using class c vehicle "battery boost" switch to jump chassis batt off house batt bank

First, you need to look at the LiFe specs (this battery?):

https://www.lifebluebattery.com/ewExternalFiles/LB12200-HCLT%20Data%20Sheet.pdf

Discharging Discharge Temperature Range -4°F~140°F (-20°C~60°C)
Discharge Current 150A, 200 for 30 minutes
Discharge Current, 3 seconds 500 Amps
Discharge Cut Off Voltage 2.0 Volts, any cell (8 Volts)
Recommended Low Voltage Disconnect 11 Volts
Short Circuit Protection 850A for 500µs
Short Circuit Protection release 30 Seconds

So, you need to first ensure that the connection from your LiFe through the cable, to the vehicle battery does not exceed the above numbers.

First, will assume a "bolted" up permanent "house to vehicle" jumper system--This is probably the "worst case" (most current) you will see. Jumper cables typically are longer (and many are smaller diameter copper cable that suggested below).

And, that issue is--Assume that the LiFe battery has almost no resistance... So that means the next two major sources of resistance are the wiring from the LiFe to the vehicle battery, and the vehicle battery itself. Note that a fuse/circuit breaker will have significant resistance too--But ignore for now.

Just for the heck of it... Say I pick 6 AWG "out of the air" for wire gauge. And this will "fuse" at (very approximatly) 600 Amps. The NEC table says 6 AWG is good for 55 to 75 amps (depending on insulation, ambient temperature, conduit fill, etc.).

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

And a typical boating spec is around 120 Amps maximum):

https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity (see bottom of page).

So, lets use 120 Amps as the peak current, and use a 120-150 Fuse/Breaker (just in case).

Lets guess that you have a "nice" vehicle battery, and assume 0.050 Ohm resistance (1/2 the 100 mOhm resistance from this link). 120 Amps will give:

https://batteryuniversity.com/learn/archive/how_does_internal_resistance_affect_performance

• V=I*R= 120 amps * 0.050 ohms = 6 volt drop

So... That makes 120 Amp surge current unlikely... Going backwards, how much current would such a battery accept:
I=V/R= (13 volts LiFi - 10.5 dead batt)/0.05 Ohms = 50 Amps (or less) max current for Battery to Battery transfer

Makes 6 AWG guess look pretty good and safe. And even a 50 Amp fuse would not probably not blow.

Using a voltage drop calculator to figure out how much wire voltage drop we need to get from ~13 volts (charged LiFe battery) to ~10.5 volts (dead battery) or 2.5 volts drop with 6 feet (one way run of 6 awg cable):

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=1.296&voltage=13&phase=dc&noofconductor=1&distance=6&distanceunit=feet&amperes=120&x=8&y=26

Result

Voltage drop: 0.57
Voltage drop percentage: 4.38%
Voltage at the end: 12.43

So the 6 AWG cable adds another ~0.57 volt drop... Which reduces the surge current (again assuming your battery has 1/2 "typical" car battery resistance):

• (2.5 volt available battery drop - 0.57 volt wire drop) / 0.050 Ohms vehicle battery resistance = ~39 Amps max current

So--A 6 foot @ 6 AWG cable, a 50 Amp breaker (in lue of switch/contactor) as the on/off switch would seem to be "adequate".

Your next choice is to "switch the LiFe breaker On" and either wait for a few minutes, or start cranking right away.... While it probably would work, you would risk tripping the breaker if cranking went on for a while. And there is the (at least on my old diesel Rabbit) the 50 amp (as I recall) draw of the glow plugs (30 seconds or so), etc...

Anyway, that is how I would rough out the calculations for such a circuit. And it would seem not to be damaging to your LiFe battery (150 Amps for 30 minutes). 6 AWG cable should carry the current fine. And for "one off" situations, a 50 Amp or so DC rated Circuit breaker would be fine.

Of course, you have to watch the secondary issues... The LiFe can only discharge down to -4F... And if you have a "dead" Lead Acid battery, you run the risk of freezing it in cold conditions (fully charged, FLA batteries can run down towards -70F or so, a fully "Dead" FLA battery will freeze just below 32F and can crack the plates/case). If you have AGM battery for your Vehicle, they do not freeze hard--But when frozen (near dead, sub freezing weather), you cannot really charge them until they are more or less thawed out.

Regarding the freezing stuff--I where I live, "really cold" is 40F on average. So, I am not the guy to ask about jump starting a vehicle in snow.

And I have made lots of assumptions above (like the resistance of a typical car battery)--All of this stuff depends on exactly what type of battery you have for the vehicle, what you call "dead", that your minimum ambient temperatures are, how much current glow plugs and starting take for your diesel, etc.

But this should get you going on a little more research/experimentation on your side... And answers that your LiFi battery should be fine in almost any practical setup for jump starting you would use.

If you use just a pair of jumper cables with alligator clamps--The clamp connection is usually "not very good" (more resistance, less current). And it depends on the AWG and Length of your jumper cables.

-Bill

You are very welcome John...

And you can always do a little testing too... Get a DC Current Clamp DMM (really AC+DC), and you can monitor the draw on the AGM and on the transfer circuit when the LiFi is switched in and the truck is starting (lights on, crank early when the glow plugs are heating, etc.) and see what current draw you measure on the LiFi:

https://www.amazon.com/UNI-T-Digital-Current-Capacitance-Multimeter/dp/B0772FYF5M (good enough for our needs)
https://www.amazon.com/gp/product/B019CY4FB4 (mid price meter)

And, as always, if you have a fuse in a critical circuit--Having a spare or three is not a bad idea either.

Oh boy, my wife wants to do a Sprinter Camper conversion Soooo Baaaadddd... 

-Bill