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I have a battery pack consisting of 4 10 amp-hour 12 volt batteries wired in parallel. We recently bought a 2000 Watt rated HammerDown power inverter to run a 500 watt window air conditioner. The air conditioner is supposed to draw 5.6 amps. But when I connect the AC to the inverter the air conditioner always shuts off as soon as the compressor starts running. I measured the current going to the inverter and as soon as the compressor started it shot up from around .3 Amps a whopping 26 Amps before turning off! Now I understand there is a surge of current needed when starting an appliance but I didn't think it would be this much. I also noted that the voltage dropped a little too (from 13.5 to 12.9 v). Given all that though, I don't understand why the inverter failed to supply the power to the AC. With 40 amp hours shouldn't it be able to provide 40 amps of current at any moment (more than needed for surge)? Another interesting thing to note is that sometimes the inverter would read OVERLOAD after one of these tests.

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    \$\begingroup\$ First off, your inverter is a piece of junk. Second off, the wires between the inverter and battery pack are too small and/or too long. Third off your battery pack is too small. Basically you have a 480 Wh battery pack and you are trying to drive a 500 Watt load. Buy a good quality inverter designed for 48V input, and put your 4 batteries in series. Use 8AWG wire for the batteries. Maybe you will be OK. But don't expect the AC to run for long on that battery bank, and don't expect to get too many charge cycles out of that setup. \$\endgroup\$ – mkeith Jun 18 '19 at 3:05
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    \$\begingroup\$ You measured a voltage drop down to 12.9 with a meter. If you check it with an oscilloscope, you will see that it briefly drops much lower than that, and that the input current is much more than 26 Amps (for a short time) and the inverter shuts down during the dip. I could be wrong, but probably I am right. \$\endgroup\$ – mkeith Jun 18 '19 at 3:20
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    \$\begingroup\$ If you use a 12V inverter, you will need much larger wire, like 2AWG or 0AWG. The only reason I suggest to use a 48V inverter is to allow you to use smaller diameter copper wire. \$\endgroup\$ – mkeith Jun 18 '19 at 3:28
  • \$\begingroup\$ Thanks. You're right about the skinny wires to inverter (10 gauge). I will try wiring the inverter directly to the battery pack and maybe replacing the battery connection wires with higher gauges. \$\endgroup\$ – st4rgut Jun 18 '19 at 5:28
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    \$\begingroup\$ Replaced the inverter and it works now! thanks everybody. My inverter is rated 2000 watts continuous 4000 watt surge. It has nice thick cables that connect to the battery pack. \$\endgroup\$ – st4rgut Jun 21 '19 at 0:42
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Your compressor is likely run by an AC induction motor, and they take very large surge currents to start.

Your inverter is rated at 2kW, and no matter how much battery capacity you have it will cut off when it reaches its overcurrent limit. If your mains voltage is 120VAC 40A would be 4080W, well above the rating of your inverter. (Even worse if your mains voltage is 220VAC).

Your inverter can likely handle short surges above rating, but starting an AC compressor is a notoriously hard thing to handle.

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  • \$\begingroup\$ thanks. the inverter says it can handle 4000 watt surge. It outputs 110 volts. How do I know what the limit is? Or how do I tell whether the bottleneck is the battery (not enough amp hours) or the inverter (not rated for such high current) ? \$\endgroup\$ – st4rgut Jun 17 '19 at 22:42
  • \$\begingroup\$ @st4rgut: if the inverter shows "Overload" after attempting to start the air conditioner, I'd say that the inverter is unable to handle the load. Note that the inverter would be attempting to draw over 56 amps from the battery when the air conditioner is drawing 5.6 amps, so the 40 Ah battery would last well under an hour, if it was possible to start the air conditioner - hardly a practical system! \$\endgroup\$ – Peter Bennett Jun 17 '19 at 22:55
  • \$\begingroup\$ where did the 56 amps come from? \$\endgroup\$ – st4rgut Jun 17 '19 at 23:18
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    \$\begingroup\$ 5.6A x 120V = 56A x 12V = 672W ... of course that is ideal, not real life \$\endgroup\$ – jsotola Jun 18 '19 at 0:55
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John D has answered the inverter overload issue. I'll answer another part.

With 40 amp hours shouldn't it be able to provide 40 amps of current at any moment (more than needed for surge)?

To understand the maximum current from the battery you need to consult the datasheet. 40 Ah is an energy rating (sort of - a better energy measure would be Wh, watt-hours which is V x Ah = 12 x 40 = 480 Wh in your case). The Ah rating is usually measured on a 10-hour discharge so a 40 Ah battery will typically supply 4 A for 10 h. At lower discharge rates the time will be higher than the rating would suggest while at high discharge rates it will be lower. i.e., at 40 A discharge, if the battery can handle it, the discharge time will be considerably less than the 1 h the calculation might suggest.

The 40 Ah rating does not tell you that the battery can actually supply 40 A. It might not or might be able to supply 400 A. Again, the datasheet is your reference.

I also noted that the voltage dropped a little too (from 13.5 to 12.9 V).

Try measuring at the compressor. If it drops more there then your cable is inadequate.


Notes: SI units named after a person have their symbols capitalised but are lowercase when spelled out. 'V' for volt, 'A' for ampere, 'K' for kelvin, 'Ω' (capital omega) for ohm, etc. Meanwhile 'k' is for kilo.

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  • \$\begingroup\$ ok i will consult the battery data sheet thanks. \$\endgroup\$ – st4rgut Jun 17 '19 at 23:17
  • \$\begingroup\$ since I was able to read 26 amps from the inverter with an ammeter, the battery should be able to supply that much right? \$\endgroup\$ – st4rgut Jun 17 '19 at 23:29
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    \$\begingroup\$ 26 A @ 12 V into the inverter or 26 A out @ 110 V? You need to be more precise in your writing. \$\endgroup\$ – Transistor Jun 17 '19 at 23:39
  • \$\begingroup\$ sorry, 26 A @ 12 V into the inverter \$\endgroup\$ – st4rgut Jun 18 '19 at 0:34
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Motors when starting draw from 3x ( soft-start 3 phase type) to 5x typ to 8x with Start-Cap surge current to 12x for high-efficiency BLDC motors. (10x typ)

Your inverter is unable to handle the startup load. It is intended for Blenders, Vacuums and Power Tools and not an air-conditioner compressor which has a much longer startup time.

For industrial use, they may choose a VFD instead of an inverter. This allows the user to choose a voltage a slow voltage ramp with a rising frequency and fixed V/f and thus it is possible to limit the surge current much lower values with high torque, instead of 8x as in your case.

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  • \$\begingroup\$ Even if the inverter can handle it, the battery would probably dip very low and might trigger UVLO. \$\endgroup\$ – mkeith Jun 18 '19 at 5:15
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    \$\begingroup\$ Agreed . 40Ah * 12V is only 480 Wh means less than hr. Op time. \$\endgroup\$ – Tony Stewart EE75 Jun 18 '19 at 6:39

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