Inverter dropping voltage

[Clueless]

I have a 1.2Kw(1.8kw surge) 12v PSW inverter which happily runs approx 50w constant (300w peak) of load @ 225v 24/7. When I change the load circuit to a constant pressure water pump that uses 650w, the inverter drops down to ~80v and of course doesn't provide enough power to run the pump. What could be the issue?

I have 2*12v/65ah AGM batteries in parallel. A meter on the inverter dc load side say the inverter is pulling 56amps @ 12.6v, which i believe to be in the ballpark to handle 650w @ 225vac (~3 amps.) for short periods at least. The pump works fine connected to the mains.

Before I start replacing the inverter (maybe upgrade to 24v inverter & changing the battery config to serial are my next moves to reduce the dc amps), can anyone point me where I'm going wrong as my beginners brain says this should be working.

Thanks

 

[MichaelK]

56A. That is a lot of amps to try pulling out of a little battery. Assuming you don't want to draw more than 1/8 of capacity from a lead-acid battery, and you have two, the math works out to be (65Ah X 2 batteries X 12V)/8 = 195W max. So, you are trying to draw way too many amps from that battery.

An additional issue might be the size of your wiring. What's the gauge of the wire running from the battery terminals to the inverter? For my 24V workshop I'm using 3/0 gauge. For the 48V cabin, 4/0 gauge.

I would advise you to upgrade to 24V AND get a bigger battery. I consider 225Ah golf-cart batteries to be small. (650W/24V) X 8fold capacity = 217Ah. So four 6V golf-carts in series would be marginally adequate.

 

[mikefitz]

The problem could be much more than the battery power inadequacy. Your 650 watt water pump will take a much higher start up power from the system. Ideally you need to measure the peak power to start and run the pomp when its operating, that is pumping water.
At a guess around 3kw peak will be needed.

Mike

 

[MichaelK]

The problem could be much more than the battery power inadequacy. Your 650 watt water pump will take a much higher start up power from the system. Ideally you need to measure the peak power to start and run the pomp when its operating, that is pumping water.
At a guess around 3kw peak will be needed.

This is what's needed to determine that. A clamp meter with "inrush current" capability. It can accurately measure the initial "blink of an eye" surge that a regular clamp meter is too slow to react to. I used it to measure the inrush of my well-pump.

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I have two, the UniT mentioned here, and a far more expensive Fluke 274. Both read within 1% of each other. Now I recommend the UniT

 

[time2roll]

When you start the pump have a voltmeter on the inverter terminals to monitor voltage. Even better have a second voltmeter on the battery terminals. Post the results during start up of the 650w pump.