Battery Charger On Sale

What makes a high amp charger a poor choice? I picked one up at a pawn shop, it says 300 amp jump starter, but it looks like a dinky little thing. It was working fine, but last night it quit showing the light for charge, there was no bubbling in the tank, and it was making a high pitched sound that didn't quit even after I turned it off. So I'm guessing high ampage for electro is harmful to the charger, but not to the cast iron?
 
I think on any car battery charger the 200 or 300 amp setting is only supposed to be used briefly to start the vehicle, not as a constant charging mode.
 
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