Can a car battery possibly kill an otherwise OK UPS?

Thread Starter

kralg

Joined Aug 13, 2017
19
In the hope that there are some experienced people here, I am trying to find the reason why and how I killed a UPS.

To make a long story short I attached a car battery to an old MGE Nova AVR 600 (360W/600VA) for testing to see if it can handle a single computer. When I disconnected the line power all transistors blew up as seen on the attached picture. I know it worked before, because first I tried it with an old unused car battery, and with that battery it was running (I could hear that typical noise) for like 5 seconds without load before shutting down.

Is it possible that MGE calculated with the internal resistance of the battery when designing this UPS?
I found some details on this page: https://www.christidis.info/index.php/blog/33-teardown-and-operation-of-an-mge-nova-avr-600
According to this the resistance of the coils have 0,1Ω, so in theory 120Amps is possible with a very low battery resistance.

Does it make sense? Any ideas?
 

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Thread Starter

kralg

Joined Aug 13, 2017
19
Thanks for the tip. The paper you quoted discusses much bigger batteries with short circuit current way above 1000Amps, but probably a 7.2Ah battery provides a much less current. Still I find it weird to rely on this as an overcurrent protection.
 

Papabravo

Joined Feb 24, 2006
16,132
Thanks for the tip. The paper you quoted discusses much bigger batteries with short circuit current way above 1000Amps, but probably a 7.2Ah battery provides a much less current. Still I find it weird to rely on this as an overcurrent protection.
Remember, that an automotive battery is designed to crank a starter motor with a VERY heavy current draw for 45-60 seconds continuous. A normal engine will start with much less cranking than that unless the battery is discharged or it is very cold. I would expect a deep cycle marine battery to work much better in this application.
 

nsaspook

Joined Aug 27, 2009
8,382
Thanks for the tip. The paper you quoted discusses much bigger batteries with short circuit current way above 1000Amps, but probably a 7.2Ah battery provides a much less current. Still I find it weird to rely on this as an overcurrent protection.
That circuit looks like a dirt cheap square-wave UPS. I don't find it weird they cut corners with an inexpensive consumer unit. It would not be the first time I've seen transistors used as fuses.
 
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