Replacement Batteries for light

[MorganPowys]

I have a Night Rider 15w light, and the 6v battery needs to be replaced. I have looked around and seen a number of posts on D-I-Y battery assembly, and it doesn't seem too difficult (though I have seen some cautions about welding/soldering tabs onto batteries).

The current battery is a water bottle with 5 1.2v NiCAD "D" batteries connected with tabs in series. No idea abut amp-hours, as the batteries have no labels.

I have seen at my local electronic store some 4.5Ah "D" NiCD batteries, $10CDN ea. I am guessing these are probably the same as the batteries in the old light (at least any other comparable lights I have seen have 4.5Ah batteries)

The store also has 2.5Ah "D" NiMH batteries for the same price. I have also seen on the web (http://www.nimhbattery.com/batteries.htm) NiMH
"D" cells with up to 11Ah (!), for about $35US a pair.

For a given wattage/voltage, can you compare Ah between NiMH and
NiCAD? I.e., would a 4.5Ah NiMH and a 4.5Ah NiCAD, connected to the same light, have the same run-time?

If building a battery, I think I would prefer to go with a higher capacity NiMH.
It seems to me that going with an 11Ah battery over a 4.5Ah makes sense. Same size battery, but much longer run-time. Downside is additional cost (about $25CDN per battery as compared to $10CDN per battery), additional charge time, and possibly some additional weight (don't know about this - are higher capacity batteries necessarily heavier, given the same physical size?). I recognize I'd also have to get a new charger if I went with NiMH instead of NiCAD. Still, it would work out to about $150 for presumably much longer run time (I haven't done the calculations, but I expect an 11Ah 6v battery could run a 15W lamp for 4 hours?

[MorganPowys]

<snip>

By "size" do you mean amp-hour rating, i.e., do you mean a NiMH battery with a 4.5Ah rating will run much longer than a NiCD battery with the same 4.5Ah rating? If so, any idea of the expected difference, i.e., a NiMH of 4.5Ah will usually run for 1.5x as long as a NiCD of 4.5Ah?

<snip>

So... does this mean that a battery charger that is designed to charge a 4.5Ah battery won't work properly for a 10Ah battery, or just that it will take more than twice as long as the 4.5Ah?