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Old 06-07-2021, 08:53 PM   #1
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Hot Cables and Connections

I recently upgraded the electrical system in my Axis 24.1, putting in 300 A-h of lithium batteries and a 3000 W inverter charger among other things. Today I was doing some work inside the coach so I thought I'd run the A/C for a while to test it out. After about a 1/2 hour the battery and inverter cables were very warm and the shunt and other exposed terminal connections were hot to the touch. With the A/C drawing 110 A, is that to be expected or is something wrong?

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Old 06-07-2021, 09:19 PM   #2
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Only 110A? My Axis 24.1 has a 15,000 btu/hr A/C unit that draws 15 amps at 110V. That will require 150A DC to power the inverter.

And yes, 110A or 150A qill make all of the connections very warm. Tighten them down first. What size wire do you have from your battery to the inverter and how far is it. You should use at least 2/0 wiring to a 3,000 watt inverter.

David
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Old 06-07-2021, 10:04 PM   #3
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Thanks for the feedback/confirmation. Everything seems tightened down and I've got 4/0 wire from battery to inverter and amongst the batteries themselves.
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Old 06-07-2021, 11:27 PM   #4
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Looks good to me, .

David
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Old 06-08-2021, 12:13 AM   #5
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No way in heck 110 amps at 12v will make quality copper 4/0 cables "very warm" unless they are 100 feet long.

You have another problem.
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Old 06-08-2021, 06:51 PM   #6
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I got the voltmeter out today and took some measurements. With 140 A of current draws, the 4/0 cables that run from the batteries and to the inverter had voltage drops of about 0.05 V each. This is higher than what should be expected for that length of 4/0 cable, but I don't know what I should expect for losses in the connectors. Thoughts?
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Old 06-08-2021, 07:46 PM   #7
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How long are your cables? You can easily deduce the connector voltage drop by subtracting the cable voltage drop from 0.05 V.

But that is only 7 watts per cable including two connectors. Not a lot of heat.

David
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Old 06-08-2021, 08:06 PM   #8
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Remember that your ground return cables are just as important as the positive terminals. Measure the voltage across the battery plus to minus under load and then measure the plus to minus voltage at the inverter battery terminals. Then you'll know how good or bad the cables are bu subtracting the two measurements. Hopefully its 2 x 0.05V = 0.1V you all ready measured.
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Old 06-08-2021, 08:54 PM   #9
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Trying to run the A/C off the batteries through an inverter, to me, is just looking for trouble.
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Old 06-08-2021, 09:58 PM   #10
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Running A/C off a 12vdc inverter is going to generate a lot of heat, especially if the compressor is constantly engaged. 2500w inverter and 400ah (4.8kwh) of battery bank and would never consider running A/C off the batts - ever.

Get the skinny. Try posting your question here in Beginner's Corner: https://diysolarforum.com. You'll get an answer. Consider a soft start if you still plan to continue down that road. I have a feeling their knowledge will change your mind.
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Old 06-08-2021, 11:22 PM   #11
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My RV guy kurt said to never run A/C off our 3000 watt inverter. He said use SP OR GENERATOR
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Old 06-08-2021, 11:33 PM   #12
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What is wrong with running an 1,800 watt appliance with a 3,000 watt inverter powered by 300 amp hours of lithium batteries?

Yes it won't run more than about two hours before the batteries go flat. But besides that, what is the problem?

David
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Old 06-09-2021, 12:18 AM   #13
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Oh we have dogs and it will not work more than a few minutes
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Old 06-09-2021, 08:33 PM   #14
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I'm inclined to agree with DavidEM in theory regarding A/C use. What's the point of having a 3000 W inverter if you can't generate 3000 W.

I do have a soft start as recommended above, but that doesn't reduce the overall power requirements of the A/C, just smooths out the curve.

I've measured the voltage drop under load across various cables, fuses and connectors and it seems like each cable or connector contributes on the order of 0.05 V of drop or about 7 W of waste heat each. Maybe 70-100 W of heating is enough to make the cables heat up?
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Old 06-09-2021, 09:52 PM   #15
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Quote:
Originally Posted by Karl404 View Post
I'm inclined to agree with DavidEM in theory regarding A/C use. What's the point of having a 3000 W inverter if you can't generate 3000 W.

I do have a soft start as recommended above, but that doesn't reduce the overall power requirements of the A/C, just smooths out the curve.

I've measured the voltage drop under load across various cables, fuses and connectors and it seems like each cable or connector contributes on the order of 0.05 V of drop or about 7 W of waste heat each. Maybe 70-100 W of heating is enough to make the cables heat up?
4.8kwh of lithium and 2500w inverter and I still wouldn't do it for any length of time.

If the A/C is not receiving sufficient amps, it's going to generate heat and that will include the wire(s) as they try to keep up with the draw. He keeps it up and he could end up with unintended consequences

Regardless of how its set up, he now knows the setup can't handle it and he should probably back off until he gets it figured out.

There's no mention of the distance between the batts and inverter and inverter to distribution panel, and the dist panel to the A/C - nor the guage of the wire used. Without those numbers, the potential volt and corresponding amp loss is not calculable.

The A/C is likely on a 20A circuit requiring a minimum of 12-2 wire. There'll be little volt drop on the 120 output side of the inverter, so that becomes less of a concern than the 12v input between the batts and inverter. If the batts are under the steps Iin front) and inverter is under the bed (in the rear), even 4/0 wouldn't be sufficient for a run like that, when trying to run a 20A load or the compressor surge (i.e. A/C)

This will help with sugested wire sizes and will by no means calculate volt drop and amp loss. Distance matters.

https://baymarinesupply.com//bosns_corner_wire_sizes
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Old 06-10-2021, 10:36 PM   #16
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Originally Posted by The_Breeze View Post
There's no mention of the distance between the batts and inverter and inverter to distribution panel, and the dist panel to the A/C - nor the guage of the wire used. Without those numbers, the potential volt and corresponding amp loss is not calculable.

The A/C is likely on a 20A circuit requiring a minimum of 12-2 wire. There'll be little volt drop on the 120 output side of the inverter, so that becomes less of a concern than the 12v input between the batts and inverter. If the batts are under the steps Iin front) and inverter is under the bed (in the rear), even 4/0 wouldn't be sufficient for a run like that, when trying to run a 20A load or the compressor surge (i.e. A/C)
The batteries are in the back of the coach with the inverter a couple of 4/0 feet of cable away. Should be sufficient for that run. I've got 15' of 6/3 AWG carrying the 120 V AC from the inverter to the AC dist panel so that should be sufficient too.

I'll keep working on this and post how it turns out.
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