You need to know how many amp-hours you took out after they were full, and how many amps your alternator
can put in. You will have some losses due to wire and battery resistance, some other basic inefficiencies, and you will also have the decrease in amps from your alternator as you approach "full", making it difficult and inefficient to get back to being "topped up".
are a great way to do the last topping up, by the way, as they provide amperes at a slow rate, use free fuel
, and are unlikely to overcharge.