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One of the very first thoughts when I bought my watch phone was: these things must be fed by crazy tiny batteries... and I was right, they are really small! So small that makes you wonder if there is about different technology... and the answer is yes, in comparison with a normal battery, the little one has to carry out way more energy in a hell cramped space.
So new technology, new charging algorithm. First of all no more common electrolytes, the only way - polymers - this type gets very well along with devices which have need of small currents for long periods - in this case, small phones.
As about charging system, <constant current / constant voltage> algorithm has been abandoned due to the crowded space inside the battery, after my measurements it looks like the mtk watch phone is equipped with a very modern and to the same extent, complex system of charging, consisting in series of short periods (5...7 seconds each) when voltage and current are increasing progressively (it is about progressive acceleration of lithium ions) followed by a pause of a second when the current and voltage are cutout (probably giving some extra time so the ions get safely to anode).
About the maximum charge voltage - it is limited at 4.16V - very safe one! I am amazed how our chinese fellows are keeping in safe our small and cheap batteries while high class brands, deliberately, are killing expensive batteries using without any shame high charging voltages...
As about currents, my advice, a low current at 5V charger is the cheapest thing you can find in a electronic spare parts shop, I've bought mine 5V 240mA spending... 2$. For a 100% capacity and safely charge it is preferable to choose a charger which provides a current [mA] of 70% or less of the capacity [mAh] you read on the battery label. Avoid as much as possible charging to the computer USB, these outputs are providing in the best case the minimum of 500mA, way to much for this kind of batteries...
In my study, in order to understand these small batteries, I tried to find DoD (depth of discharge under constant load)... I was surprised to find that there are no diagrams made even for the most known batteries! Hence I have decided to build one by myself... The principle was simple, battery charged to maximum, phone in standby, voltage measured at every two hours until battery gets drained - and of course all these put together in a diagram
- Opening Engineering Menu we find under device\set default level\battery a very interesting row of parameters
[LEV 1] 3200000; [LEV 2] 3400000; [LEV 3] 3550000; [LEV 4] 3640000; [LEV 5] 3740000; [LEV 6] 3870000; [LEV 7] 9999999; [LEV 8] 9999999; [LEV 9] 9999999; [LEV 10] 9999999;
Very obvious these values makes us think that is about trigger-flags in the form of a series of voltage values... Checking some I discovered that [LEV 7] to [LEV 10] are neutral, [LEV 4] to [LEV 6] controls the level battery bars shown on display upper right corner and the first ones triggers the system audio alert or shutdown. We have now a DoD diagram, we have as well these values, so let's project them over the diagram
So it looks like the default values are not the best choice (from my point of view there is no logic to display when battery is 76%, 44% and 9% of full capacity), most probably they are copied from normal bar mtk phones - obviously are not applying in our case but now having the diagram you can set by yourself the best way of displaying the battery level changing [LEV 4] to [LEV 6] according with your needs and/or preferences
Warning: do not change LEV 1, it triggers shut down if the battery voltage is lower than the set value, imagine what is happening if, by mistake, you put there a value bigger than 4100000... you'll never be able to start back again your watchphone, therefore keep it exactly as it is, 3200000 is perfect!
Hope it helps!