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

Hello. Does jumping the chassis batt off the house batt bank hurt the house batt bank? I have a 2006 Sprinter with a 2.7L diesel engine. The house bank is a single LiFeBlue LB12200-HCLT 200AH lithium battery. Thanks. 

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • outwestbound
    outwestbound Registered Users Posts: 38 ✭✭
    Bill, Thanks so much for your time. I follow your calculations and can plug my figure into that methodology. I have a nice AGM chassis battery. At the chassis battery, there is a 100 amp fuse and #1 cable about 6 feet to a blue sea post, then 2/0 for another 6 feet from that post to my house battery bank. Very good info about house batt temp. The "batt boost" switch on my dash closes a 300A Cole Hersee solenoid to connect the two batts. Thanks again. John 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • outwestbound
    outwestbound Registered Users Posts: 38 ✭✭
    On the Sprinter conversion, and it's sad to say, but the market will likely be so flooded if this economy continues to decline. You may be able to buy a converted campervan for a price that may make a DIY effort unnecessary. 
  • fratermus
    fratermus Registered Users Posts: 48 ✭✭
    > Does jumping the chassis batt off the house batt bank hurt the house batt bank?

    It'd be easier on the system to connect the two for a while to allow the starter battery to come up to something sane.  Then start (with or without the two combined).  This would avoid pulling so much across the wiring and from the house battery at once.  

    Digression:  my voltage-sensing isolator has a button that combines the two for 5 mins.  I used it once when I ran an air compressor off the starter batt long enough to run it down.  

    > the market will likely be so flooded if this economy continues to decline

    Maybe.  The lemmings that follow YT/Insta influencers might bail.  If so, it sounds like a business opportunity for someone.  Buy'em cheap and arbitrage to where they will be used.  

    To me the situation reads this way: 

    1.  keep an asset one can live in offgrid and follow job opportunities, etc; or 
    2.  sell it at a loss

    Not a tough call for me.  We're talking about useful things here, not ludicrously fat-tired "custom" choppers or tulip bulbs.  


  • outwestbound
    outwestbound Registered Users Posts: 38 ✭✭
    Thanks fratermus, 

    Checked out your WordPress site. Sweet rig for sure. I've been fulltiming since 2014 (early retired).  I'm into finance and investments, so despite my willingness to spend on RVs, I tend to look at the issue practically. The Sprinter based rig I just built for specific international adventures has a basis of $50,000 and has 80,000 miles today. If I can get another 80,000 miles and salvage it for $15,000, then that's a capital cost of .44 center per mile in the form of recreational expense. Heck, you can take a family of 5 to Europe and piss away $35K in a month easy, so the Sprinter lasts longer. 

    I have another fifth wheel/F350 rig (my home) that I look at a little differently, wherein I delude myself into thinking that it's a savings of "economic rent" if I can boondock. No financial fiddling can escape the reality that the thing will be worth nearly zero in 10 years, and have to be replaced. RVs are a chain pull...…..but they are fun, like horses, boats and other fun things. 

    As it always does, the used market will flood in a bad recession. Everyone and their mother bought built vans and/or converted them in the last 7 years. IF one were in the market, best to wait and buy low; maybe a nice Roadtrek. This is how I bought my fifthwheel. I paid the fellow about .45 cents on the dollar for what he'd paid 3 years before. It's just reality and he was happy to sell it for cash. 

    And in 10 years, the $125 thousand I dropped for my "home" will be worth $35,000 - a chain pull, but a fun time!