Lithium batteries are designed to be charged at a somewhat constant rate. Chargers are designed to limit that rate to whatever is safe for a battery. The charger in the nanoVNA is probably designed to deliver the current need to charge the 500 mah battery and no more. A larger capacity battery will require more charge time but there should be no issue with charging it with the built in charger. Just don't try to substitute a battery with higher voltage. You will destroy the battery and the charger.
Bruce, K4TQL
