My phone charges upto 18% and start to discharge to zero .

check your battery on a voltmeter. It should read around 3.8V. My bet is that you have a bad battery.

I just experienced the same issue with my Galaxy Grand Prime-----aaannnndddd several other phones over the years. They were all samsung devices but this might work for you

Discharge your fully until it turns itself off.

Turn it on again and let it turn off.

Plug your phone into a charger and, without turning it on, let it charge until the on-screen or LED indicator says 100 percent.

Unplug your charger.

Turn your phone on. The battery indicator will likely not say 100 percent, so plug it in (leave your phone on for this) and continue charging until it says 100 percent on-screen as well.

Unplug your phone and restart it. If it doesn't say 100 percent plug the charger back in until it says 100 percent on screen.

Repeat this cycle until it says 100 percent (or as close as you think it's going to get) when you start it up without being plugged in.

Now, let your battery discharge all the way down to 0 percent and let your phone turn off again.

Fully charge the battery one more time without interruption and you should have reset the Android system's battery percentage.