Use a current-mode buck
DC-DC converter. You sense the current in a low-side resistor below what you are driving (in your case the batteries). It is used commonly for LED drive, since that is current-based: http://www.national.com/pf/LM/LM3424.html#Overview Although, I guess you'd prefer a high-side sense, so that the batteries can be solidly ground-referenced... You can use cycle...