Machavity's answer is perfect regarding a pure 120V 15A vs. 20A issue. 16A is 80% of 20A, so that is the NEC maximum for continuous usage (like EV battery charging) on a 20A circuit. If you simply had a 20A 120V device with an improper 5-15 plug, this would be the solution.
But there is a MUCH BIGGER PROBLEM. 3300W at 16A is 206V. Which means this charger is likely designed for a 200+V installation - either 240V in the US/Canada, 208V in the US (common in large buildings) or anywhere from 220V to 250V in most other parts of the world.
With small electronics - laptop or desktop computer, monitor, inkjet printer (but not usually laser printer), TV, etc. it is quite common to have a power supply which can handle a wide range of voltage - e.g, 100V to 250V and frequency - 50 Hz/60 Hz. That power supply will automatically and safely adjust to provide the desired output voltage (typically 5V to 20V but depends on the device) and everything works just fine.
However, there are two types of loads that simply don't work that way:
- Large motor loads - that's why you generally can't easily convert large power tools (table saw, dust collector, etc.) or motorized appliances (washing machine, vacuum cleaner) from 120V to 240V or vice versa. There are some exceptions, but typically those are on relatively expensive power tools and converted by moving power wires on the motor.
- Large resistive loads - water heaters, space heaters, etc. For these frequency is generally not an issue but using the wrong voltage can result in burning things up (120V device on 240V power) or really poor performance (240V device on 120V power).
Battery charging gets interesting. On a really advanced system, including most EVs with large batteries and sophisticated battery charging/management - Tesla, Rivian, etc. but actually any EV made by any major manufacturer (Ford, GM, BMW, etc.) the EV itself handles different voltages - 120V AC and 240V AC but also DC up to 400V or even 800V or more, depending on the model.
However, what you have here appears to be a much more basic system. Presumably it is designed to convert ~240V AC into an appropriate DC voltage for charging the battery. If you feed it 120V instead of 240V it could:
- Run at ~ 1650W and overload a 15A 120V circuit but be perfectly safe on a 20A circuit. Note that this will likely cut the charging speed to about half of what is listed in the advertising materials for the charger/double the time to charge.
- Not work at all. Really.
- Attempt to pull as much as ~ 3300W and trip a breaker (hopefully!) pretty quickly.
- Run at somewhat lower than 1650W and charge even more slowly.
The bottom line is that a device sold in the US with a 16A rating and a 15A cord/plug is simply wrong. And there is no easy way to know how many other things might be wrong:
- Wires too small
- Unsafe design that could lead to exposed 120V AC
- Unsafe design leading to over-charging of batteries
etc.
UPDATE
OP provided a link for the charger which led me to the manual for the motorcycle. On page 35 it says:
- Charger - External charger AC220V, output current 15A
which basically confirms what I thought: this is designed for 220V charging. Now the US, for far longer than EVs have been a big thing, has been a 120V/240V country. But the terminology persists. And it is likely that if it is designed for nominal 220V that it will work perfectly fine on the full range from 208V (some, mostly large, buildings) to 240V (most single-family homes). But that does not mean it will work well at 120V.
ASK THE MANUFACTURER what voltage range and current parameters actually apply to this charger.
