I had an onyx 245 a few years back and I'm 99.9% sure it has the lipo storage mode, read your manual carefully and I think you will find it. this is basic maintenance for your lipo, it will keep it healthy and make it last a lot longer!
What charger do you have?
If your charger actually doesn't have a storage charge your going to want to store them at 3.8v per cell or 7.6v for a 2s pack.