Should I bring the battery of my Ryobi 18v grass trimmer inside for the winter?

Either the latch has broken on the battery or on the trimmer housing. Or the spring loaded contacts need cleaning.

https://www.google.com/search?q=qualcast+18v+cordless+grass+trimmer+spares

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Try PartsTreeRearClinc

Depends on the battery. It's age
Is the charger topping it up really full.
I bought a Ryobi grass trimmer and the book says it would work for twenty minutes. I used it for ten minutes and it was slowing down so I took it back to the store and bought a gasoline one. they said it was not for heavy use.
Maybe now they have changed but I was not satisfied with what I had to do.

The motor may have overheated and burned open. Use a multimeter to check resistance across the battery contacts in the tool with the switch closed. If it reads open, the motor has failed.

Ryobi Trimmers unscrew halfway down the shaft (so it fits in the box). If this gets loose, the trimmer will stop working. Simply tighten the connection between the upper and lower half.

the wiring maybe disconnected, either at the switch or down near the head of the trimmer

You sure the battery is fully charged. A bad battery will run a bit and then stop providing enough power. Then if you wait it will have "rested" and work another tad. Then rest again.
Almost sure that your battery is bogus.

Usually means grass is too long for that trimmer or you are cutting too close...try trimming with just the tip of the cord...or eyelets are worn sharp

All you have to do is check the sizes of the batteries. If they are the same size, by that I mean shape and dimension, there is no reason you cant use them on different equipment. ...Lee

Does it not hold a charge, or will the charge not indicate "fast charge"? I had to apply a initial jump charge on the battery to get it up to 20 volts, then I put it in the Ryobi Charger, and it started charging it.