Voltage drop using inverter

[PNW_Steve]

Originally Posted by o1marc

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.

I received an Instant Pot for Christmas 2018. I love it. They are now offering an Air fryer top as an accessory.

I may have to give it a try.

Next thing you know, we will be swapping recipes.

[Mat7783]

The gauge wiring provided is probably a little too small. May want to use something like 2 AWG wire which handles heat better . Heat is the enemy of voltage. There is a YouTube video out there about your inverter which clearly showed a difference with 2AWG wire running full 3kw of power at 108v compared to 104v with the smaller wire provided.

[Danjo]

Voltage drop can become a big problem with long runs @ 12V..

So is that to say that In my minimal setup where I’m only going to have a 120v AC outlet that I should mount the inverter as close to the batteries as possible and run a 120v AC home run to the outlet inside the bus?

[TheHubbardBus]

Originally Posted by Danjo

So is that to say that In my minimal setup where I’m only going to have a 120v AC outlet that I should mount the inverter as close to the batteries as possible and run a 120v AC home run to the outlet inside the bus?

I'm probably a glass too far into this wine bottle, Danjo. But I'm not following.

[neversayneverbus]

Depending on what the AH of the batteries are, you may be using up the power available in the batteries. In home wiring, you are supposed to run a microwave outlet all by itself on its own circuit, because it draws so much wattage. My math may be wrong, but I think a 1kw microwave would actually be using 83 amps at 12v.

If nothing is getting hot, then your connections are probably good.

If you are measuring voltage at the batteries and it is showing low voltage, then you are just draining them. Batteries also have different levels that you can drain them to. Lithium can be drained much lower than lead acid or AGM batteries. If you have drained them too far, it may have damaged the batteries. Then when you charge them, it shows good voltage, but falls much quicker than an undamaged battery would.

[Danjo]

Originally Posted by TheHubbardBus

I'm probably a glass too far into this wine bottle, Danjo. But I'm not following.

Well I’m 7 into a six pack, so let me restate my question:

Is it better to have the inverter next to the batteries with a short DC run and do allonger homerun with AC?

[TheHubbardBus]

Originally Posted by Danjo

Well I’m 7 into a six pack, so let me restate my question:

Is it better to have the inverter next to the batteries with a short DC run and do allonger homerun with AC?

LOL. Look forward to relaxing with you someday bro.


Absolutely. Line loss is one of the main reasons AC became the standard for long-distance power transmission. In the context of an RV/Bus electrical system, the length of your AC runs doesn't matter. The same cannot be said on the DC side.

[Danjo]

Originally Posted by TheHubbardBus

LOL. Look forward to relaxing with you someday bro.


Absolutely. Line loss is one of the main reasons AC became the standard for long-distance power transmission. In the context of an RV/Bus electrical system, the length of your AC runs doesn't matter. The same cannot be said on the DC side.

Hehe. I’m on a rare night out

Thanks for the reply.

Nothing ever goes like one would expect in the beginning and I would say that’s especially true with ambitious builds of short buses. I was going to make a battery drawer To go where the secondary battery is in the side of the bus and the power center somewhere around 10’feet back and with service loops in the cable it would be maybe a 20’ run. Time To rethink...

[john61ct]

did not read the thread

voltage drop is normal for high C-rate loads

need a bigger bank or a healthier / newer one or both

or LFP, drops a lot less

But running a high C-rate for more than a few minutes will empty out the Ah

Run a genset to support such usage

or get back to shore power

[Ninjakitty]

Originally Posted by Danjo

So is that to say that In my minimal setup where I’m only going to have a 120v AC outlet that I should mount the inverter as close to the batteries as possible and run a 120v AC home run to the outlet inside the bus?

Yes. Inverter as close as possible to batteries and then you can run a 120v run to an outlet.

[Ninjakitty]

Originally Posted by Danjo

Well I’m 7 into a six pack, so let me restate my question:



Is it better to have the inverter next to the batteries with a short DC run and do allonger homerun with AC?

Yes it is
You are correct.

[matthews2001]

Lot of good comments.

Still wonder how/why are inverters sold bigger then 1000 watts. 100 amps @12volts. 10% battery drain idealy requires a battery bank of 1000 amp hours at 12volts.

(2)t-105 have 250 amp-hours@ 12 volts, so ideally you need 8 total batteries. Seems to me MOST rv/buses doing off the grid use 4 t-105 ($800 total cost).

So even a 1000 watt inverter is usually too big for most?

[john61ct]

If it's for just short bursts, Ah draw isn't that great.

Lots of use cases require 3kW or even higher.

Just not the usual ones in **this** context.

But even 10kW is not a problem, assuming you have the energy storage and/or live input to support it.

[Kubla]

You running the microwave is discharging the batteries at a rate they can not sustain for moeee than a short burst (like starting the engine) I am pretty sure the recommendation is no more than 10% of the rated amp hours for a sustained draw, so you need at least twice as many batteries and your hook up wires should be at least 2 gauge and short as possible, for your 15 feet, you would want 1/0 gauge wire

[Doktari]

Use short, under 6’, 4/0 cables. Tape and twist them together as much as possible. Then check your battery capacity if there is still a issue. Also if the lead acid batteries are very cold the capacity is reduced significantly. They will be more sluggish.

[kidharris]

Originally Posted by matthews2001

Attachment 40377
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?

To me this seems"normal" especially since all of your "data" is so vague.

You don't list the watts of any of the appliances, but from my experience microwaves run from 800-1500 watts depending on size. That's a lot and is similar to turning on an air conditioner.

If it is a new tv then it doesn't use enough watts to even worry about, same for the fridge. One thing to note about fridges is that they are not on all of the time and many are so quiet that it is hard to tell if it is on at all ( for hours at a time it may be off, then run for a short time).

You need to plug each appliance into a watt reader (KillAWatt or similar) and find out what it is drawing and if it is actually running (fridge)

In addition it takes very little to drop battery voltage when it is at 13.8v. and there is a possibility that the battery is not as strong as it shoiud be. 14-14.2 v suggests that you are reading it while the bat is charging.

"down to below 12" means nothing. How much below 12? are you reading it while the microwave is running or after it is turned off? Does the bat come back up some in a little while (after the load is turned off) without being charged? No expert, but I think most inverters have a preset "turn off" voltage around 10.5-11v dc.

What "is wired using pre exisitng wiring to the removed handicap lift"? "It" does not have any meaning here. I'm guessing inverter, maybe microwave?.

The dc wire to the inverter should be heavy as per the inverters instructions.

I'm thinking that the wires to the appliances should be sized according to building codes - or larger - for house wiring and use heat tolerant insulation like automotive wiring with a minimum of maybe 250-600v rating and be flexible, multi strand (wire with a ground wire? - I'm not sure how to properly ground ac in a mobile vehicle, I plan on cruising the boat sites to find out for mine, they have had many years to work out what is best.

[kidharris]

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.

Attachment 40391

Attachment 40392

Attachment 40393

Not trying to be critical, but skimping on the wiring to the inverter may not be the best place to save $$. You think that you have problems now and the inverter is no where near loaded.

[kidharris]

Originally Posted by Danjo

So is that to say that In my minimal setup where I’m only going to have a 120v AC outlet that I should mount the inverter as close to the batteries as possible and run a 120v AC home run to the outlet inside the bus?

Don't forget that the inverter needs to be kept cool the engine compartment may not work.

[kidharris]

Originally Posted by matthews2001

Lot of good comments.

Still wonder how/why are inverters sold bigger then 1000 watts. 100 amps @12volts. 10% battery drain idealy requires a battery bank of 1000 amp hours at 12volts.

(2)t-105 have 250 amp-hours@ 12 volts, so ideally you need 8 total batteries. Seems to me MOST rv/buses doing off the grid use 4 t-105 ($800 total cost).

So even a 1000 watt inverter is usually too big for most?

So now you do your homework? LOL. keep it up and research the ground issue for me?