Inverter help

BB. said:

Generally, if an inverter beeps and shuts down, it is because the battery bus voltage is low on its DC input terminals.

Assuming this is a 12 volt AC inverter running 650 Watt heat pump system (that is not bad, what brand/model of heat pump?):

• 650 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff = 73 Amps max continuous on 12 Volt DC bus

You need short/heavy copper cables to carry that current. If they are too long, too small of AWG, and/or there are poor/loose/corroded connections, any of those can cause the inverter to have a low voltage shutdown.

The other typical issue is the state of charge/health of the Battery Bank. A small battery bank, older batteries, batteries that have not been properly recharged/maintained can all cause problems too.

Anyway--First, use a volt meter and check the voltage on the AC inverter while trying to run the heat pump. If you have low voltage (anything under ~11.0 volts is pretty low), check the voltage on the Batteries/Battery Bus. You are looking for excessive voltage drop (like 12.5 on the battery bus, vs 10.5 on the DC input terminals for the inverter).

If you are using a temporary connection (like jumper cables) to try and run the inverter--No good. Jumper cable clamps (alligator clamps) are just not reliable enough to run (or sometimes even start) an AC inverter....

To give you an idea about the copper wiring. I would suggest a maximum of 0.5 volt drop at 73 Amps. Say you have 5 feet (one way run for this calculator), the minimum cable size would be:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=5&distanceunit=feet&amperes=74&x=53&y=26
5 feet of 8 AWG (one way run) cable with 74 amps gives:
Voltage drop: 0.46
Voltage drop percentage: 3.87%
Voltage at the end: 11.54

That is not too bad... But you also need to look at how much current the cable can safely carry. Using the NEC (simplified) table:
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Depending on insulation type, 8 AWG to 4 AWG cable would be suggested for 73 Amps. For Battery Bus/AC inverter inputs, I like to derate by 1.25 (NEC continuous current derating):

• 73 Amps * 1.25 NEC derate = 91 amp rated branch circuit wiring/Circuit Breaker/Fuse

91 amp rated wiring--4 AWG or 2 AWG would be nice...

But, you can also look at Marine Wire rating and see that they are not nearly as conservative:

https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

8 AWG or 6 AWG would be allowed.

So, that is where I would start.

If you work with a fair amount of DC wiring/power systems, getting a (AC+)DC current clamp DMM would be really nice:

https://www.amazon.com/UNI-T-Digital-Current-Capacitance-Multimeter/dp/B0772FYF5M (inexpensive, good enough, this unit may not be available on Amazon for the moment)
https://www.amazon.com/gp/product/B019CY4FB4 (mid priced unit)

Note there are many AC current clamp meters--Very nice--However, does not measure DC current. I am suggesting AC+DC Current Clamp DMM for this use. They safely and easily measure upwards of 400 Amps DC current safely, and you do not have to "insert" a current meter or current shunt to do the measurement).

-BillI 

M using 2 and 4 gage battery cables with Copper fittings on the end.  They're short. One is long because I have one of the batteries in the front under the drivers seat and those cables are like 2 feet. The batteries are full I charged then and checked them. I also have a DC to DC charger. It's only a 20 amp and in use it just when they're full to maintain. The other thing I forgot to say is I moved the inverter and how I switched sides on it I accidently shorted out the fuses inside. But I changed then and put in 6 new 30 amp fuses. Everything works perfect except the AC.  The microwave uses more power. So is the AC unit bad or could the inverter be the problem? It's the big 3 prong grounded plug with a test and reset button on it. The microwave has a regular 3 prong. Could it be that I need a different socket for it? I know I have enough power so it's either something the av unit doesn't like it my inverter isn't putting out what it should. I did measure and it said only 103 v then I measured the small inverter and that one said 119. I'm about to buy a 3000 watt inverter just to see if it will work then. I just don't understand how it could run longer on the smaller 300 watt modified sine then on the nice big one with pure sine.  what's causing it to stop? The weird thing is the microwave works. And that's where I get stuck trying to solve this. How can something more powerful work and the lesser one not work. 

It's the AC unit that makes the noise not the inverter. Just bought a 3000 watt inverter. And finally stopped driving. Going to install it how and see if the AC will work now.