Here's more on the subject of seizure of a bike or battery - this is from a top attorney in special litigation in NYC:

Forfeiture of a vehicle can only be for its use in a "crime," which covers a misdemeanor but not a violation. So, assuming that riding an unregistered vehicle like an ebike is just a violation, NYPD has no legal grounds to confiscate it.
Can a cop actually seize one? Yes, if he wants to punish someone for insufficient deference, or whatever. You can ultimately get it back in a few days, if it is still at the precinct, or longer, if they remove it to some pound, after you have dealt with some functionary who doesn't have a personal stake in sticking it to you. So that is the reality; there is no guarantee against hassle.

So then, is riding an unregistered motor vehicle a misdemeanor or a traffic violation? Well, here's the text of VAT section 319:

§ 319. Penalties. 1. Any owner of a motor vehicle registered in this
state, or an unregistered motor vehicle, who shall operate such motor
vehicle or permit it to be operated in this state without having in full
force and effect the financial security required by the provisions of
this chapter and any other person who shall operate in this state any
motor vehicle registered in this state, or an unregistered motor
vehicle, with the knowledge that the owner thereof does not have in full
force and effect such proof of financial security, except a person who,
at the time of operation of such motor vehicle, had in effect an
operator's policy of liability insurance, as defined in section three
hundred eighteen, with respect to his operation of such vehicle shall be
guilty of a traffic infraction and upon conviction may be fined not less
than one hundred fifty dollars or more than one thousand five hundred
dollars or may be imprisoned for not more than fifteen days or both. In
addition to the penalties herein set forth, such person, upon
conviction, shall also become liable for payment to the department of
the civil penalty provided in subdivision five of this section.

Operating without insurance is not even a misdemeanor. This article says that an unregistered vehicle is a violation.

I conclude that it is not legal to confiscate either the bike or battery. You can get fined however. I am also not a lawyer so don't sue me or anything, but I think we're on pretty solid ground here.