Problems charging older iPad The iPad was bought 5 years ago, it runs iOS 9.3.5 and claims to be up to date.

The battery is fine - it does hold the charge, and it does charge fast (when it decides to charge).

The cable is also fine - I bought a new thick cable and plug it into a "travel adapter" which outputs 5.1V/0.7A (same behavior with a 5V/2A adapter).

Problem: when I plug the cable into the iPad and adapter, the charge indicator (lightning next to the battery icon) does not come up.

However, after I 


Unplug the cable (either from the iPad or from the adapter)
Turn off iPad
Plug the cable back


the iPad turns on right away, and usually starts charging (and then it charges fast and holdw the charge fine, as mentioned above).

What do I do to get the device to charge reliably?
I suspect the output of the travel adaptor is too low. 

I admit that I've had several iPads from that era, and on occasion ignored Apple's guidance (they would list groupings that would work, but didn't have a "don't do this list of combinations). 

But .7 is pretty low. If the model is using lightning (iPad original (4th gen) and iPad Mini (original)?) then you might want to check the both sides of the connector for lint or material that is interfering with the connection. (This was a big improvement over iPod/style, but it is more prone to contact problems over the lifetime of the unit).

There's a very good discussion about iPads and charging here -> 
Can I use my iPhone charger with my iPad?