Shouldn't it be "not good" instead of "no good"?

No good is correct. But I have no idea why or how to explain why.

Your intuition is correct: "The battery is not good" is grammatically correct and *would* be the more correct way to say this. However, "No-good" is a phrase that means "worthless" or "useless". If this wasn't a phrase, then "the battery is no good" *would* be grammatically incorrect. https://dictionary.cambridge.org/dictionary/english/no-good "No good" can also mean "untrustworthy" or "immoral", but in this case it means broken or worthless (see definition 3)

“No good” is a phrase that is acting as a single adjective modifying “battery”. “No good” as an adjective is actually closer to “bad” than it is to the phrase “not good” If we were to say “The battery is not good” we are specifically saying that the battery isn’t good but we aren’t saying it’s bad. In the case of batteries this doesn’t really make any sense. (Which is probably why it wasn’t used) But imagine a cake. “This cake is not good” means the cake is not good, but maybe it’s just ok. It’s not good, but it’s not necessarily bad either. But if we say “this cake is no good” it means the cake is positively bad.

They're a little different. I would say no good is stronger. It means not good at all, useless or worthless. Not good simply means...not good.

Both are correct but have slightly different meanings (most people don't even know and use them interchangeably).**"Not good"** is more neutral and descriptive. It simply indicates that something is of poor quality or unsatisfactory.**"No good"** is idiomatic and often implies that something is useless or completely inadequate. In your sentence, **"no good"** works well if you want to say the battery is defective or unusable. If you’re describing a milder issue, such as reduced efficiency, **"not good"** might be a better choice. But honestly, it doesn't matter because whatever you pick it conveys the same idea.

Both are correct. “No good” just means “not good” or “worthless/useless.”

Both sound correct to me

One meaning is "not, not at all". The examples are: You’re no better than a common thief. Look no further than one's nose (From Wiktionary) Tbh, I don't see this usage very often.

“No good” is correct. Using “no” like this is an old usage that remains only in set phrases. You use it specifically for broken or used up parts and in a few other very specific cases. “Not good” would mean the battery isn’t high quality.

“No good” is a common phrasing/idiom in informal contexts, but I would not say it is grammatically correct. “Not good” would definitely be a better way to write it.

“No good” is correct, it’s an English idiom

"No good" is a very common expression. "Good" is a noun in this context. In "not good", it's an adjective. Apparently, the earliest known use of "no good" was in 1838: https://www.oed.com/search/dictionary/?scope=Entries&q=no%20good

With negating most adjectives, you’d be correct. Here, however, “no good,” is a single phrase used when something doesn’t work as it’s supposed to, e.g. (1) is shoddy in quality, (2) not right for the situation, (3) broken, or (4) worn out, as is likeliest in the case of this battery, which has probably been drained through use. It’s different than when something is simply “not good,” which has a broader set of meanings that could, although idiomatically wouldn’t, encompass the meaning of “no good,” as well as other metaphorical meanings or comments on quality.

"no good" is a very common but informal way to say something is useless, unfit for the purpose, ineffective.

No good is a phrase meaning something is literally bad, not good means that, simply, its not good

*Good* can be a noun, as it is here. If something is no good, it is/provides no benefit at all. If your idea is no use, it provides no solution. If your sister is no fun, she doesn't foster any jollity. You can do this with a vast array of abstract nouns, including concrete nouns used metaphorically. It's no problem! (Colloquially, it ain't no sweat!)

“Not good” could mean the battery isn’t good relative to other batteries. If they work for a year, but this one for only six months, I might describe it as “not good”. “No good” would mean the battery doesn’t work at all.

Both work. Unlike the battery

I feel like not good could mean anything from underperforming (how? Idk) to depleted. No good pretty much means the battery is empty.

"No good" is a phrase, much like "it's gone" or "it's dead." Just a fancier way of saying the battery is in terrible shape.

not good implies there's still room for usage. no good means there is nothing good about it, its time to throw it in the bin

No good ≠ no + good, unfortunately.

Both are correct. However “not good” is more a neutral statement of fact, while “no good” is more idiomatic and gives off the sound of disgust or irritation towards the thing.

Not good will sound more correct to you but somewhere someone started saying no good and now it's an idiom, that means basically the same as not good.

No good is an adjective that means something is not good

Both are correct, 'no good' is more of a euphemistic term than a 'not good', the later of which is very literal. Google defines the term as meaning worthless and means the same thing as 'good for nothing' or 'good for naught'. It could describe the uselessness of an object or a person who is nefarious, evil, self interested, or a criminal. Inversely, 'good' in the context of 'the battery is good' would mean that whatever you are describe as 'good' is in functional or working condition or a person who is generally honest. If I say 'That's Jerry standing by the table. Jerry is a good guy.' it may mean he is a the kind of person I would can trust to feed my dog while I'm out of town for a few days or pay me back if I lend them money. However you could say it as a way to down play one of there faults. If I say 'Jerry is a cigarette smoker, but hes a good guy', that carries the specific implication that Jerry is a better person than what that trait would imply culturally.

“No good” is a slang/idiomatic expression for broken or bad. 

If a battery is not good, then it is of poor quality. If a battery is “no-good,” then the battery has no charge. Both are correct, but no-good is a little informal.

Not good would mean the battery is of poor quality, no good would mean it's not fit for use

"not good" means something is fundamentally/permanently wrong with whatever you're talking about, while "no good" means less of an issue. in this case of a battery, if someone told me it was not good I'd assume the thing was broken and would never work again. while no good i would interpret as it just doesn't have a charge currently. make sense?

En “not good,” good es un adjetivo. En “no good,” good es un nombre. Los dos son correcto, pero tienen significados un poco diferentes. “Not good” = no bueno. Puede ser “evil” o “bad quality” o “unfortunate” o más. “No good” solo significa “bad quality.” “His intentions were not good” ✅ (he had malicious intentions) “His intentions were no good” ❌ (no tiene sentido, suena como él no sabe tener las intenciones) “That idea is not good” ✅ “That idea is no good” ✅ (I really can’t detect a difference between these) “These potatoes are not good” ✅ “These potatoes are no good” ✅ (to me, the first one sounds like the potatoes were prepared badly and the second sounds like the potatoes are rotten or spoiled, but I think either sentence can mean either) “This password is not good” ✅ (you need a more secure password) “This password is no good” ✅ (the password is wrong and I can’t log in) In general there is a lot of overlap between the two, but “no good” is a bit more specific

"Not Good" == 'not'-modifier 'good'-subject "No-Good" == noun-phrase, itself the subject ...at least that's kinda the vibe I get

No good = useless Not good = faulty, doesnt yield desireable result. I.e. "This battery is not good" = this battery is bad, it has many faults but still useable. Whether or not you use it is up to you. You shouldnt use it. "This battery is no good." = you cannot use it at all. Either due to dead battery, or differences in voltage and ampe. You couldnt use it.

While “not good” is technically correct, “no good” is also correct because “no good” is an English phrase meaning that something is worthless or useless. So it is being used as an idiom. You may see similar usages with phrases such as “no problem” or “it’s no hassle”. But generally, you are correct about the usage of “no” vs “not.”

It’s native but it’s probably stolen from another language English is a thief it beats up other languages in back alleys and rifles through their pockets for loose grammar and spare vocabulary…

“No good” is one phrase meaning “not fitting” It’s no good = it doesnt work we should try something else

I'm more surprised that Duolingo would be teaching this at all, unless it's specifically in an idiom lesson. Both are correct, with different meanings. **'No good' = bad** (within the context) - this could be something like 'rancid', 'off', or (tastes) 'awful' for food. The battery, in this case, is very likely being called 'broken'. **Not good = *not* good** - by saying something isn't good, it doesn't mean it has to be bad, just that it isn't 'good'. Something could be average/medicore or just 'okay'. It gets a bit confusing because this could definitely be used in the same context, it would just mean something different. If the battery was 'not good', it could mean that the brand or type of battery is bad or not functioning as intended, rather than necessarily being broken or unusable.

Jfc, I thought I was looking at a German word for a second

The battery is “no good” means, colloquially, it’s not working. You would say it in the specific context of talking about your/someone else’s car not starting. If you are shopping for a car battery and evaluating the quality of batteries, you could say, this battery is not good, implying there are other, better car batteries available.