why do they always say that if one bettery is dead, you need to change both of them regardless?

Question

i meant ot say BATTERY not bettery!

Answer 1

Because batteries are typically connected in series this means that if you change one of two flat batteries with just one new one this will quickly become drained by the remaining flat battery touching it. Try to think of these as either full or empty bottles so changing one empty bottle with a full one will mean it will immediately drain into the remaining empty one.

Answer 2

If two cells are connected in series the current passing through the load completes a circuit which effectively tries to charge the discharged cell backwards. This results in internal breakdown which could result in overheating and venting (the technical term for leaking) or even explosion.

Don't mess with Lithium cells - they can kill.
I have seen a video of an attempt to charge a Lithium cell, one frame the lab looks ok, the next frame - no lab.

Answer 3

Medic_13 is talking complete twonk. Usually batteries are replaced in pairs such as in a remote control for your telly. It makes sense to change both at the same time. Even if one is showing some signs of life it'll probably die in the near future (and not in a ball of flames). More so with things like electric doorbells which have 4 batteries +. If you don't swap all of your doorbell batteries at the same time it'll sound like a wounded Avon lady before you know it.

Answer 4

they say this because if you have a circuit, with a battery and whatever you are powering, there must be a continuous flow with no breakages if there is a break then it is not a complete circuit which means the electrons inside the wire can't go any where so the circuit is broken

Answer 5

Because if you have one dead and one good battery in the same circuit, the good battery will begin to recharge the dead battery. Since regular alkaline batteries overheat when they are recharged, the dead battery will then overheat and burst, causing damage to the unit.