Time to take a new project. Many times I have noticed on the outside, that Nikon D200 pulls out effectively the full charge out of batteries. The supplied EN-EL3E is not provided with a big capacity. In cold temperatures like -20...-30C, image count could be as less as 20...30.
Insteadt of using older lead-acid technology, I selected Litium technology. This provides higher capacity in same volume & weight. The downside of this, is higher price tag.
Purpose of this project, is to provide capacity at least 3 times, compared to normal setup with Nikon D200 drived with single or two EN-EL3E LiIon batteries. I selected LiPo (litium polymer) battery type, because those are easily available in stores selling stuff for RC hobbyists. Microcontrolled chargers, provide accurate charging of LiPo battery. More will follow, when project continues. Stay tuned :-)