Could someone else in this thread please confirm if they see this battery recharge issue? Please plug HA-2 into a computer, turn HA-2 on, don't play any audio, and leave it fully charged for a few hours. Does it start recharging the battery after some time?
It sounds normal to me. The battery will self discharge even with the unit turned of and sitting in a drawer. The charger brings it to full and stops charging. The charge drops and it starts charging again.
Even if the HA-2 is drawing zero power from the battery, I would expect it to go into charge mode periodically. I see the same with mine, but have never worried about it. Why is it concerning to you?
^ I agree.
The greatest rate of self-discharge (steepest portion of the curve) happens immediately after reaching a full charge. So, it doesn't take long for a charging circuit to detect that the battery it is monitoring has lost some voltage.
The HA-2 most likely uses a Li-Po battery - see the
orange curve in this chart:
http://nordicgroup.us/battery/
When a charger automatically senses a voltage drop and reacts by topping off the battery AND you leave the charger plugged in for long periods when the battery is not in use, the battery is repeatedly shallow-cycled between 100% and something like 99% of capacity, depending on how the charger is designed. This happens over and over and over again if you leave such a charger connected while the device is not in use.
This can literally wear out the battery, slowly but surely. It's not as bad as deep-cycling a battery, as when taking it all the way down to the voltage at which the device shuts off, and only then putting it back on the charger. That regimen will force you to replace the battery far sooner than if you pampered the battery by routinely taking it down to 75% of capacity between charges.
The more shallow each depth of discharge, the longer the battery will last before it must be replaced - and it's not a linear relationship, meaning you will not get twice as many charge cycles out of a battery by taking it down 25% each time instead of taking it down 50% each time. The relationship is exponential. You might enjoy something like 10 times as many charge cycles by routinely going only half as deep as you would otherwise.
So, shallow discharges are better than deep for getting a lot of use out of a battery, but topping off to 100% every time the battery loses only 1% because of self-discharge is also bad for the longevity of the battery - because it can happen thousands of times across a year of leaving the unused device plugged into its charger.
This is best evidenced by considering the familiar and, to some, peculiar death of laptop batteries when the laptop is left plugged into its charger all the time. A year of countless automatic recharges to repeatedly top off the battery as it self-discharges is abusive. Even though the depth of each self-discharge between each recharge is very shallow (perhaps only 1%), keeping the laptop on the charger slowly destroys the battery by using up its ability to be cycled - like shaving away thousands of tiny slices from a bar of soap while it's not in use.
Many an infrequent business traveler has discovered this when he takes his relatively new laptop on the road for the first time in several months and the battery just refuses to hold a charge. It's not memory effect (as was the case with NiCd chemistry). It's due to abusive, repeated, shallow self-discharge and recharge cycles. Thousands of these minimum depth, shallow discharge cycles can be just as destructive as a few dozen maximum depth, deep discharge cycles.
So, the best thing you can do with a laptop or an Oppo HA-2 or any other rechargeable device that is equipped with a charger that tops off the battery automatically as it reacts to self-discharge is...
Unplug the charger after the battery has been topped off. Allow the battery to self-discharge without suffering repeated recharging. 
