Little inverter runs a fan better than my new 1100W??

[positiv4]

Hi all! New to the board here. My apologies if this has been discussed, but I couldn't find it here or on the www.

I have had a 400W Cobra inverter for a couple years and recently bought a 1000W of the same brand. I tested it with a desk fan and did not get full speed from the fan. I tried the fan with my 400W and got full speed. I figured the inverter defective (as modest inverters can occasionally be) and sent it back. I ordered an 1100W Power Bright and experienced the same exact thing - lower fan speed! This time I checked voltage with my kill-a-watt and I see that my 400W Cobra puts out ~120v while the 1100W PB puts out ~112v. I'm guessing the 1000W Cobra was putting out lower than 120v as well because the behavior seemed identical, but didn't yet have a kill-a-watt with which to test it. From what I can figure, voltage seems to be the only difference I can find which could explain a slower fan speed? But I am diving right into all this and learning as I go. The 1100W Power Bright seems to work well; it will kick my fridge compressor and run a larger TV (albeit with a little buzzing). I just don't get why the larger inverters that I tried won't run a simple fan as well as a little $25 inverter! I am not ready to invest in a PSW inverter yet.

Any thoughts, explanations or <$100 inverter recommendations appreciated!!

[Sunking]

Voltage is within acceptable limits.

What size battery are you using to run the inverted. A 400 watt inverter needs roughly a 200 AH battery @ 12 volts. A 1100 watt inverter needs almost a 500 to 600 AH 12 volt battery. Try this little test. With the fan running, measure the battery voltage at the input of the inverter. I and do mean at the inverter input terminal, not the battery post. Then after you measure the input at the inverter, then measure the voltage on the battery post.

Lastly after you do the two above test, turn off the inverter and measure the battery voltage again.

You should find your issue.

[positiv4]

I have a 12v deep cycle 101 ah battery.

with the 400w inverter fan running: 12.56 @ inverter 12.59 @ batt 12.66 @ batt inverter off good fan speed
1100w inverter fan running: 12.57 @ inverter 12.58 @ batt 12.64 @ batt inverter off weaker fan speed

I'm fairly new to this stuff. The two tests seem pretty similar to me, but I could definitely be missing something! Does it say anything to you?

Thanks!

[positiv4]

I did a little digging about battery requirements for inverters and it does make sense that the larger inverter requires more current and so more ah. Does the inverter then somehow demand that the full current be available? I've been testing these with <100w loads, so it seems like the power needed is available. I'm here to learn so any help is appreciated.

[Sunny Solar]

You are going to get the same result with the 1000 w inverter if you connect it to a 12v motorcycle battery or 10x 1000ahr deep cycle batteries using 000 cable..
The problem is the inverter many but not all big inverters behave very badly if a very small load only is connected to them. Add another 200w load and then the fan and you most likely find the problem gone.

[Sunny Solar]

To run a desk fan on either a 400w or 2000 w inverter you only need something the size of a battery out of a toyota corolla and some fig 8x 14awg cable to connect the battery to the inverter.
BUT if the lodas are much closer to the inverters output. that you need a more substantial battery and much thicker cable ie. 8awg for 400w and 0 for 2000w

[positiv4]

Simple as that. Nice. I did fire up some incandescents and the fan did as you predicted. Turn off the lights - fan ran slower; lights back on - fan faster. Voltage even jumped up and Hz straightened out to 59.9. Thanks!

I bought the larger inverter as an estimated twice what I would need so that I won't be pushing things too hard. Good to have learned a couple other things along the way here. Much appreciated!

[Sunny Solar]

positiv4.. glad could solve your problem.. It was very simple and obvious.. I have no idea why another poster was asking and saying "" A 400 watt inverter needs roughly a 200 AH battery @ 12 volts. A 1100 watt inverter needs almost a 500 to 600 AH 12 volt battery""". Inverters of those sizes only need batteries that size if they are going to be used at near their rated output.. You were only driving a desk lamp with a battery draw of 2a plus the inverters self use of mabe 2a worst case. Total 4a..Not much of a loads and almost any 12v FLA battery and fig 8 light cable could successfully be used.

[Sunking]

I have no idea why another poster was asking and saying "" A 400 watt inverter needs roughly a 200 AH battery @ 12 volts. A 1100 watt inverter needs almost a 500 to 600 AH 12 volt battery""".

Hogwash John you know exactly why I said that. Battery capacity has to me matched up to inverter wattage. It is something we talk about all the time. For a FLA battery maximum current at discharge should be roughly no greater than C/8. Otherwise internal resistance is going to cause excessive voltage drop at the battery.

As for motor speeds, well if it is an AC motor, voltage only determines the speed if the AC motor is an Induction motor, otherwise line frequency and the number of poles in the motor determines the RPM.

[positiv4]

I did see the tutorial here about battery / inverter size. Do I understand correctly that a typical FLA discharge is C/8 meaning that a 101ah battery should be discharged at ~12A (101/8), and so drawing >12A will result in a decrease in voltage due to resistance of the battery itself? Obviously I wasn't drawing over 12A for a fan, but good to know as I learn this stuff.

[Sunking]

I did see the tutorial here about battery / inverter size. Do I understand correctly that a typical FLA discharge is C/8 meaning that a 101ah battery should be discharged at ~12A (101/8), and so drawing >12A will result in a decrease in voltage due to resistance of the battery itself? Obviously I wasn't drawing over 12A for a fan, but good to know as I learn this stuff.

C/8 is the rule of thumb for FLA batteries. Really depends on what type of battery it is. For example a hybrid type made for golf carts and floor sweepers can supply quite a bit more than C/8 without excessive voltage sag. SLI aka starting/cranking batteries can deliver very high currents for a short period of time.

The difference is in the way they are constructed. A starting battery is made of many thin spongy like plates to increase suface area which allows them to have very low resistance. A True Deep cycle battery has much fewer but huch heavier and thicker plates to enable the battery to support high cycle life but the trade-off is higher resistance. Hybrids are a cross between Starting and Deep Cycle.

When you are working with low voltage like 12, 24, and 48 volts voltage loss is critical. a 1 volt drop on a 12 volt system is 8.3% loss in voltage and power. Not only do you have to account for voltage sag of the battery, but wiring losses becomes very critical. When added together if you put a undersized battery on a large inverter where the inverter is drawing rated capacity, you will have an Under Voltage trip even though the battery is fully charged up. Undervoltage input to th einverter can also lead to under voltage on the inverter output. That is why I had you do the voltage checks to see if you were sagging or losing voltage on your wiring.

Now here is the other thing to think about is Peukert Law. Your 105 AH battery is rated for a C/20 Discharge rate which means just over 5 amps for 20 hours before the voltage reaches 10.5 volts. However if you discharge at the higher discharge rate of C/8 or 13 amps your AH rating goes from 105 AH to about 80 AH. So at the C/8 rate one would think you could pull 13 amps for 8 hours. Not going to happen, more like 6 hours. This is why systems need to be sized to do what you want..

[Mike90250]

Inverters have "overhead" looses when they are on, whither or not you are using power, it is. Larger inverters have larger overhead.

Mod sine inverters also vary the wave form they produce somewhat, to regulate the power, and you may be on the low side of the useable area with the large inverter/small inductive load (fan) Motors generally, do NOT like mod sine power, and will run hotter and consume more power from the battery.

[positiv4]

Good stuff! I appreciate the knowledge. So when I calculate how much/long I can run is 10.5v where I can safely end up? What's the best way to monitor state of charge? I have an LCD volt meter on my battery as well as a meter between the controller and battery. I do understand that I will only get true voltage after 24hrs of resting, but can't I get a feel for SOC from working voltage? I've read not to discharge my deep cycle FLA lower than 50%.

Lots to learn here. This is why I'm starting with a simple and inexpensive setup that I can build on in the future. Thanks again!

[Sunking]

I do understand that I will only get true voltage after 24hrs of resting, but can't I get a feel for SOC from working voltage? I've read not to discharge my deep cycle FLA lower than 50%.

Do you have a hydrometer? You can get a working voltage using a hydrometer to record voltages at the 90, 80, 70%.... SOC

For now just do not go below 12.1 volts aka 50%

[russ]

10.5 volts is deder than ded!