The LG CordZero is one of the better cordless vacuums available now. A new CordZero with 2 batteries costs $300 new. I agree with u/Administrative_Map25 , first I'd try the 2nd yellow battery in the base. If both batteries are worn and don't hold as much charge anymore it's totally worth it to buy a new battery. Low power is pretty useless. Medium power lasts 20 minutes and High power lasts 6 minutes. A cordless vacuum won't replace a corded vacuum for it's ability to properly deep clean carpets but it's a nice runabout for daily cleaning crumbs and stuff. More on reddit.com

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6v might be to much, best case it works but at reduced power or very short run time. Looking at the description I'm guessing the brains of the vacuum wont like it and consider it a dead battery. More on reddit.com

Tbh it's tough to diagnose remotely but I suspect it's more likely something is properly broken than it is easily fixable (if you get what I mean). If it's appearing dead then it's probably either a PCB is blown or there might be a physical issue....like something jamming the motor fan or maybe an interlock switch not working. I see replacement main bodies are around $120 so all things considered might that be the easiest option? If you want to double check the replacement battery you got is good, measure the voltage on its terminals....but I think that is unlikely to be the issue if you just got it. More on reddit.com