Maximum charge current for iPad battery based on voltage and battery size

Certified USB charging cables are both recommended and adequate. Using an Apple USB charging cable will ensure that you have a compatible cable suitable for charging your iPad or other Apple devices.

Most models of iPad require a Power Adapter rated at 12W (i.e., 2.4A @5V) or greater to reliably charge. 5W Power Adapters (1.0A @5V) intended for older models of iPhone are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow. Recent models of iPad are supplied with a 20W USB-C Power Adapter.

Some third-party USB-C power adapters (and portable PowerBanks) may not support USB PD (Power Delivery). Power Delivery mutually negotiates the charging voltage; the source, load and cable are all elements involved in the negotiation protocol. Unless the Power Adapter explicitly supports USB PD, it will almost certainly fail to charge the iPad; proprietary charging standards, such as QC (Quick Charge), are not supported.

Many computer USB ports also cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, computer USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).

The higher charging voltages employed by USB PD, where supported by the device being charged, reduces the current needed to transfer the equivalent power. Volts (V)x Amps (A) = Power (W). As such, raising the voltage for a given current allows greater power transfer (W) without exceeding the current capacity (A) of a given conductor's cross-section area (mm squared).

User Attempts to manage charging current are usually an indication that User knows enough to be dangerous. Please see the additional advice in When to charge your iPhone or iPad - Apple Community. As Lawrence Finch points out, the actual charge control is in the device being charged. The external parts are simply a source of volts and amps.

You don’t even say which model iPad you are using. The charging cable for older models uses the Lightning connector capable of handling at least 2A from the power adapter supplied with the device (5v/10W or 12W). More recent iPads have USB-C connectors and can use a power adaptor with PD rating (multi voltage, multiple power level negotiated by the iPad). The default USB-C cable from Apple is rated at 60W but of course the Amps to provide this power level will depend on the voltage negotiated by the device being charged. If you don’t also know the voltage in a PD charging setup it is pointless to know the charging current.

My only other comments are around measuring charge current. Don’t Do That because it indicates you are trying to use unapproved power supply sources. The cables supplied with Apple devices or sold by Apple are adequate for their intended power ratings but they don’t have a meter connection facility.

We are not apple, on this forum there are only apple users participating. If you want to get an answer from apple directly you might want to contact apple support for further reference