If you have some power-hungry applications that require D-cells, you can usually save money in the long run by using rechargeable NiMH cells instead of alkaline cells. One approach is to buy a set of AA to D-sized adapter shells, and use rechargeable AA NiMh cells in those. This is the cheapest method because AA NiMH cells typically cost less than ten dollars for a package of four, and you can use low-cost AA charger for them (instead of buying another charger that accepts D-cells).
However, don't expect to get the same operation time as before when you were using alkaline cells. This is because the capacity of a typical NiMH AA cell is only about 2500mAh. In comparison, a typical alkaline D-cell is rated at about 20,000mAh for low current applications. At higher current, the actual capacity may drop to half. But this is still four times as much as that of a NiMH AA cell. In other words, you can only expect one-quarter the operation time after switching to NiMH AA cells.
Another complication is that: the negative terminal of an AA cell is smaller than that of a real D cell. So the larger spring in a battery compartment may not make good contact with the AA cell bottom. Usually this problem can be solved by inserting a copper penny between the spring and the battery.
There are rechargeable NiMH D-cells available with capacity up to 12,000mAh. Those cells can out-last alkaline D-cells in large current applications. However, those cells are expensive (around $25 for two) and you need special charger that are designed to charge high capacity D-cells. (Most low-cost chargers, even those that can accept D-cells, cannot fully recharge cells with capacity greater than 3000mAh)
Finally, be carefull not to fall victim to fake D-cells offered by Energizer and Rayovac. Those are just 2500mAh AA cells enclosed in larger D-sized cases. Prices of those fake D-cells are often ridiculously high, even though you still get very short operation time from those. Plus you need to buy a charger that can accept D-cells.
All things considered, using AA cells in D-sized adapters is still a good approach. Just make sure that you lower your expectation, and DON'T PAY TOO MUCH for those cheap plastic shells. These have failed to work for me in every item where I have tried to use them. The problem was that they did not make electrical contact with the metal springs inside the battery compartment. If you compare these adapters with a real D size battery, you will see that the typical real battery has much larger metal areas for the springs to touch.
It's a nice idea and I'd love to find some adapters with a better design but these unfortunately fail the test. Buyer beware. I've used a similar product by Panasonic which was MUCH better made, but I haven't found the Panasonic brand adapters sold outside of a Costco rechargeable battery kit.
The 4 Lenmar AA-to-D adapters do work but they are inconsistent in how well your AA batteries fit inside. 2 of the 4 adapters worked normally for me while the other 2 were too loose or the AA battery didn't snap into place when inserted - and I tried several AA batteries with the same results.
These Lenmar's are cheaply made but they will work. I don't recommend putting these adapters into service where you're going to have to change-out batteries all the time
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