Can I use my iPad overnight connected via USB without draining the battery, or will it still drain slowly?

Your iPad will always charge from a connected source of power if it can supply sufficient power. Most iPads require at least 12W (2.4A @5V) to reliably charge.

At ~10W or less (2.0A @5V), the iPad will not charge its battery - and will draw power that it needs to run from both its internal battery and the connected power source. As such, the external power source and the internal battery are both supplying power for the iPad. A 5W power source will be inadequate to power the iPad at all.

In summary, just connect your iPad to power source of 12W or greater. Your iPad will charge and also be fully charged when you need it to be.

To reiterate an earlier reply, your iPad will suffer no ill effects through being connected to a capable USB power source for extended periods. Your iPad will automatically regulate and optimise charging - and will not overcharge the battery. 

If the USB power source is incapable of delivering at least 10W, it is unlikely to be able to maintain the iPad's remaining battery level, let alone charge its battery. If you don't know the power output (look for a label on it), then contributors here won't be able to tell you anything about it.

Friendlyguy78 wrote:

Why are the posts displayed out of order?

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As for your battery draining, if your iPad is connected to an external source of power (e.g. your USB Power Adapter or PowerBank) rated at 12W or greater, the iPad battery will not drain at all.

Keep it connected… the battery won’t drain. When you wakeup you have fully charged iPad.

Simply connect your iPad to its Power Adapter.

Your iPad will suffer no ill effects through being connected to its Power Adapter for extended periods. Your iPad will automatically regulate and optimise charging - and will not overcharge the battery. 

When used in kiosk applications - where external power is connected for extended periods of time - the iPad may temporarily modify its charging strategy and limit battery charge to ~80% of maximum. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times.

Nominal 100% charging will be restored when a normal charge/discharge routine is restored.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

Help me, please, can I use physical overcharge protection devices? For example, I have Li-IQ Casual (from here) - I bought it because of the additional functions (protection against voltage surges - and in my region they are mostly at night, I can't control), but what about overcharge protection and draining the battery?

You're very welcome.