MPPT Boost Converter - Output Voltage
I think you have to give up the idea that an MPPT charger outputs a constant voltage.

It outputs a controlled voltage that 

(a) is safe for charging the batteries
(b) results in a safe charging current for the batteries
(c) gives the highest current to the batteries that the solar cells can provide at the moment.

In practice this means measuring the input voltage and current, calculating the power, and adjusting the output voltage to maximise power (goal c) while also considering the output voltage and current AND the state of charge of the batteries to meet goals (a) and (b).

State (b) probably means you need a bigger battery with a higher charging rate - or it is close to fully charged (and only permits trickle charging)  and state (a) means the battery is fully charged. The rules for these depend on the battery technology. Lead acid batteries have characteristics allowing safe MPPT charging; lithium batteries may have stricter charging regimes... In either case these rules give you some leeway to charge at different rates, and the MPPT charger uses that.

Normally an MPPT controller will be in state (c) - the MPPT state - until the batteries are close to fully charged. And here it is controlling the charging current to the batteries - by adjusting the charge voltage.