This maybe useless information (my brain holds a lot of that)

The deposits you are "cleaning off" are either of two
Chemical compounds, depending on temperature and
(Indirectly) electrical potential during formation.


1) CuMg2 - a hexagonal crystalline structure, which if
alloyed is specific proportions with lithium (yLix) forms
vacancies or "holes" in the crystals, which can accept
H2 molecules, which are released upon heating.
This makes this substance an ideal Hydrogen storage substrate.
The proportional relationship between variables x and y control
the storage capacity and temperature dependent rate of
H2 release.




Or


2) MgCu2 - a cubic, piezoelectric crystalline structure,
With similar properties to SiO2


Not sure which one you are creating in your water batteries.


Many people will try to tell you that the copper does not break down
during this process. This is actually an inaccurate statement.
The Cu<->Mg galvanization in fact corrodes both Anode and Cathode
At either of two specific rates, in the higher of temperature ranges
The cathode will corrode faster than the Anode.


This is not unique among galvanic reactions, however it is not
common, as most galvanic reactions take place under conditions
that promote cathode stability.

Around the time/date of the above last post (Febr. 2017) i setup a small waterbattery using a magnesium bar with around it some copper wire.
It was submerged in normal tap water and was/is powering a battery (1.5V) clock.

Now after more then 14 months the clock i still running on time.

I only just recently added some tap water and the voltage on the clock still reads 1V.

Regards Itsu