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Non-rechargeable 1.5v batteries only start at 1.5v. As you use them up, the voltage drops, going down to about 1v just before the battery poops out. The curve is not linear.

These batteries are down to 1.2v around 30-50% of the way through their life.

The rechargeable batteries start around 1.2v, but their discharge curve is much more level. They stay near 1.2v right up to the end.

If a device has trouble with rechargeable batteries because of the voltage, that device is also going to have problems with non-rechargeables, as the non-rechargeables spend at least half their service life below 1.2v.

There are some good graphs showing discharge curves here: http://www.powerstream.com/AA-tests.htm

The rechargeable batteries can confuse battery meters on some devices, though. If the device assumes non-rechargeable batteries, and then tries to estimate remaining battery by comparing the voltage to a typical non-rechanrgeable discharge curve, and you put rechargeable batteries in, the device is going to think they are starting out at 50-80%, and then is going to think they are draining very very slowly.




It's a good point, but in practice I have found some devices won't even power on with the 1.2v batteries.

A good example being the Microsoft Bluetooth Mobile Keyboard 6000. The batteries don't last for 5 minutes even. Alkaline batteries last for months.




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