Voltage drop using inverter

[matthews2001]

I bought a 3kw inverter to run our full size fridge in my new build. Also to use the microwave occasionally without starting the generator.

When I use it I get a dc voltage drop from 13.8 down to below 12 within 5 minutes of microwave use (and ac volts down to 104). Using just fridge and tv, no drop at all.

It is wired using pre exisitng wiring to the removed handicap lift, all connections very clean and tight. It uses 4 new 6v trojan batteries (the voltage there is 14-14.2, so I do use some voltage on its way!)

BTW I get over 5 days fridge only (and lighting, water pump, electronics) use this way without recharging.

Any ideas?

[o1marc]

That's a 3K kw. I'm told the inverters suck power when they are on and may consume more than needed if it is too large for the system.

[PNW_Steve]

What do you have for your house battery bank?

What inverter are you using?

[TheHubbardBus]

Is this a stand-alone battery bank you charge from generator/shore from time to time, or part of a solar system? I ask because your voltage numbers (14.2 / 13.8 ) don't make sense, unless they were taken while being charged. A fully-charged 6V should be ~6.4V @ room temp, so 2 in series (or the 2 parallel, 2 series as I suspect you have to have) would be 12.8V.

Also, what, exactly, is wired to what using the existing wiring? What gauge is the wiring? What type of insulation? In most cases, wire that's used for 12V DC circuits is unsuitable for use powering AC loads.

[matthews2001]

inverter: wzrelb about $350, high efficiency, amazon

4 new 6 volt trojans t 105
full charge 12.8 volts (charges at 14.2), intelli-charger
no solar, charge batteries when on shore power,

[sometimes charge direct to alternator when driving but that doesn't work well (alt voltage is only 13.4 i think)( gonna reconfigure wiring so when driving alt will power inverter and charge house batteries thru the intelli-charger at 14.2+) (I realize alt voltage may be low but never have had a starting issue so haven't 'gone there' yet to explore that issue)]

12v wiring to inverter is preexisiting handicap lift wiring, very large wire. All a/c wiring is professionally done boat type (former bookmobile)

So, does the draw of a 3000w inverter at say 2000 watts (large micro and refrig) all wiring adequatelly sized, clean connections really drop voltage down to 10=11 volts and 105 ac after 5-15 minutes usage? Inverter manufacturer thinks bad connections but I dont see that.

[TheHubbardBus]

Originally Posted by matthews2001

12v wiring to inverter is preexisiting handicap lift wiring, very large wire. All a/c wiring is professionally done boat type (former bookmobile)

So you're saying the leads from the battery bank to the inverter is this 'very large wire' you scavenged? If so, what is large (in AWG), and what is the temp rating of the insulation? And how long is the run, from bank to inverter? What are these leads fused at (amps)? Did you use the same wire for wiring the bank itself? If so, how did you create your terminal connections?

I'm concerned that since you're referencing your cabling by a subjective description rather an actual measurement, you possibly didn't properly match the wire gauge to the load it needs to support. Whether or not this is your problem I don't know yet, but if that's the case, it's certainly a problem (and possibly a very big fire hazard). Also, if you don't know what the wire gauge/type is, there's no way you could have fused it properly.

[matthews2001]

the wire and house battery wiring is all pre exisiting. dont know gauge but it is the largest wire diameter on the bus excepting battery/starter. dont know its fusing. It used to run the 12vdc motor on the power wheel chair lift.

the run from the battery bank to the inverter is 12-15 feet.

Note: wire and connections do not get warm/hot under use.

The connections at the inverter are prexisitng spade connectors that I drilled out for bolt connections on inverter.

[TheHubbardBus]

Originally Posted by matthews2001

dont know gauge

That's a problem.

dont know its fusing.

That's a big problem.

the run from the battery bank to the inverter is 12-15 feet.

That is far from ideal, and - depending on your wire gauge/composition/insulation - could be part of the problem. Ideally you want this run to be as short as possible, You also want to run the cables side-by-side (touching) the entire length, to minimize loss.

Note: wire and connections do not get warm/hot under use

Certainly a good sign, & perhaps an indicator I'm barking up the wrong tree, but even so, that doesn't negate the importance of addressing the problems above. You need to match wire to the load, & fusing to the wire, and consider voltage drop based on the length of your run. Your cabling may be sufficient - or it may not - but not knowing is no bueno. Unfortunately I can't tell from your pics whether your cabling is up to snuff or not. Are there any markings on it? What's the OD? Is it copper?

The connections at the inverter are prexisitng spadeconnectors that I drilled out for bolt connections on inverter.

I can't say your connections are the problem, but I will say their appearance & construction (drilling holes in spade c onnectors) is not confidence inspiring. They may be perfectly fine, but considering your unresolved issues, it would be nice to see proper lugs there. From what I can see, it looks like you have a poor (and inconsistent) interface between the terminals & lugs. It also looks like you have a lot of oxidation or something else (is that light reflection?).


Finally, those cables running through holes in (plywood?) the way they are appear to present a hazard in regards to insulation abrasion.

[Drew Bru]

Originally Posted by matthews2001

the wire and house battery wiring is all pre exisiting. dont know gauge but it is the largest wire diameter on the bus excepting battery/starter. dont know its fusing. It used to run the 12vdc motor on the power wheel chair lift.

the run from the battery bank to the inverter is 12-15 feet.

Note: wire and connections do not get warm/hot under use.

The connections at the inverter are prexisitng spade connectors that I drilled out for bolt connections on inverter.

You'll want to measure the wire diameter to determine the gauge of the wires. There are charts online that will give you the gauge. I'm far from an expert in things electrical, but I seem to remember that you should keep the distance from battery bank to inverter as short as possible, less than 5' is ideal. The longer the wire, the larger it should be. As the voltage drops, the current (amps) will go up and if not properly sized and fused it could be a fire hazard. There's others with way more electrical knowledge that, I'm sure, will chime in with both encouragement and chastisement. We're all here to learn and help when we can. I'm not saying this is a death trap or anything...just that you should know the particulars of your wiring so you don't put yourself at risk.

[TheHubbardBus]

I don't know the specs on your inverter (If you have the rate would DC input voltage that would be handy), but looking @ a random 3000W inverter I pulled a manual for, they list a DC input voltage of 375A. Taking 125% of that (NEC requirement) that gives you 466A. According to said manual, that would require 2 x #3/0 cable (90 degree, copper) per lead, @ a max run of 10'. For reference (check out page :


https://www.samlexamerica.com/docume...-0819_Hrez.pdf


Likewise, if you take the 466A figure & compare it to the NEC ampacity specs, you'll see that #4/0 cable, which is probably the max your cable could be, only supports 260A w/ 90-degree insulation. 230 w/ 75-degree. So as a match for your inverter, it's likely well undersized (assuming it's #4/0, & your inverter is close in specs to that above. Both are just guesses at this point).


https://www.usawire-cable.com/pdfs/NEC%20AMPACITIES.pdf


That's not to say, however, it's undersized for your loads. It doesn't sound like you're pulling anywhere close to the max input. But if it were me, I'd want to match my cabling to the inverter capacity (and fusing to the cabling!!!!), so I could ultimately use it to its full potential if/when you wished.


Voltage drop can become a big problem with long runs @ 12V. Curious to know what your cabling actually is, to see if that's an issue.


Could be as simple as a bad battery (even one showing good voltage). But with the wiring question up in the air...

[matthews2001]

I left the bus in anchorage this winter and flew back. Anyone up there wanna run by and measure my cable? haha

ANYWAY do people with 3000 watt inverters on 4 6volt trojans experience similiar voltage drops when using their microwaves? BTW my inverter shows in and out volts. Might I have a bad inverter?

Only get voltage drop when using microwave

[PNW_Steve]

Originally Posted by matthews2001

I left the bus in anchorage this winter and flew back. Anyone up there wanna run by and measure my cable? haha

ANYWAY do people with 3000 watt inverters on 4 6volt trojans experience similiar voltage drops when using their microwaves? BTW my inverter shows in and out volts. Might I have a bad inverter?

Only get voltage drop when using microwave

This link should help answer your questions:

https://www.solarpaneltalk.com/forum...-size-tutorial

Good luck.

[PNW_Steve]

And another:


https://www.solarpaneltalk.com/forum...utorial-part-2

[Ninjakitty]

I had similar issues at one point. Batteries would show 12.8~ on the victron monitor. Add almost any 120 volt load and would start plummeting to dead, very quickly. All my wiring was good. Gauge of wires were all over-sized. I took apart my system 3 times, checking and cleaning and tightening connections.

Turns out, my battery bank was bad. These were 4 agm deep cycle 200 ah batteries. Bought new, less than a year old. At some point they got charged at too high a voltage or were just duds.

I got lucky, mine were under warranty. They were renogy batteries and the company just needed them load tested and pictures that the tester said they were bad. Shipped new ones right away. You may one to get yiur batteries tested. You will need a load tester to do it.

[matthews2001]

PNW Steve GREAT POST on Inverter sizing.

So basically the maximum inverter size is 1000 watts for a 12 volt system.

Now I read as much as I could building my bus last winter (and am a former Mechanical Engineer haha) but kept seeing those 2-5k inverters selling for not much more. Why are we buying them, where are they getting used? And what about my dream of quiet microwave use without a generator?

I will forget that nothing got hot or tripped a fuse drawing the 150 amps I must have been drawing! I will pretend it did get hot so I dont put my micro on the inverter again.

But who is USING those large inverters? Seemed impressive when I said I had a 3000 watt one, now I know if someones inverter is over 1000 watts they havent read PNW Steve's link (or done an easy calculation W=VxA

[bubb, the real one]

2000w at 120v is about 20amps, this means you are pulling about 200 amps at 12v from your bank, x10, a 400ah bank can barely sustain a 40 amp draw, your bank is undersized,
You only want to pull up to 10% of a batt bank capacity if you want to keep the volts up, this ignores your cable and inverter loss

[PNW_Steve]

Originally Posted by matthews2001

PNW Steve GREAT POST on Inverter sizing.

So basically the maximum inverter size is 1000 watts for a 12 volt system.

Now I read as much as I could building my bus last winter (and am a former Mechanical Engineer haha) but kept seeing those 2-5k inverters selling for not much more. Why are we buying them, where are they getting used? And what about my dream of quiet microwave use without a generator?

I will forget that nothing got hot or tripped a fuse drawing the 150 amps I must have been drawing! I will pretend it did get hot so I dont put my micro on the inverter again.

But who is USING those large inverters? Seemed impressive when I said I had a 3000 watt one, now I know if someones inverter is over 1000 watts they havent read PNW Steve's link (or done an easy calculation W=VxA

I have run my microwave (900 watts) on my inverter but don't make a habit of it. Too much drain on the batteries.

I have developed a routine while traveling. When I get up in the morning I start the generator and make coffee. My wife makes breakfast and then runs her hair dryer and curling iron. Then I shut down the generator.. To make it less offensive I have run very quiet generators.

[JDSquared]

Originally Posted by PNW_Steve

I have run my microwave (900 watts) on my inverter but don't make a habit of it. Too much drain on the batteries.



I have developed a routine while traveling. When I get up in the morning I start the generator and make coffee. My wife makes breakfast and then runs her hair dryer and curling iron. Then I shut down the generator.. To make it less offensive I have run very quiet generators.

I did a bunch of baking this past month, and yeah I could do it with a hand mixer, but I love my KitchenAid. Not sure how many watts it draws but it's coming with me. I've decided to give up my fryer. I love that thing too for convenience. Regardless of power draw, hauling and sloshing and disposal of all that oil on the road sounds about as much fun as a black water tank.

[PNW_Steve]

Originally Posted by JDSquared

I did a bunch of baking this past month, and yeah I could do it with a hand mixer, but I love my KitchenAid. Not sure how many watts it draws but it's coming with me. I've decided to give up my fryer. I love that thing too for convenience. Regardless of power draw, hauling and sloshing and disposal of all that oil on the road sounds about as much fun as a black water tank.

We are pretty much on the same page. The fryer is not going with us. I really want to take the mixer. I just need to find a good spot for it. Some of the accessories are staying home though. I can't see making ice-cream on the road

[o1marc]

Originally Posted by o1marc

That's a 3K kw. I'm told the inverters suck power when they are on and may consume more than needed if it is too large for the system.

Originally Posted by PNW_Steve

We are pretty much on the same page. The fryer is not going with us. I really want to take the mixer. I just need to find a good spot for it. Some of the accessories are staying home though. I can't see making ice-cream on the road

I recently bought a new deep fryer. My family is talking me into getting an Air Fryer instead, my heart will appreciate it. Sounds like that would be the way to go in a bus, no splashing grease.