Questions about power inverter wires for negative terminal and chassis?

bkw11bkw11 Registered Users Posts: 6 ✭✭
I have a 2000W power inverter (CNBOU B12P2000W-1, 2000W Pure Sine Wave Inverter) with 1/0 power wire and negative wire running to a 150AH deep cycle battery.  I plan on using this to run in my van.  My questions are as followed (please forgive my ignorance on the subject):

As I understand there is a negative terminal on the inverter that runs from the inverter to the battery, AND there is also a chassis terminal that runs from the inverter housing to the van (in this case)?  The chassis terminal is WAY smaller than the negative terminal that runs to the battery.  Why is that?  And do I need the same size wire for the chassis terminal as I do for the negative battery terminal?  Can't I just use a 1/0 gauge wire for the chassis ground wire?  Or do I need another size?

Also, can the chassis terminal on the inverter be in used at the same location of the negative terminal that runs to the battery?  Or can I not do this because one is AC power and one is DC power?  I am confused.  Do I need a chassis wire install in the first place!?

Sorry for allt he questions.  Just trying to paint the overaall picture in my head.  Thanks.


Comments

  • BB.BB. Super Moderators Posts: 26,837 admin
    Chassis ground is always a bit confusing for vehicles... Normal car devices (radio, etc.) use the body of the car/truck for return power path (the negative lead). And for smaller loads, it can work OK. However, there are issues.

    One is that if you have a true frame of the vehicle and the chassis--many times there are rubber washers for noise and vibration insulation. And there is some sort of bonding/jumper wire from "chassis" to "frame" to make a good electrical connection (sometimes HAM radio folks will even bond doors, trunk lid, engine hood, and across rubber donuts to reduce radio interference issues).

    And that gets us to large devices like AC inverters... For your inverter, the maximum current is probably somewhere around:
    • 2,000 Watts * 1/0.85 inverter eff * 1/10.5 volt battery cutoff = 224 Amps (accounting for voltage drop, less than fully charged battery, etc.).
    That is quite a large current (probably more current than your starter draws)--So incidental electrical connections (spot welds for body panels, a jumper from chassis to frame, etc.) may not be low enough resistance to carry that large current without overheating (or excessive voltage drop). So, the reason most large DC devices (like AC inverters) have their own dedicated DC return line--And do not use the inverter's metal chassis for return current flow.

    But--You still have a metal chassis for the AC inverter that could short to internal wiring/devices inside the AC inverter. So you want a heavy enough chasses "safety ground" connection that the Fuse or Circuit Breaker from the battery to the AC inverter's positive input will open if there is a short (for example) inside the AC inverter to the chassis.

    Since the chassis ground is a "safety ground", it does not have to run for minutes to hours at high current--And can be of lighter/smaller wire than the DC input cables.

    For safety reasons, some inverter mfg. to recommend the same awg cable for chassis safety ground as the DC mains cables... But it has also been a standard (such as for house wiring) that you could use a smaller "green wire safety ground" cable to trip the breaker if there is a short. Roughly a 6 AWG cable will carry around 600 Amps before it "fuses". And (if I recall correctly), you can use a 6 AWG cable on a 200 amp AC mains service for a house's safety ground connection.

    And, it would seem that you could use a 4 AWG cable for a ~250 Amp fuse/breaker cable to your AC inverter's chassis "safety ground". (the chassis ground wire should be OK with 4 AWG or heavier--Heavier chassis ground cable should not hurt anything--other than trying to find a lug to fit both the cable and the chassis ground connection on the inverter).

    Now--There is another design issue--Pulling ~225 Amps from a 150 AH @ 12 volt flooded cell battery is a lot of current... Using our typical rules of thumb--A 150 AH @ 12 volt inverter will reliably supply a 375 Watt rated output inverter.

    Or, you would need close to a 800 AH @ 12 volt deep cycle lead acid battery to reliably supply a 2,000 Watt AC inverter (roughly 400 AH @ 12 volt battery per 1,000 Watts of AC inverter output).

    Remember that many AC inverters are rated to drive upwards of 2x their rated output power for a few seconds (to a few minutes)--Or around 448 Amps for your 2 kWatt inverter--Pretty heavy amount of current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bkw11bkw11 Registered Users Posts: 6 ✭✭
    Wow, OK.  Thanks!  I'm going to have to mull over what you wrote to digest it for a bit then I'll get back to you, but while I do that just a couple things to add:

    1. My inverter is a 2000W inverter with 4000W (peak watts).  The reason I chose 2000W inverter with a 4000 Watt Peak was because I wanted to make sure if any of my appliances were to surge the inverter could handle it.  I did not buy the inverter thinking the inverter would also surge to the battery.

    2.  What I plan on running in my van, I do not plan to exceed 300W at once.  And I know this because as of now I am running this desktop computer, a mini fridge, 2 KRK strudio monitors, etc. and am only currently getting around 106 watts usage from all those combined as read from an electric watt reader.  The watt reader says as of now I am also running 120 V or 1 amp as I write this.  However after looking at the high watt peak it has shown a high of 1659 watts (yikes), which must of been the high watt surge; the low was 2 watts  The only other thing I plan to run inside the van is a cooling van, really.  So my expectations is that the wattage used from the appliances will not exceed 300W unless they surge, which my understanding they typically surge 2xs their average running amount, which would be 600 W surge with a 300W combined of appliances running. 

    A 2000 W inverter will not surge the battery unless the appliances act on it first?  Or will the inverter surge the battery without influence from the appliances?  Because if the inverter surge to the battery depends on the appliance's influence, then I would be fine?

    I'll look more into it and get back to you.  Thank you for your response btw :)
  • BB.BB. Super Moderators Posts: 26,837 admin
    You are very welcome BKW,

    Yep--Over-sizing the AC inverter is a common thing folks do... Not thinking that any power out of the AC inverter needs to be supplied by the battery bank (including surge current).

    Besides cost for "too large" of AC inverter that cannot really be used--The other major issue is that larger AC inverters tend to take more Tare or "idle" current just to turn on and run the internal electronics. A small AC inverter (like this 300 Watt 12 VDC 300 Watt inverter) will take around 6 Watts, a larger inverter may take 10-20 Watts (or more). That is energy your battery needs to supply, that you cannot use.

    For example, a 12 volt @ 150 AH battery, nominally, we recommend to avoid discharging below 50% (for longer battery life)--A 20 watt Tare load will use up that battery in (roughly):
    • 20 Watts * 1/12 volts = 1.7 Amps average DC load
    • 150 AH battery * 0.50 maximum discharge * 1/1.7 amps = 44 hours (without any other AC load)
    Regarding your loads--I do have a question about your refrigerator... Is this an RV type refrigerator (uses propane with DC/AC backup power)--Or a typical 120 VAC "mini-fridge"?

    Unfortunately, an AC refrigerator it what pushes an off grid/battery based power system from small to "medium size"... And, unless you drive the van all of the time (many hours per day) and/or have an extension cord from a house/business to power your van (such as on a job site)--Many times an RV type propane refrigerator is a better solution (get one from a wrecked RV at a reasonable price)--Or a DC RV designed refrigerator (lower power requiremetns).

    For example, a 120 VAC refrigerator that takes ~290 kWH per year (typical):
    • 290 kWH per year * 1/365 days per year = 0.8 kWH per day = 800 WH per day
    • 12 volt * 150 AH battery * 1/800 Watt*Hour load per day * 0.85 AC inverter eff * 0.50 maximum battery discharge = 0.96 Days of usage (fridge from battery)
    • 0.96 day usage * 24 hour per day = 23 hours worth of stored energy
    Note that AC refrigerators (and induction motors in general) have very high starting surge current (power). And typically you need a minimum of a 1,200 Watt AC inverter, plus a large enough battery bank to support that surge (you would need a ~480 AH @ 12 volt flooded cell battery bank--Something like, at least, 4x 6 volt * 220 AH "golf cart" batteries).

    And if you discharge your battery bank to 50%--You will need a good size charger (10% rate of charge nominal) and >8 hours of charging energy to fully recharge the battery bank.

    We highly recommend that you understand your loads (Watts, Watt*Hours per day, etc.), pick very energy efficient loads/appliances, and then design the battery bank+charging sources to support your needs.

    Using a Kill-a-Watt type meter to measure your loads (and/or an DC Amp*Hour / Watt*Hour meter for your DC loads) is very important and gives you the information to design your system.

    Take care,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bkw11bkw11 Registered Users Posts: 6 ✭✭
    Well, I am using this mini fridge as an example because it's hooked up inside my room at the moment.  If I were to use a mini fridge inside my van I would likely go the gas route, so we can scratch out the need for the mini fridge in the van using current.  Currently, the one in my room is running normal 120VAC.

    I have used this calculator from this website to do the math:
    https://www.batterystuff.com/kb/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html

    I am using 120VAC system with 1 Amp AC usage with a 12v battery, which comes out to 11.04 Amps DC using this calculator.

    Once I found the DC amps, I then used that information and plugged it into this calculator to estimate the battery:
    https://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html

    11.04 Amps running for, say, 5hrs with AGM battery.  It tells me I need a battery size of 126 AH @ 20 hrs.

    If Amps = Watts/Volts then, say, 300 Watts/ 12V = 25 Amps.  With 150 AH/ 25 Amps, that is 6hrs * .80 eff = 4.8 hrs running time.  My reader tells me right now that I am running 180 Watts with my desktop, LCD screen, speaker monitors running with music (not including mini fridge which likely just turns on when needed).  I don't know why the wattage is so low, but that is why I am using 300 Watts in my calculation above.

    ^^BUT I'LL TAKE YOUR CALCULATIONS OVER MINE!  This is just me trying to put it together with what I currently understand!  Things I am not taking into consideration are somewhat blind to me which I'll keep researching.

    **Also, to charge my battery I am using a 200 amp battery isolator that charges from the alternator of my van, so when the van runs it charges the battery (http://www.sonicelectronix.com/item_16886_Stinger-SGP32.html).  In the future I will likely also have a solar panel to charge the battery, but not for a while.  I just want to rely on isolator for a while to charge the battery.  Everything is hooked up with 1/0 cables and 200 amp fuses.


  • bkw11bkw11 Registered Users Posts: 6 ✭✭
    edited September 2016 #6
    Whoops.  I guess 300 Watts / 12V = 25 amps....... to get it back to 120V system I need to multiply 25 * 10?  So, 250 amps?  Ugh
  • BB.BB. Super Moderators Posts: 26,837 admin
    I really like to deal with Watts (P watts = Volts * Amps). When you have multiple voltages (120 VAC and 12 VDC) it gets really confusing (1 amp * 120 VAC = 120 Watts = 10 amps * 12 VDC--Ignoring conversion losses for the moment).

    For example, you are using 180 Watts for your desktop computer system--An average amount. However, an average laptop (not the high power gamer type) can use around 30 Watts--Or about 1/6 the amount of power--Very handy if you are going to run with a battery bank (and solar power or what for battery charging). They can use upwards of 90 Watts to recharge the laptop's batteries--But that is only while charging, and is fairly efficient -- Power to battery to unplugged laptop energy usage).

    I highly suggest you do a few paper designs--First trying to figure out what your minimum power needs (per day) are... And then what your system should do (recharge from solar panels, genset, driving, plugging into wall outlet after a ouple days on road, etc.).

    (and before I forget--Remember a propane fridge needs a chimney/vent to the outside for combustion exhaust--Or bad things could happen to you--such as carbon monoxide poisoning).

    I know that it is very exciting to design the power side of the system at the same times you are researching your loads... However--Believe me when I say that (generally) your first step is to reduce your electrical loads as much as possible first--(laptop, not desktop, tablet instead of laptop, LED lighting vs other type of lighting, small vs large AC inverter, small DC fan vs larger AC fan, etc.). Battery powered devices (such as laptops, tablets, LED lighting, RV water pumps, and cell phones) tend already to be much more energy efficient than standard AC appliances.

    For most people, they only have a small space for a battery bank, and cannot really carry that much weight. And if you want to use solar panels--There is limited roof space.

    There are other solutions out there for off grid power systems used in RV/Vehicles--Such as using LiFePO4 batteries vs Flooded Cell or AGM (much lighter, much smaller, able to "surge" much more power if you have high/short term loads like starting a power tool)--But those tend to be more expensive and a bit more difficult to use (worrying about Batter Management System, very careful to fuse/breaker your wiring to prevent short circuits/fires, if you "kill" a LiFePO4 battery bank--Much more expensive to replace).

    But before we go down the power system road--Getting a handle on your Loads first will help a lot. We can do some simple paper design to see if it will meet your requirements (both power, space, and costs), or not.

    And some solutions are easier to get lots of power out of (a Honda eu2000i and 1 gallon of fuel can supply you with more energy than 4x golf cart batteries--And weight much less too).

    Sometimes the answer is a small battery power system (for lights, cell phone/tablet charging--quiet time/at night)--And the genset when you need to power the "big stuff".

    But without better knowledge about your loads and how many hours per day you run them and what the charging source is (solar, not solar, etc.)--I cannot even begin to guess what any solution will be best for you.

    We do all of this with simple "paper" design (and modeling with math). We really do want to save you money and have a working system (for you) at the end... We are not in this to sell you hardware (and almost all of us here are volunteers--With no financial incentives at all).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bkw11bkw11 Registered Users Posts: 6 ✭✭
    Thanks for all your support!

    The more I research about this the more I am confused.  Every "calculator" seems to give me different results, some being close and some not close.  Every article I read is different than the last.  And there really isn't THAT much information on the internet about my specific question so it's hard to choose who's right and who's wrong if I don't know for certain myself.  I suppose the best bet since I already have all these devices I talked about is to just test it out for myself and see what happens.  If the battery dies or drains very fast, or for how long, then I will know.

    But then again, just ditch the 120 appliances and go for 12V sounds like a MUCH easier solution :wink:  Then I can just do easy formula amps = volts * watts and plug and chug.  My battery will also last as I intended with this inverter mess I have found myself in.  Honestly, I just don't want to ditch my desktop computer as it is a custom built one with expensive parts.  I'll be int he van a lot so won't have anywhere else to put it unless sold...but anyway...

    To give a little more information on carbon monoxide issues in my van...I will have a fantastic vent circulating air through the top of the van and I will also have a carbon monoxide detector to go off if levels are too high.
  • BB.BB. Super Moderators Posts: 26,837 admin
    I tend to avoid the "calculators"--They hide assumptions and make it difficult to figure out where they are including deratings/safety factors/and assumptions about various solutions (inverter losses, battery losses, etc.).

    The equations we use here are relatively simple and we are clear about fudge factors--So you can decide which ones apply to you or not (days of no-sun storage, depth of discharge, and such).

    However--We really need to start with your loads and how many hours per day you use them. And your charging sources.

    I don' want to say "you cannot do that"--However, I do want you do understand that there are costs associated with your choices. And remember the batteries are not a source of energy--They are simply storage, and you must recharge them every day (or every few days) and those recharging resources are a critical part of the design process.

    You have not said what your power usage is planned to be... It could be 3 day weekend (say you run a business that goes to weekend fairs during the summer)--Or you could be living full time off grid. This could be fixed location (national forest areas a few weeks at a time, or you travel daily)--And where they system will be used (forest in deep shade; coastal regions with lots of marine layer; desert south west with lots of sun; up north of Chicago, or what)?

    Just to give you some ideas--Off grid power, when you take all costs into account run roughly ~$1-$2 per kWH (more or less, but a good starting point--Remember you are pre-paying 5-10 years of energy usage).

    Here is a very quick design based on your request vs what I would do just using "generic" paper parts (i.e., just sizing the system, not a full design).
    • (180 Watt computer + 25 Watt satellite receiver + 25 Watt router) * 10 hours per day usage + 10 watt LED lighting*5 hours per night = 2,350 Watt*Hours per day
    • 2,350 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery * 2 days storage * 1/0.50 max discharge = 922 AH @ 12 volt battery bank
    • using 6 volt @ 225 AH "golf cart batteries"-> 2x 6 volt batteries in series for 12 volts @ 225 AH; 4x parallel strings of 8 total G.C. batteries => 12 volt @ 900 AH battery bank
    • 922 AH * 14.5 volts charging *1/0.77 panel+controller losses * 0.10 rate of charge = 1,736 Watt solar array nominal (full time off grid) based on battery bank size
    And based on Chicago "hours of sun" per day:
    http://solarelectricityhandbook.com/solar-irradiance.html

    Chicago
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:
    Jan Feb Mar Apr May Jun
    1.68
     
    2.41
     
    3.43
     
    4.45
     
    5.25
     
    5.93
     
    Jul Aug Sep Oct Nov Dec
    5.93
     
    5.12
     
    4.29
     
    3.03
     
    1.83
     
    1.48
     
    Toss the bottom three months (use a genset during bad weather):
    • 2,350 Watt per day * 1/0.52 off grid system eff * 1/2.41 hours per day = 1,875 Watt array minimum (February break even month)
    Now--"my system suggestion" of using low power devices:
    • (30 Watt computer + 8 Watt cell phone wifi hot spot) * 10 hours per day usage + 10 watt LED lighting*5 hours per night = 430 Watt*Hours per day
    • 430 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery * 2 days storage * 1/0.50 max discharge = 167 AH @ 12 volt battery bank
    • using 6 volt @ 225 AH "golf cart batteries"-> 2x 6 volt batteries in series for 12 volts @ 225 AH (2x batteries total)
    • 220 AH * 14.5 volts charging *1/0.77 panel+controller losses * 0.10 rate of charge = 414 Watt solar array nominal (full time off grid) based on battery bank size
    • 430 Watt per day * 1/0.52 off grid system eff * 1/2.41 hours per day = 343 Watt array minimum (February break even month)
    So--More or less, using a laptop+cell phone wifi, your solar power system is now 1/4 the size of a full power desktop system+satellite networking at 10 hours per day.

    If you only need 1 day of storage, you can cut the battery bank by 1/2 (not the solar array)--But that leads you to more generator use (takes more hours of sun per day to recharge a deeply cycled lead acid battery). You could use a Lithium Ion battery bank (LiFePO4) and even get smaller/lighter than two golf cart batteries (more costly battery technology--But vary nice for RV use).

    Anyway--An example of how your choices affect system design. Notice that 120 VAC vs 12 VDC is basically the 85% inverter efficiency--Not a big deal, and many times, a 120 VAC system is much easier to use and wire (at the cost of 1/0.85 or 1.25x larger system). An off grid solar power system runs from 10.5 to 16.5 volts--That wide of voltage range can cause may "12 volt car adapters" to fail--A single (good quality) AC inverter can make things much easier (and you don't have a bunch of 12 volt adapters--which can have their own losses too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bkw11bkw11 Registered Users Posts: 6 ✭✭
    edited October 2016 #10
    Wow, thanks again!

    I plan on living off grid for a while (1 year minimum) full time in the van.  I'll be living in and around Chicago in the van.
  • EstragonEstragon Registered Users Posts: 1,543 ✭✭✭✭
    Note that we want to avoid discharging a lead acid battery more than 50%. Fully discharging them will greatly reduce lifespan and may also cause problems with equipment with low voltages (and higher currents) at very low state of charge.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • MenaMena Registered Users Posts: 4
    BB. said:
    Chassis ground is always a bit confusing for vehicles... Normal car devices (radio, etc.) use the body of the car/truck for return power path (the negative lead). And for smaller loads, it can work OK. However, there are issues.

    One is that if you have a true frame of the vehicle and the chassis--many times there are rubber washers for noise and vibration insulation. And there is some sort of bonding/jumper wire from "chassis" to "frame" to make a good electrical connection (sometimes HAM radio folks will even bond doors, trunk lid, engine hood, and across rubber donuts to reduce radio interference issues).

    And that gets us to large devices like AC inverters... For your inverter, the maximum current is probably somewhere around:
    • 2,000 Watts * 1/0.85 inverter eff * 1/10.5 volt battery cutoff = 224 Amps (accounting for voltage drop, less than fully charged battery, etc.).
    That is quite a large current (probably more current than your starter draws)--So incidental electrical connections (spot welds for body panels, a jumper from chassis to frame, etc.) may not be low enough resistance to carry that large current without overheating (or excessive voltage drop). So, the reason most large DC devices (like AC inverters) have their own dedicated DC return line--And do not use the inverter's metal chassis for return current flow.

    But--You still have a metal chassis for the AC inverter that could short to internal wiring/devices inside the AC inverter. So you want a heavy enough chasses "safety ground" connection that the Fuse or Circuit Breaker from the battery to the AC inverter's positive input will open if there is a short (for example) inside the AC inverter to the chassis.

    Since the chassis ground is a "safety ground", it does not have to run for minutes to hours at high current--And can be of lighter/smaller wire than the DC input cables.

    For safety reasons, some inverter mfg. to recommend the same awg cable for chassis safety ground as the DC mains cables... But it has also been a standard (such as for house wiring) that you could use a smaller "green wire safety ground" cable to trip the breaker if there is a short. Roughly a 6 AWG cable will carry around 600 Amps before it "fuses". And (if I recall correctly), you can use a 6 AWG cable on a 200 amp AC mains service for a house's safety ground connection.

    And, it would seem that you could use a 4 AWG cable for a ~250 Amp fuse/breaker cable to your AC inverter's chassis "safety ground". (the chassis ground wire should be OK with 4 AWG or heavier--Heavier chassis ground cable should not hurt anything--other than trying to find a lug to fit both the cable and the chassis ground connection on the inverter).

    Now--There is another design issue--Pulling ~225 Amps from a 150 AH @ 12 volt flooded cell battery is a lot of current... Using our typical rules of thumb--A 150 AH @ 12 volt inverter will reliably supply a 375 Watt rated output inverter.

    Or, you would need close to a 800 AH @ 12 volt deep cycle lead acid battery to reliably supply a 2,000 Watt AC inverter (roughly 400 AH @ 12 volt battery per 1,000 Watts of AC inverter output).

    Remember that many AC inverters are rated to drive upwards of 2x their rated output power for a few seconds (to a few minutes)--Or around 448 Amps for your 2 kWatt inverter--Pretty heavy amount of current.

    -Bill
    thanks for your nice answers. I just got a 1000w power inverter and wondered how to connect the  chassis terminal  and negative terminal. But I still have a question. In which circumstances should I use the terminals? or I should connect it with the car battery once I need to use it for my devices?
  • BB.BB. Super Moderators Posts: 26,837 admin
    In general, larger power devices (like AC inverters) are going to have dedicated + and - DC input terminals. Those should be conneccted directly to the battery (bank) with the appropriate size cables, switch/fuse/breaker in (typically) positive lead. The chassis ground on the AC inverter should be connected to vehicle chassis (if metal chassis of car/RV/truck) or directly via the "green wire ground" to the negative battery terminal.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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