It's US where LiIon is presumed 4V nominal, so effectively 42 elements. Pretty nice margins.
Indeed battery pack is where the tool companies make their profit with all of them being incompatible with each other, including Dewalt/Black and Decker (same company different brands) having just a small plastic notch to prevent interchange usage.
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Edit: durability depends on max charge and min/cut off discharge voltage (something like 4.13v/3.3 would be decent) - you can test those with voltmeter easily, temperature the batteries are run at, and discharge current. At around 400 cycles they should be at ~85% capacity.
Indeed battery pack is where the tool companies make their profit with all of them being incompatible with each other, including Dewalt/Black and Decker (same company different brands) having just a small plastic notch to prevent interchange usage.
-- Edit: durability depends on max charge and min/cut off discharge voltage (something like 4.13v/3.3 would be decent) - you can test those with voltmeter easily, temperature the batteries are run at, and discharge current. At around 400 cycles they should be at ~85% capacity.