For questions about component-level electronic circuits, tools and equipment.
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4: Be safe.
I'm trying to use it as a water pump for my aquarium. I'm using a 6V 4.5AH battery with a DC step up convertor to power the pump. After about 5 minutes it gets really hot. Is this expected?
It was probably a marginal product deliberately overdriven beyond "good design norms" to make it look good for the currently-missing data sheet.
Put it on a variable voltage power supply, run it at 6v to 12v in one volt steps, and measure its output volume for ten seconds at each step.
You might find that you still have adequate flow for your situation at a lower, cooler, voltage.