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Thread: Where to put the power inverter...

  1. #11
    Join Date
    Oct 2008
    Also, what are some preferred brands for inverters? When I was picking them up for the acura I was in a hurry, so I swung by home depot and picked up a 750w black & decker for cheap.
    Then realized that I couldnt run 750w so I ended up returning it and buying the cheaper black & decker for 300w.
    I imagine there are some higher quality brands around.

  2. #12
    Raw Wave
    Join Date
    Nov 2009
    Sorry - just back from a gig. Some Hollywood band called Evil Beaver. (Yeah - some band! I saw them last week at another gig. Very impressed. Not everyone's cup of coffee being merely a drummer and a bassist/singer.)

    Re inverters (and amps etc), a rough rule (or ROT = Rule Of Thumb) is to divide their output (Watts) to get input DC Amps. Hence a 2,000W inverter (or amp) will take up to or about 200A at typical 12V vehicle voltages under FULL load. That's more than most highbeams and may be of a similar Amperage to starter motors.
    But that's at FULL output. (And it's RMS aka average output - peak Watts is irrelevant!)
    And if you only run a 50W or 90W PC then it's only 5A - 9A instead of 200A... PLUS the inverter overhead ie standby current which these days is usually small - maybe a few Amps.

    FYI - My first inverters were 150W etc for the odd 240VAC chargers I had. And of course the all important coffee grinder.
    Though those 150W inverters have now been replaced by 600W inverters, my current consumption is the same because the loads have not increased. In fact the load is probably less since the newer 600W inverters have a lower standby (aka idling) current as well as better efficiency. (The latter despite inverters typically being most efficient at say 70-89% of full load, but since overall efficiency of the newer is better, a 10-20% loading of the 600W inverter is less than or similar to the 40-80% loading of the 150W inverter.)

    I think you understand re heat and lifecycle/lifetime. If NOT running, the temp essentially does not matter. (Provided its "storage" temp specs are not exceeded. But whilst a device may only be rated to 85C for OPERATION, it may have a storage temp up to 110C etc.)

    And essentially a device has a certain temperature rise during operation (at a specific load). Put that in an engine bay which is say 50C hotter than the cabin, then add 50C to its cabin operating temperature - hence a 32x shorter lifespan in simple theory based on half the life per 10C increase. (It will actually be cooler due to increase infrared heat losses, but hey, this is supposed to be a KIS else worst-case deign.)
    But if a component exceeds its max operating temperature, then it's instant death. Many components have an 85C rating thou some have 105C etc (eg, some capacitors).
    And engine bay temps vary greatly. Since most thermostats are rated at 85C, that means a coolant exit temp of at least 85C. If 85C, the air temp of air coming thru the radiator will be a MAX of 85C, but it might only be 60C etc depending on air speed. But then there might be cooler air from under the engine, but maybe (or probably) not in the upper engine bay where <whatever> is mounted. And then there's the MUCH hotter air coming from the hotter (than 85C) exhaust and splugs and maybe head. But if the sides of the engine bay are cooler due to wheel-well cooling and front venting... [Tho radiator cooled vehicles are usually designed to pull air thru the radiator - hence no other venting, and hence radiator shrouds etc. (Hence why removing shrouds or having bonnets partly open may result in engine overheating.)]

    Geez I can go/ramble on. And on...!
    In short, avoid the engine bay. Tho as SNO suggested, an enclosure - which is ALSO vented from outside - WITHOUT letting in water or dust & contaminants... (And IMO filters have their own hazards, but I'll skip that ramble!)

    How would I design it? Let's assume a rear mount for all the reasons above.
    A 22' run - lets make that 10m (33').
    Assume my (ideal) design limit of 0.5V max drop.
    And let's go the full 2kW for now.
    Hence 200A at 0.5V (max drop). V=IR hence R=V/I = 0.5/200A = 2.5 mR (milli-Ohms) for 10m (=0.25mR/m) = 250mR = 0.25R per 1km.
    From powerstream's 'merkan Wire Gauge Table, 00G is 0.255512 R/km.
    Damn - too big (a cable) for my usual design approach.

    Ok, defeated for now...
    Let's assume 2G and be more accurate.
    2G = 0.1563 R per 1000 feet or mR per foot. (Dear Lord, forgive me for using Imperial! But hey Man Esq, let's face it - it makes sense in this case! PS - IMHO of course!)
    Hence 22 x 0.1563 = 3.4386 = 3.5mR.
    200A a 3.5mR = 700mV = ~0.7V for that 22' run at full 2kW output.
    Ok, not too bad...
    Let's assume a total 1V drop at full load taking into account a fuse & various connection losses BUT assuming a negligible GND resistance - ie, a short run of 2G or better (2 x 2G?) to body/chassis which gives a near zero resistance to battery -ve.

    Many inverters cut out at 10.5V (usually to protect the battery, but can also be to protect the inverter), hence with 2G, full 2kW output should be achievable with a battery terminal voltage of 11.5V and higher.

    I'll leave it there because the next step needs definition - eg, what is your battery terminal voltage @ 200A - and that depends if the alternator is charging (and what it outputs at the relevant RPM) or not.
    But assuming it's the only load with engine off and the battery is fully charged (say 12.7V internal voltage), that means a batter internal resistance of less than [12.7-11.5V = 1.2V/200A = ] 6mR which essentially means an AGM battery - and that's only for start up. How long it lasts depends on batter capacity. (Tho AGMs are usually ~12.8V internal voltage, but that's only minutes difference.)

    Ooops - I was going to leave it yet I started the next step's confustigation.
    Last edited by OldSpark; 04-11-2014 at 12:39 PM. Reason: Evil Beaver ate too many letters & words!

  3. #13
    Join Date
    Oct 2008
    Wow, that is a lot of useful information.
    Funnily enough, I'm in school as a mechanical engineer and the imperial unit drives me insane as well. Unfortunately, it's hard to reset the hands on portion of the brain to metric when everything is written in imperial. I'm not averse to using 00 gauge wire if that's what needs to be done. I was just hoping for an easier solution. Alas it is better to run 00 and run it once than run too small a wire and spend the remaining time kicking myself for being cheap.

  4. #14
    Raw Wave
    Join Date
    Nov 2009
    Whether it needs depends on many factors.
    I've given an example for a 200A load with a max drop of 0.5V - ie, max 2.5mΩ total - and assumed it's all cable - ie 33' of cable. But even that is too thin if you're running solely off a fully charged AGM battery with ~6mΩ internal resistance. And since your cranking battery is probably a flooded type and hence probably over twice that internal resistance (eg, ~14mΩ), it still won't work.
    Remember - when cranking, a battery's terminal voltage might drop to 10.0V or lower but an inverter at that voltage would simply turn off.

    However, if the alternator is charging and outputs say 250A at idle or 2,000 RPM hence maybe a 400A alternator etc...

    Or if you only have a 500W load instead of 2kW...

    And there is a point where the battery becomes the issue - ie, drawing 200A for a minute may severely effect battery life - as well as leaving you stranded somewhere. So hence dual batteries which ~halve the internal battery resistance, or maybe 2 separate AGM batteries in parallel each with similar capacity to your wet-cell cranker which would have 1/4 the internal resistance of the cranker.
    And since the extra battery(s) might be in the rear & hence a shorter run to the alternator... However if the inverter is to run for a long period whilst the alternator is charging, then there is no saving in cable sizing. In fact the cable needs to be larger if battery recharge currents are involved.

    But that's typical Engineering. There is no single design nor solution - it depends on what is desired and what is used etc.
    The first step however is to decide what load you are designing for - ie, how many Watts? (Then how long for, with or without the alternator, etc.)

    If you have the 2G already, I'd do a temp install or mock-up and test that.
    Last edited by OldSpark; 04-11-2014 at 02:43 PM.

  5. #15
    Join Date
    Oct 2008
    Standard use for the inverter will be for xbox & TV, etc for road trips. Small battery chargers (18v), the laptop, when working on stuff. Small mini fridge for camping trips (not even sure about the mini fridge).
    Larger more non-standard use will be... plugging in blender when at the beach, powering a small pump to clean out the little horse trough, running small air compressor when the tractor tire pops off in the field.

    Currently my plan is as follows...

    AGM battery installed in the secondary battery position in the truck (There already a mounting place for that). Isolator to prevent the truck from being drained.
    2 Gauge wire running from the battery to the isolator to the alternator (most likely excessive, but I have the cable so why buy anything smaller, right?)
    000 gauge wire running from the battery to the back of the vehicle for the inverter. I will also line off the 12v for a set of lights off the back. They are smaller lights (3") and only use 55w so it really shouldn't be an issue, but I upgraded the wire size for that reason.

    Ideally I should never need to pull all 2000w from the inverter for more than a few minutes and I most definitely wouldn't do it with the truck off. In the event the truck does die (as is my luck) the starting battery is isolated anyway. I would worry about the inverter damaging the battery, but seeing as most do an auto kill at 10.5v (i remember mine doing that) im not too worried.

    Whatcha think?

  6. #16
    FLAC PhilG's Avatar
    Join Date
    Mar 2009
    Ontario, Canada
    Quote Originally Posted by Rogue909 View Post
    I was thinking about that, the passenger panel is taken up by the rear HVAC system (blower stuffs) but the drivers side is mostly clear. Unfortunately that leaves me running the length of the whole suburban (22ft) and across.... conservatively were calling that 25ft-30ft of run. For 2000w thats going to be a large cable.
    What cable did you end up using when you ran the 1500w inverter? and How long was the run?
    Ran from the driver side battery to the rear wall in the cab, around 20' with down to the frame and back up to inside the cab if I recall. I ran 2/0 like this:

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    My 2007 Ford F350 Work Log located HERE

  7. #17
    Raw Wave
    Join Date
    Nov 2009
    Quote Originally Posted by Rogue909 View Post
    Whatcha think?
    I don't think - I know it all and merely regurgitate!

    The 000G will probably not help. Tho it has 2/5ths the resistance of 2G, the 2G resistance over 22' is probably less than the (internal) resistance of the fully charged AGM. (And as a battery discharges, its internal resistance increases.) And there's still connection and protection resistances.

    The inverter solution solution is to use a dc-dc converter at the inverter end... (Yeah - so for an 18V laptop the electrical path is dc-ac-dc - dc-ac - ac-dc as opposed to a mere 12V dc-ac-dc). And people wonder why people are so confused about the sexuality of ions & electrons!)
    However IMO the main solution is to convert your loads to dc...
    Get a 12V air compressor (I use one daily thanks to a tyre I recently bought).
    A 12V TV if it's smallish (a LED LCD of course!).
    And I'm sure I've written ages ago about my FIGJAM $145 Peltier fridge (... laughing at those that spent 10x as much on equivalent size fridges) and why I chose later to spend ~$1,499 on an Engel MT45FP (RRP; $AUD, but got a preloved unit with bag for $900).

    Or use your loads selectively - not all at the same time. It doesn't sound like any load is particularly big except perhaps for the compressor - and as Rodney pointed out, inductive/motor loads are an issue for many inverters. A general rule is to use an inverter with at least twice the motor rating tho for compressors they often suggest tripling the rating. (Again, these are continuous ratings. "Peak" ratings are irrelevant except where f.ex a motor or load quotes a peak inrush current. Otherwise 'peak' power is merely twice the continuous Wattage (aka RMS) rating, except where wankers use their own peak rating method.)
    And in any case, for critical loads, you probably would not want to share a motor's inverter with other loads.

    Another change I'd make is to place the AGM in the back with the inverter. That's twofold - reduce the voltage drop between the battery and inverter (since at 2kW it will be the battery supplying the inverter), and remove the AGM from the hotter engine bay (hence it should last 8 to 32 times longer using the 10/15F rule).
    Of course if this was a caraudio forum I'd suggest placing the AGM in the engine bay so you get a 20-30% capacity increase, but for you guys I suggest TWO AGMs elsewhere hence costing you at least 4x less in batteries longterm, AND giving you 100% extra capacity instead of a mere 20-30%. (Some audio forums are just so cute!)

    With a remote mounted battery you can use a smaller isolator if charging during operation is not an issue - ie, it only connects to recharge the battery and run small loads. But then there's the fusing or how to cut it out during overload issue...
    But upping the rating of an isolator is usually no problem - just have the isolator energise whatever relay is required (200A, 400A etc) hence the isolator need only be rated for a few Amps.

    With luck you've already saved isolator expense by using a UIBI (charge light controlled). That is also easy to configure for forced off, forced on, etc.

    And then there's the usual protection of the secondary battery - ie, a low voltage cut out aka battery protector - but that is independent of whatever configuration you use.

    There are other tricks like having the alternator sense the AGM battery voltage instead (but ONLY when NOT isolated; when isolated you must sense the main battery!) hence providing maximum voltage to the AGM, and possibly adding the 0.2 to 0.4V extra voltage that can be used to charge an AGM (compared to a typical wet cell battery) but then the alternator power cable should go to the AGM (eg, to the main battery side of the isolator which is collocated with the AGM) and is provided the main battery and system tolerates the higher voltage (eg, the main to AGM cable drops the AGM's (say 14.8V) voltage to a max of 14.4V to the main).

    Oh yeah, there are so many implementations! Tho I find them simple and straightforward, unless the desires and design constraints are defined, I can't recommend which to choose. (Hence why I point out the various considerations...)
    But I'd recommend first reducing your AC load requirement. You can then determine your dc voltage drops and maybe your battery or system voltage and compare that to the minimum inverter voltage required.
    If it was sized for 2kW with a dual battery, I'd be implementing the UIBI with a 'connected during cranking' to extend battery life.
    And I have a dash 3-digit voltmeter that sits across the main battery (via a 1A IGN or manually controlled relay) and I'd be monitoring the secondary battery voltage(s) as well - preferably live, but otherwise at regular intervals, and certainly after use or before connection whilst still getting used to the system behavior. (I only do the odd maintenance voltage inspection of my remote AGM. But that usually only has the 2.5A peak Engel connected, and has its own battery protector. My 60-140A UIBI with its dual 50A self-resetting circuit breakers handle parallel cranking and battery recharge peaks so I'm really only checking for a failed AGM (it's only 14 years old).
    Last edited by OldSpark; 04-11-2014 at 09:55 PM. Reason: A few missed words & a dc-ac-dc-dc-ac correction.

  8. #18
    Join Date
    Oct 2008
    Perhaps it is my naiveness of electrical engineering. But I am a little confused by the following.
    You're saying that using 2gauge wire will be sufficient for the run and that upgrading to a 000gauge wouldn't net much gain because the majority of the resistance issue that is being faced at this point is internal to the battery.
    Yet you're also saying I should move the battery to the back of the vehicle to reduce the length of the run.
    I'm confused.

    Unfortunately, wiring a battery up the rear of the vehicle would be difficult, there may be space in the little storage area of the vehicle but it would definitely be tight. And I still do not understand what I am gaining by moving the battery there. I do see the point about heat effecting the life of the battery but the batteries are designed to operate in the temperature ranges inside of engine bays. Furthermore, buying 2-3 extra batteries starts breaking my budget. Not to mention the customization required (where am I to put these extra batteries?)

  9. #19
    Raw Wave
    Join Date
    Nov 2009
    Moving that battery to the back means negligible voltage drop between the battery and the inverter - ie, a short cable length as opposed to 22'. (If 2', it's ~1/10th the voltage drop.)
    But understand that that is from the battery. If supplying the load from the front alternator & battery, you still have the same voltage drop to the battery.

    Assume a typical FULLY charged and new AGM battery internal resistance of ~4mR (for a size equivalent to typical cranking batteries) and that is is 12.8V internally.
    At 200A its terminal voltage will be 12.8A minus the internal resistance drop (V=iR) = 200A x 4mR = 800mV = 0.8V, hence 12.8 - 0.8 = 12.0V.
    The 000G voltage drop is 200A x 0.0618mR/foot = 0.01236 V/foot.
    So for a local 2' cable length, that's a 0.024V drop versus (0.12 x 22 = ) 0.27V for 22' from the front.
    But notice that even the 22' 0.27V drop is only 34% of the battery's internal voltage drop.

    A 2G would likewise be 0.063V for 2' versus 0.69V for 22' - ie, roughly 2.5x higher than 000G.
    And whilst the 22' 2G run loses almost half its voltage in the cable (the other >half is lost in the battery), the 2' 2G run is again almost negligible - ie, 0.063V versus 0.8V or ~8% of the battery loss.

    But if rear mounting is not an option, then with a 2kW inverter output, the 000G cable loss will be 0.27V so the inverter only gets 12.8V - 0.8V - 0.27V = 11.73V.
    That's about 1.2V higher than the inverter's presumed cut out at 10.5V which might mean operation until the battery is near fully discharge (100% discharged being typically ~1.2V lower than the fully charged terminal voltage tho that can vary from 0.8V to 1.6V depending on the battery), but the battery's internal resistance increase with discharge so it might cut out when 50% discharged. The latter isn't a bad thing since a deep cycle battery should be limited to 50% discharge, but a cranking type battery should be limited to 20% discharge.

    And assuming a battery capacity of ~80AH, at 200A 50% capacity might be reached in ~5 minutes or less.
    And that rate of discharge is not good for AGMs, especially deep cycle types.

    As to budget, are you talking initial or life cycle. Buying 2 batteries instead of 1 means the each battery will last about 1.5 time longer - ie, spend up from for 2 batteries that will last about the same time as 3 successive single batteries.
    Or like that caraudio (forum) example: Put 2 batteries in the rear for twice the capacity versus a single battery in the engine bay that will be replaced 7 times for a mere max 30% increase in capacity?

    But hence why I say reduce your AC load. Get dc PSUs or loads instead. My $1,500 fridge is cheaper than my $150 fridge based on the (then) cost of $1,000 of extra solar panels required, plus the extra batteries I would have needed (eg, 4 batteries versus one, or 8 versus 2). Then there are other factors like the cheap Peltier fridge (cooler!) only cooling to 20C below ambient (hence to 20C on a 40C day) versus the Engel which can cool to -19C on a 40C day, AND use well under 1/4 the power doing it (it's more like 1/10th or 1/20th the power based on my shallow observation).

    But as you'll learn with battery systems, it is usually far cheaper reducing the loads than it is to compensate with more batteries, bigger inverters, etc - especially long term when batteries need replacement.

    And of course I have described non-alternator assisted inverter operation, and at its full 200A input. But in the absence of load and alternator capacities and duty cycles...

    PS - further to the relatively large battery resistance, that resistance also increase with age. (And varies with temperature etc.)
    Last edited by OldSpark; 04-12-2014 at 05:08 AM.

  10. #20
    Maximum Bitrate
    Join Date
    May 2012
    Grandville, MI
    I don't have time to read all of OS messages so I will not respond to what he said. But yes that is the same console I have. I have a spare console that is at a shop getting modified. At the front of the console in my truck there are two cig lighter outlets in a back plastic panel with two pockets. I was told in some Tahoes this has a 6 disk changer in it.

    If yours is the dual pocket design with 2 12 volt sockets in it like mine that piece will also pull out.. The frame around the piece that you can pull out will also pull out easily as well.

    Behind this is what you might think is equal to a dual din space. You can cut all of this area out if you desire to mount an inverter if yours is small enough. Just realize the "floor" of the area behind the two pieces I just mentioned you could pull out is just above the subwoofer. But you could cut out the whole area and the inverter could poke under the dash a little bit. You could replace the black panel with the two 12 volt sockets with a flat black piece of plastic and lose the pockets if you can use that area.

    I have totally disassembled my spare console but the main piece is at the audio shop as I mentioned getting my screen mounted in the front piece. I am also having the bracket that is under the console storage compartment powder coated because it is just bare steel. It will be a month or so before I get mine back but I could take some pictures of the disassembled console if you are interested.

    Another option is in the back on the sides you could probably build a box to house the inverter or if you are good at welding you could make a pocket in the floor to house the inverter as well. The in floor pocket might be interesting to cool but could be done.


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