Inverter low voltage at 12.6 Volts?

stmoloud · October 2018

I have a new 600 PSW inverter which I was hoping would be sufficient to run a small fridge. The plan is (was) to use this inverter dedicated solely to running the fridge.
When I disconnect the AC and then connect to the inverter, the inverter does indeed start the fridge. According to the inverter panel, the fridge draws 8.1 amp.
However every subsequent time, approx every 30 minutes, the fridge will attempt to cycle but cannot. I can hear the motor begin but then it will fail and the inverter will throw an E3 fault which according to the manual is low voltage. But the voltage is reading 12.6 which I have always thought is OK.
Am I missing something?

Raj174 · October 2018

Hi stmoloud,
I believe the low voltage referred to, is the 120 volt output and is sagging due to the load requiring more power than the inverter can handle. I understand that it started the fridge, but it may be that the size of the inverter is borderline when it comes to handling subsequent surges.

Rick

stmoloud · October 2018

Raj174 said:

Hi stmoloud,
I believe the low voltage referred to, is the 120 volt output and is sagging due to the load requiring more power than the inverter can handle. I understand that it started the fridge, but it may be that the size of the inverter is borderline when it comes to handling subsequent surges.

Rick

Thanks. So even if the voltage on the panel reports 12.6 DC, the E3 fault is referring not to the DC but to the AC output. Would it help if I were to increase the size of the AC cables? Also, as the inverter is borderline would it help if I were to introduce some additional cooling?

BB. · October 2018

What could help... Heavy/short DC battery to inverter cables (low voltage drop). As much voltage and current to the inverter input as possible.

And, actually try a 50 or 100 foot "cheap" (14 AWG or 16 AWG extension cord) from the inverter to the refrigerator (reduce surge current on the AC circuit). That will help "unload" the inverter+battery bank a bit.

What is the AH rating of your battery bank? What is the battery (flooded cell car battery, deep cycle flooded cell lead acid, AGM, etc.)?

In general, for standard refrigerators (US, standard AC compressor, bar fridge or full size fridge), in general, a 1,200 to 1,500 Watt inverter will be more reliable.

Assuming you are getting 8.1 amps on the 120 VAC circuit (~960 VA/sort of Watts)--That is sort of in the range of a typical starting current for the standard fridge (a bit high sounding).

And, it is possible that your (FLA?) battery is near 13 volts for the initial starting bus voltage, but when the voltage drops to 12.5 or so, the inverter simply cannot support the surge current requirements.

Adding the extension cord to the fridge, reduces starting surge current (while still allowing enough to start the fridge). There are hard start compressor kits you can try. Or even look at the motor start capacitor and get a couple smaller sizes (mFd) and try one of those to reduce starting surge.

One of the issues with an "undersized" power system... Even if you get it working now--A few days or months from now, it may again not start-And you end up with spoiled food...

-Bill

stmoloud · October 2018

Thanks Bill. Battery is FLA 85 A/Hrs. It reads 13.1 volts initial starting. The fridge is 240 VAC. I will try your fixes. But it looks like a bigger inverter is needed.

BB. · October 2018

85 AH @ 12 volt FLA battery... That is probably not near large enough to provide a useful amount of energy for even a small fridge...

110 Watts * 24 hours per day * 1/3 cycle time = 880 WH per day (pure guess)
880 WH per day * 1/12 volt battery bank * 1/0.85 AC inverter eff = ~86 AH per day @ 12 volts

So--Even if everything goes "right", in 24 hours, your battery will be dead (and ruined).

You need to estimate your power needs (hot weather, fridge uses more energy) and size the system to supply the needed energy... For example, a nominal setup for the battery bank would be (assuming 1,000 WH per day for standard fridge and our "rules of thumbs" for a reliable off grid power system):

1,000 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 maximum discharge (longer battery life) * 1/12 volt bus = 392 AH @ 12 volts

That would be 2x 6 volt @ 200 AH "golf cart" batteries in series * 2x parallel strings = 12 volts @ ~400 AH

A 400 AH battery bank will support a 1,000 Watt AC inverter (with 2x surge current)... It is close, but a 1,200-1,500 Watt AC inverter would probably be OK to (keep DC cables short and heavy). There are many >200 AH 6 volt batteries, or if you use AGM (much better at surge current support)--Some options.

Problem is that a refrigerator pretty much requires a mid-size solar power system... That is one of the largest energy users for most people.

-Bill

BB. · October 2018

8.1 amp draw at 240 VAC???? That is a lot of current... Is that 8.1 amps @ 12 volts (an "inverter refrigerator")?

-Bill

stmoloud · October 2018

Yes I thought twice about this. Previously I measured just under 4 amp with a kill-a-watt but the inverter panel says different. Thanks for the numbers Bill, appreciated.

Photowhit · October 2018

stmoloud said:

I measured just under 4 amp with a kill-a-watt but the inverter panel says different.

There may be additional loads you aren't aware of unless it is directly wired and nothing else connected. I didn't know there was a 240 volt version of a Kill-A-Watt meter. Even at 120 volts, a 4 amp load is huge for a small fridge, That's 480 watts. My 20 year old full size fridge uses about 2 kWhs a day during the summer. and <300 watts running.

480 watts will be a serious load on a 600 watt inverter and I'd be surprised if it started it once. I think some value isn't being measured correctly.

Also when was the fridge at 12.6 volts? Was this before hooking up the fridge, but during the day when charging? If so your battery was likely less than 1/2 full!

BB. · October 2018

Assuming Stmoloud is somewhere Auckland New Zealand... There are 230 VAC Kill-a-Watt and other brands out there.

Measuring surge current is always difficult... Most digital meters sample the voltage and current once a second or so... A true peak reading meter--You have to specific get a DMM with that function. It is matter of luck if you get a true peak current reading with a typical meter (surge many be only a few handfuls of cycles--1/10ths of a second.

A surge current of 5x to nearly 10x running current for an induction motor is not usual (i.e., 120 Watt motor will probably have an easy 5x120w=~600 Watt minimum surge to as much as >1,000 Watts)...

Technically, the surge current is probably 600-1,000+ VA (volts * amps)... Watts (power) is:

Watts (power) = Volts * Amps * Cosine current phase angle = V*A*Power Factor...

A typical induction motor, the Power Factor (PF or Cosine of the angle between voltage and current) is typically around 0.6 to 0.8 ... (Watts is either equal to or less than VA). Designing wiring/transformers, you need VA. Most residential AC inverters have Watt Max = VA Max...

AC power math is a whole 'nother post... (what Thomas Edison was not great at, but Professor Tesla was--the Physics and Mathematics of AC energy).

-Bill

stmoloud · October 2018

Photowhit said:

stmoloud said:

I measured just under 4 amp with a kill-a-watt but the inverter panel says different.

There may be additional loads you aren't aware of unless it is directly wired and nothing else connected. I didn't know there was a 240 volt version of a Kill-A-Watt meter. Even at 120 volts, a 4 amp load is huge for a small fridge, That's 480 watts. My 20 year old full size fridge uses about 2 kWhs a day during the summer. and <300 watts running.

480 watts will be a serious load on a 600 watt inverter and I'd be surprised if it started it once. I think some value isn't being measured correctly.

Also when was the fridge at 12.6 volts? Was this before hooking up the fridge, but during the day when charging? If so your battery was likely less than 1/2 full!

Actually maybe it is possible I misread the meter and it draws 0.4 Amp. It's only a few years old. So, 240 x 0.4 would come out to 96 watts? That will make it more likely to start but not very well I guess.
The 12.6 volts was the reading before the subsequent attempt just prior to the inverter failing. I did though use a fully charged jump starting battery (12v 19 amp/h) just for proof of concept, so no, it was not hooked up to any panels.

BB. · October 2018

96 Watts is closer to what I would expect for a running refrigerator compressor.

Induction motors are notorious for high starting current. More or less like starting your car (or a bit more) current from your battery.

Another issue, if you used alligator clamp jumper cables, they generally do not make good enough connection to pull substantial amounts of current (many AC inverters will simply DC voltage fault with jumper cables).

Anyway... The above numbers or a "minimum" battery bank of FLA "golf cart" batteries is where I would start (golf cart batteries are generally cheap and pretty rugged). About the cheapest way to get started with this type of system (in the US, roughly USD $100 each).

Have fun,
-Bill

stmoloud · October 2018

Thanks Bill, in my second attempt with the jump starter I had it out of the casing and was bolted directly to the terminals. Yes I agree I am going to have to spend more.

petertearai · October 2018

try an inverter fridge

Photowhit · October 2018

stmoloud said:

Thanks Bill, in my second attempt with the jump starter I had it out of the casing and was bolted directly to the terminals. Yes I agree I am going to have to spend more.

A jump start battery of 19 amp capacity, would be less likely to start it than standard FLA 85 A/Hrs. In my opinion, if they are both in good shape. You are drawing a higher percent of it's total capacity, so the voltage drop should be dramatic.

mcgivor · October 2018

The main problem in my opinion, is the conversion ratio of a 12V to 230V ~20:1, this puts extreme stress on the battery to provide any useful amount of current on the AC side, use the refrigerator as an example, 0.4A running current with an inrush of 4A. Running would be 8A with 80A inrush on the DC side, lead acid batteries can provide enormous amounts of current however the voltage drops significantly, higher voltage is definitely an advantage to avert this phenomenon, or use a chemistry that suffers less by being able to support the demands.

stmoloud · October 2018

So maybe I should try connecting the batteries in series? Probably not.
Furthermore upon reading the specs they say it is good for a 1200W output power surge but I suspect for only so many micro-seconds.
Perhaps that's the reason the subsequent start only starts the motor a quarter turn but goes no further.
Thirty minutes from a cold start-up the logic chip in the inverter has warmed up and no longer accepts what did get by thirty minutes previous.

BB. · October 2018

No, on series batteries, your bus voltage would be around 24 volts.

That would ruin your inverter (assuming 12 VDC input inverter).

Bill

Inverter low voltage at 12.6 Volts?

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