half amp, so it is likely a relay operated by a microcomputer chip that controls the on and off switching based on battery voltage. 

The 130 amp alternator and 150 amp isolator switch will not make 130 or 150 amps happen.  When the switch first turns on there could be a large current spike wearing or possibly damaging the switch contacts.  The current will fade very quickly.  Look for 10 amps to actually happen after a few minutes.  The engine computer runs the alternator at a high voltage (14+) to recharge the starter battery for a very short period of time then cuts back to 13.5 to 13.8 to minimize water loss in the maintenance free battery. 

The first question in the original post is "Is 4AWG adequate to charge the auxiliary battery?"  Given the low charging voltage and short driving time (less than 4 to 6 hours daily) there is no AWG that will charge the battery if it is significantly discharged.  If the max load of 30 amps never happens all at once and if there is no 20 amp hour per day refrigerator or CPAP machine and the actual load is just a cell phone, some led lights, and a couple of hours of small laptop it might do what you need.  if 4 AWG is not enough, 2 AWG won't be a good fix.
 
highdesertranger said:
I do not recommend isolators.  either a solenoid or a battery to battery charger.

Also those car audio components with the screw pinch terminal are junk IMO.

Also some 12v breakers only pass power in one direction.

highdesertranger

Hmmm. I appreciate you bringing the battery to battery option to the table. Although it may be more costly, it will most likely be a better option to future proof my van. I’m planning on adding a solar panel in the future, so if I opt for a B2B charger with MPPT, it will decrease future costs and headache of integrating 2 separate systems. I see Renogy has a 30A dual input with MPPT for $250.
 

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