Reliable Inverter Big Voltage Shuts Off Everything Under Medium Load

st4rgut

Hi all, I bought a Reliable 2500W 48V pure sine wave inverter 6 months ago for off grid living and has served me well until recently. The power would cut off intermittently usually when high energy consumption appliances turned on like a water heater or small oven. I was confused at what was happening, but finally caught the inverter in the act as the problem recurred more frequently under smaller loads. Here's a snapshot of the low voltage problem, but I've linked a youtube video as well.

If you watch the video, the voltage displayed by the inverter drops from 120 to 53V, when a heavy load (in this case a water heater) turns on drawing ~ 30 A from the battery. This power consumption seems well within the inverters capabilities (48 * 30 < 2500 ) yet the inverter suffers this tremendous voltage drop. After everything turns off due to the low voltage, the inverter seems to recover quickly, but the blackouts seem to be getting more frequent. I have the feeling the inverter is slowly dying, and want the manufacturer to honor the 1 year warranty, but they're based in China, and the salesperson has to go back and forth with the engineers and comes back with suggestions like "try taking out the breaker", which I don't know what to make of. I've only seen good reviews about the inverter which is why I bought it, and its the cheapest 48V inverter out there. Has anyone had a bad experience with Reliable? I'm thinking about replacing this inverter with a bigger one, but would like to know why this happened first.

BB.

It looks like the three meters are:

DC Battery Bus voltage (where is this voltage taken--At the Battery Bus or on the DC input terminals to the inverter)?
AC Bus Voltage
DC Bus Current

Can you measure the voltage on the DC Input terminals of the AC inverter with your DMM during the Power Droop?

30 Amps * 55 volts = 1,650 Watts DC Input or so--Should not be a problem for the inverter.

What is the battery bank (Battery type and AH rating of bank)? What is the AWG (wire gauge) and one way length of the cable from the battery bank to the DC inverter?--Any Voltage drop issue--Again where is DC bus voltage measured).

Check all DC terminals are tight/clean. After running a "heavy" AC load (not crashing the inverter output) for a while--Are cables and terminals cool to the touch (be careful--A poor connection can get burning hot pretty quickly--Don't burn yourself).

Those are my first debugging questions--Assuming AC inverter is still good.

-Bill

mcgivor

The inverter is likely not the cause but rather voltage drop due to under capacity batteries, under sized or too long DC conductors, poor treminal connection, failing batteries, or a combination of all, electronic equipment such as an inverter do not gradually fail, they either work or don't.      

st4rgut

@BB. Thank you for your reply. According to the manual, the DC voltage displayed on the inverter is the "Input DC voltage display". The voltage is taken at the DC input terminals to the inverter. The battery chemistry is Lithium Iron Phosphate and it is 100 Ah. Here is a pic. While drawing it I realized the bms cable may be the weak link. But the specs say 60 amp continuous discharge current (https://www.ebay.com/itm/233779221863). On a related note, I've had the BMS turn off I guess to overheating and flipping the breaker in the process.

mike95490

Whoa !     awg10 is rated for 30A  Pushing 90A through it will cause it to both overheat and reduce the voltage.

200A will require OO wire, or larger - depending on the distance

A inverter that needs 200A, will likely trip your 100A breaker.  Often.

The nasty thing about inverters, is as the battery voltage sags (discharging battery / or / voltage loss in small cable  ) the inverter will try to draw more and more amps, till "something" happens.  ( breaker trips, wire melts, battery damaged, BMS shuts system down )

mcgivor

The 30A rating for 10 AWG is taking into consideration length of run in typical installations, the actual current it is capable of carrying is significantly higher over short runs. Since the BMS pigtails are probably no more than a few inches 10 AWG is more than capable of handling 60A without significant voltage drop, as shown below using the minimum 1 foot, value the result is 0.14 V

BB.

A 2,500 Watt will draw, continuously, worst case:

• 2,500 Watts * 1/0.85 inverter eff * 1/42.0 volt battery cutoff = 70 amps "worst case @ 42 VDC inverter cutoff)
• 2,500 Watts * 1/0.85 inverter eff * 1/56 volts battery bus voltage = 53 Amps with "good voltage" on inverter DC input

As you can see, inverter current draw does depend on DC input voltage (constant power device: P=V*I ... Voltage goes down, I(current) goes up to supply constant Power).

Also, a 2,500 Watt inverter can supply something like 2x rated output wattage for several cycles to several minutes--Depending on inverter usage.

mcgivor

The symptoms in the video are consistent with an overload, if the inverter can support the heating loads most of the time but intermittently drops I would suspect there is a cyclical load such as a pump for example, can you confirm either way? 

High frequency inverters suffer with induction motor starting being there is no heavy transformer to absorb the inrush, if there are other loads eliminating them temporarily would eliminate their influence 

The sound your  inverter makes is identical to my cheap 2000W backup inverter makes when it attempts to power a microwave, also an induction load.

st4rgut

@mcgivor you're right, I do have a pool pump running in addition to 2 chest freezers, 1 fridge and microwave which are all inductive loads though I can confirm at the time the microwave was not on. Also the pump was already primed and running using less than 300W. I've removed all heavy loads from the inverter now (little oven and water heater) and the inverter works fine. 

mcgivor

Too many possibilities for one device  being the culprit for tipping the ballance when running the resistive heating load, the fridge, along with the freezers, not to mention the pool pump, are all inductive loads, any one starting could induce an overload.

My personal thoughts are there is nothing wrong, with the inverter, it's just being asked to do more than it's designed for. Taking the time to eliminate certain device loads will provide valuable information to operate within it's  parameters,  or consider a larger low frequency inverter capable of absorbing the intermittent  inductive  cycles.

Unfortunately it's a common misconception users make, basing useage on wattage  ratings alone, without factoring in power factor, a phenomenon associated with inductive loads which influence the generating capacity independent of the battery capacity.

The subject is extremely complicated for the average layperson  without prior education, so without prejudice, don't assume the inverter is the cause of the intermittent failure, the engineering department would come to the same conclusion when provided with the same information.