Do I need to restrict current to charge a dyson Li-ion battery directly with my photovoltaic system
Please note that any voltage source, including your step-up converter, only tries to supply the output voltage, it is up to the circuit it's connected to (your Li-ion pack in this example) to determine what current will flow. E.g., if it's an open circuit, the current drawn will be 0; if it is a resistor, then you can apply Ohm's law. The current rating (3A in your example) only determines the maximum amount that the voltage source can supply (if the circuit "requests" it), above that it may fail to work, overheat, etc.

In your case, the best way to see if everything looks normal is to apply those 26.1V from the step-up, and check what the current is with a multimeter:







simulate this circuit &ndash; Schematic created using CircuitLab

If you're measuring 780 mA or less, then it's all good.