iPad model 2602. Plug into USB charger. Shows charging for a few seconds, then stops.

These support pages should help with troubleshooting battery and charging issues:

Charge and monitor the iPad battery - Apple Support

If your iPad won't charge – Apple Support

Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item. Be aware that most models of iPad require a Power Adapter rated at 12W or greater to reliably charge. 5W Power Adapters intended for older models of iPhone are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow.

If you continue to experience difficulties when using an appropriate Power Adapter of 12W or greater, you might be best advised to seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad, Power Adapter and cable(s):

Genius Bar Reservation and Apple Support Options - Apple

My iPad mini 4 keeps losing charge rapidly (from 100% to zero) soon after start up. I have checked chargers and cables are working with other apple devices. It’s os 15.3.1.. it could not do the most recent update as it closed itself down before completing.

HadNhet wrote:

My iPad mini 4 keeps losing charge rapidly (from 100% to zero) soon after start up. I have checked chargers and cables are working with other apple devices. It’s os 15.3.1.. it could not do the most recent update as it closed itself down before completing.

As a protective measure, to prevent update failure or system corruption though power failure, it is strongly recommended that your iPad remains connected to its Power Adapter whenever you perform an over-the-air software update from iPad settings.