Most models of iPad require a Power Adapter rated at 12W or greater to reliably charge.
Be aware that many computer and vehicle USB ports cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, these USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).
You’ll need a suitable USB charging source. If your car does not have a USB charging outlet, you’ll very likely find that it has a 12V accessories power socket - into which you can insert a suitable USB charger.
Many USB chargers are available. You should chose one that is rated at 10-12W or greater. Here are two examples marketed by Apple:
https://store.apple.com/uk/xc/product/HN7Z2ZM/A
- mophie Dual (USB-C/USB-A) Car Charger
https://store.apple.com/uk/xc/product/HN822ZM/A