Small AC motors and Inverters

Hi,

Voltage drop may be linear but amps drawn definitely is not.

I don't think you can predict it that way, but the simple thing is to try it for your set-up. It won't be quite the same for somebody else's.

My new Can Tire 2000w model says the voltage alarm is for 11v +/- 0.3v. So even the same model inverter can have a different point at which the alarm goes off. BTW 0.3v is a lot in this game. If it is at 11.3 instead of 11.0, there goes a bunch of margin for running loads with lower SOC batteries.

SoundGuy wrote:

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That'll mean a run of about 5' from the battery terminals to the inverter but I have friend who I think can supply me with 2 gauge at cost so that's what I'll use, along with an 80 amp fuse I bought last year when I was experimenting with another inverter.
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Nice, little line loss there, and the 80amp fuse will protect both the line and the Inverter.

SoundGuy wrote:

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As for running cables, that's easy too - one hole in the storage compartment floor near the front wall for the battery cables and another (because I've decided to go "whole house") for a 120 vac cable running from the inverter output to the outside world. No need for a hatch or expensive weathertight receptacle ... instead I'll wire a Leviton 15 amp WetGuard Receptacle from Home Depot on to a short length of 120 vac cable, run that cable up to the inverter (sealing the hole with caulking), and wire on a 15 amp male plug that will plug into the inverter.
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And that too is a neat solution. Really like that WetGuard connector.

But think I'll still try the auto relay and see how that works. Have always wanted to open up my trailer's basement and see what's inside. :)

BFL13 wrote:
I have found that whatever the voltage drop is at high SOC it stays about the same all the way down. So eg, if the inverter draws 100 amps and that makes for a 0.9v drop, then if batts were at 12.7 they will drop to 11.8 and if at 12.2 then to 11.3
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Thanks. Good info. I haven't really looked at how my batts respond to loads as mine (loads and batts) are pretty small. IIRC -0.2V is all I've seen when the furnace fan comes on (not sure)?

Anyway if 100A causes a 1V drop, maybe my ~20-25A (250W) coffee grinder will cause a 0.25V drop, max? Add 0.15V max for the 10' 6ga cable drop and get 0.4V total drop? (That line loss is actually a greater proportion than I thought it would be). And if the batts are at 50% SOC, say 12.0V in the morning, that means coffee grinding will drop it to 11.6V, worst case. Add in the furnace fan ... 11.4V?

And the coffee grinding takes 15s. :)

All back-of-the-envelope B.S. but still, the inverter's alarm doesn't come on until 11.0V and it doesn't turn off until 10.6V

I have found that whatever the voltage drop is at high SOC it stays about the same all the way down. So eg, if the inverter draws 100 amps and that makes for a 0.9v drop, then if batts were at 12.7 they will drop to 11.8 and if at 12.2 then to 11.3

So in that case to stay above 11.0 the batts would have to be 12.0 going in. EXCEPT, once that initial drop happens the voltage then slowly drops some more as the load stays running, so now you need more margin depending on how long it takes to do the job.

It is easy to test for this while at home to see what your set-up can and can't do, so you know what to expect once out there camping.

Don't forget while you are teetering on the edge with the voltage at 11.00001, and you think you are getting away with it, all of a sudden the furnace comes on and adds more load! Oops there goes the inverter alarm (if you can hear it above the furnace noise) :(

Manufacturers of CHEAPO Battery CHARGING Jumper Cables, wheedle and whine

"Our cables are 10 gauge (SAE) because we expect consumers to know that the cables are intended to be connected between a vehicle with the engine running and the disabled vehicle and REMAIN CONNECTED with the intention of RECHARGING the disabled vehicle's battery before a start attempt is performed"

"Oooooooooo! Start another car with a BAD battery? Nooooooooo! Our cables are not intended for that!"

They omit to mention this irrelevant factoid on the packaging so as to be a green manufacturer and save trees and ink.

brulaz wrote:
Think part of the issue here is you guys think big. Many have big inverters with big loads. I just want to run a coffee grinder, maybe run a computer. And don't want to spend a fortune doing it.

I agree, let's be practical here. ;) I decided to buy that Xantrex 1000 watt PSW from Crappy Tire that some have been talking about here ... just tested it and of course it's solved my squealing fan issue completely. Sure, overkill for the purpose but the sale price was right for a sine wave inverter and heck, I recently sold $80 worth of "junk" on Kijiji so I applied that to the cost. :B

As for installation, that's easy - I'll mount the inverter to a piece of plywood and screw that upside down to the joists in the front passthrough storage compartment where it'll be completely out of sight, out of mind. That'll mean a run of about 5' from the battery terminals to the inverter but I have friend who I think can supply me with 2 gauge at cost so that's what I'll use, along with an 80 amp fuse I bought last year when I was experimenting with another inverter. As for running cables, that's easy too - one hole in the storage compartment floor near the front wall for the battery cables and another (because I've decided to go "whole house") for a 120 vac cable running from the inverter output to the outside world. No need for a hatch or expensive weathertight receptacle ... instead I'll wire a Leviton 15 amp WetGuard Receptacle from Home Depot on to a short length of 120 vac cable, run that cable up to the inverter (sealing the hole with caulking), and wire on a 15 amp male plug that will plug into the inverter. To use, I'll simply dogbone the trailer's 30 amp male plug to convert it to a 15 amp U Gnd and plug that into the WetGuard ... when not in use I'll simply cap the WetGuard. I've been using these weather resistant connectors for years and they're perfect for this application.

Love it when a plan comes together! :B

MrWizard wrote:

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6ga wire fused at 60amps.. may allow the inverter to operate safely
but now you look at the performance factor , and will it meet you use
or it will it be shutting off ?
that will be up to you to decide after you try it out on a camping trip

I suspect that performance will depend upon my 220Ah battery bank more than the 6ga cable.

60A voltage drop is maybe 0.3V for 6ga, 10'. For my coffee grinder that may be only 20A or 0.1V drop. I dunno. But won't my battery voltage drop a lot more, depending upon its SOC? I just don't know how much.

And like I said, when I upgrade my battery bank, I may even upgrade the Inverter and then all the wiring will be upgraded to match.

Think part of the issue here is you guys think big. Many have big inverters with big loads. I just want to run a coffee grinder, maybe run a computer. And don't want to spend a fortune doing it.

battery cable under the hood, see high current only for seconds
then only see high charge for a few minutes
plus they are exposed to open air cooling from underneath, when the engine is shut down

they are in an enclosed space where the heat will build, but will cool when stopped ( coled from the compartment being open at the frame )

they are not usually called upon to carry a 100 amps for 6 minutes while baking a potato or 80 amps for 10 minutes while making a pot of coffee
or hours of battery charging a bank of batteries (while it could do so)

that #2ga has no problem carrying 40 or 50 amps from the alternator while you drive all day , its usual job

so why is battery cable 2ga instead of 6ga
because 6ga can not be expected to maintain integrity and safety, even though it will never have to see the sustained loads that inverter does

why do they sell cheap 8ga jumper cables,
because they are seldom used and the use only lasts for "SECONDS"

Again.. it is the context of use and conditions of use that determines what is safe and allowable

6ga wire fused at 60amps.. may allow the inverter to operate safely
but now you look at the performance factor , and will it meet you use
or it will it be shutting off ?
that will be up to you to decide after you try it out on a camping trip

BFL13 wrote:

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To stay above 11v loaded, it is best to have fat wires and a big battery bank. With just two batteries, you have little margin to run the loads when the batteries are near 50% before you start the inverter. Having fat wires with just two batteries gives you more margin.

You are right. With the 60A, 10ft cable scenario, Ampacity is not relevant (10ga would be good enough), but for max 3% voltage drop I need 6ga cable. And a max 1% drop specification would require a 3ga cable at 10ft, according to the Circuit Wizard.

But 3% of 12V is only 0.36V, not huge but that has to be subtracted from the battery voltage. What sort of voltage can I expect at the battery if it's at 50% SOC (12.0-12.1V) and 20A (for the coffee grinder) is withdrawn from a 220Ah battery?

I suppose "it depends" on battery condition and other factors too numerous mention. Guess I'll find out, probably not until November in Smoky Mtn NP though; that's usually when I start wishing for a bigger battery bank.

The inverter's low battery alarm is at 11V, and shutoff occurs at 10.5V.

BFL13 wrote:

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It was shown on here a while back that AGMs can take a big load and stay above 11v much better than Wets can. If you have a small rig that cannot carry more than two batteries, and you want to run big inverter loads, having AGMs would be the way to go.
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Thanks. Another plus for AGMs. I could carry more than two back in the living area at the foot of the bed.