Re: MPPT implementation ,how to balance
When it comes to battery charging, there's only three main stages: bulk, absorption and float. Depending on the actual stage (by reading the battery VOLTAGE), your (MPPT) charge controller will adjust the output CURRENT.
That's it, the charger will always act as a variable CURRENT SOURCE. Moreover, the PV panel itself it's a COURENT SOURCE, too.
But, depending on the PV panel output current, the voltage across it will also change.
Now, if you want to maximize the PV panel output power you could
(a) measure the charger input current AND the voltage across the PV panel (the charger input voltage) or
(b) measure the charger output current AND the battery voltage (the charger output voltage).
In practice, it's obvious that the second option is the way to go, as you need to monitor the battery charging process anyway.
So, the charging alghorithm should look like that: the battery needs (could accept) more current? Then increase the charger PWM duty cycle. If the charger output current increase, then you're heading in the right direction (toward MPP). If the output current is decreasing, then you're going in the opposite direction (away from MPP) hence you need to decrease the duty cycle.
In simple terms, that's all about the MPPT alghorithm (P&O - perturb and observe).