Less voltage than expected from 400w Cobra Inverter out of a 12 V battery

[zcapozzi]

Hi all,

I'm having some issues with a used Cobra Inverter connected to a 12V battery. The AC voltage available from the inverter (Cobra CPI 430 400W/800W) is measuring at 107V. The battery is fully charged (measuring 12.7V). I am using a Morningstar Sunsaver charge controller (10A/12V). So far, I've been able to run a cell phone charger from the 5V USB outlet, but haven't been able to run anything from the normal 120V outlets.

The inverter beeps when I plug in the other loads (i've tried a laptop and iPad charger), indicating that the voltage is not sufficient. This prompted me to measure the available voltage using a multimeter, which is when I saw it was at 107V.

The inverter is used (got if from a buddy to test out my system), but aside from that, I don't have any reason to think that it wouldn't work. Any ideas about why I'd be losing so much voltage between the battery and the inverter?

Thanks in advance.

- Zack

[thastinger]

what size are the cables from the battery to the inverter input?

[zcapozzi]

what size are the cables from the battery to the inverter input?

The cables are 10 gauge copper wires. Also, the battery is not directly connected to the inverter. The charge controller takes in the battery cables, then separate inverter cables are wired to the controller. The charge controller is approximately 40 inches from the inverter.

Also, I tested the voltage with the inverter on and with it off. When the inverter is off, the voltage measured on the charge controller outputs for the inverter are 12.7V (so full capacity). When the inverter is switched on, the voltage on those same leads goes down to 12.4V.

Thanks for the response. Hope this helps to clarify.

[zcapozzi]

Hi all,

I'm putting this thread on hold. I believe I misspoke when I said that the battery was fully charged. I've got my panel out and operating nicely, so once the battery is fully charged, I'll check the inverter output to see if it's still not giving me 120V.

- Zack

[Mike90250]

mod sine inverters can often fool meters designed to measure pure sine waves. I think that is your issue, not battery voltage.

The beeping is related to low voltage at the inverter terminals, and I can say that 10ga wire is undersized.

[zcapozzi]

mod sine inverters can often fool meters designed to measure pure sine waves. I think that is your issue, not battery voltage.

The beeping is related to low voltage at the inverter terminals, and I can say that 10ga wire is undersized.

Interesting. I actually calculated that I would need 16 gauge, but based on a potential for expansion, I went with 10 gauge. Can you tell me why you think 10 gauge is undersized?

[Naptown]

400w / 12 V = 38 amps
Over the rating for a 12 V system and causing a voltage drop
Second the inverter should connect directly to battery posts with fuses same with charge controller

[thastinger]

Is the 12V battery a flooded or AGM battery and how many Ah is it? All that makes a difference in how much juice you can get from it without it sagging in voltage.

[zcapozzi]

Is the 12V battery a flooded or AGM battery and how many Ah is it? All that makes a difference in how much juice you can get from it without it sagging in voltage.

It's a 35Ah and it's an AGM. Also, the charge controller had directions for connecting load to the controller, so I had my inverter hocked up to the cc instead of to the battery directly. I have since connecting it directly to the battery terminals, which I think is helping also.

[Naptown]

At full AC load your battery will be 100% drained in well under 1 hour
Look up what the capacity of the battery at the 1 hour rate.
I would suspect less than half.
Draining it at the 10 hour rate you could pull 3.5 amps which is 42 watts on the battery.
Now take out about 20 watts which the inverter uses or more and you see what you have left to work with.
Not much

[PNjunction]

That inverter already has a low-voltage dropout/disconnect of it's own, so connect it directly to the battery.

You'll need an RMS capable multimeter to measure the ac voltage properly. Also note that unit is rated at 115v ac +/- 5%, so you may also have one that is a tad bit on the low side, but still within spec.

Be very aware that many usb ports on a lot of cheap equipment stick to the original 500ma current standard. Trying to use a device that draws more current like a modern cellphone, ipad, etc may either blow the fuse, or in some cases, fry the logic board so that the entire inverter no longer works properly. This is one reason I NEVER use the usb port on a device, but rather stick to a seperate high-power usb mobile dc converter. Now THAT might be useful to connect to your charge controllers load outputs.

[thastinger]

It's a 35Ah and it's an AGM. Also, the charge controller had directions for connecting load to the controller, so I had my inverter hocked up to the cc instead of to the battery directly. I have since connecting it directly to the battery terminals, which I think is helping also.

Yeah, so your system isn't capable of doing what you want it to do. If you discharged your 35Ah AGM battery at a C/4 rate, it could only power an 80W load once you factor in the inverter losses, 105W load if there were no losses at all.

[john8750]

I have three Cobra inverters. All read about 97 volts, with or without load. They seem to work fine, although are very lightly loaded running fluorescent lighting and some small power tools. Also runs 1/8th HP motor with no problem, for swamp cooler. Inputs are properly connected, and batt's are up to voltage. Might just be the inverted sine wave reading different.

[Mike90250]

I have three Cobra inverters. All read about 97 volts............. Might just be the inverted sine wave reading different.

An "inverted sine wave" is still a (wait for it) sine wave. But the Modified Square Wave (which is what the "mod sine" inverters produce) is likely screwing with your meter.

[inetdog]

I prefer to call it Modified Square Wave rather then Modified Sine Wave (still MSW either way) just to emphasize that it is far closer to a square wave than to a sine wave.

It is also true that an AC meter which senses the peak voltage and uses that to compute the equivalent RMS value of a sine wave with the same power output will NOT give a correct number when reading either square or MSW.

The value which is reads will be a factor of 1.414 lower than the RMS voltage in the case of a square wave, and a smaller error in the case of MSW.

In addition, some cheap MSW inverters will have a wildly varying duty cycle for their output waveform depending on the DC input voltage. Take a look at the scope traces in this article: http://forums.woodalls.com/index.cfm...g/26680637.cfm

A low DC input can cause the RMS value measured from a peak-reading AC meter to be much lower then it really is, and the RMS output voltage of such an inverter can be surprisingly low over the full range of input voltages.
Low output voltage may not be too bad for a light bulb or power supply, but it can cause serious overheating in motors which are running near full load.