I see. Again, I am sorry to hear about your situation.
Charging off a loan is an accounting procedure. That is, typically a company has a debt on the "liabilities" side and an accounts receivable on the assets side. If no money comes in, the books are "lopsided" and don't balance. Changing off a debt means essentially taking it off the books to make it balance. It does NOT mean that the debt is forgiven or no longer owed. It really has nothing to do with the collection of the debt itself or repossession of the car.
The lender can seek full payment if you have missed a payment in full. Even making partial payments means that you have not complied with the terms of the contract (which state that you have to make the entire payment). Now if the contract specifies that they will first give you notice and they have not, then they'd be in breach of contract. But they don't have to give you notice to indicate that you breached the contract. They can still pursue you for breach of that contract. And the promissory note that you signed gives them a right to "accelerate" the loan upon breach. That means that if you miss one payment in full, they can tell you that everything is now owed.
So in short, yes, it does sound like they can do this. The charging off has nothing to do with the actual debt. It's an accounting procedure on their side. It's not true that they only can accept the full amount. They can still continue to accept monthly payments. But they can require that you pay the full amount. It's really up to them, I'm sorry to say.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (good or better). Thank you, ***** ***** luck to you!