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Mixing New and Used Battery

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Boris_yo:
Hello,

Found this saying on emergency flashlight. I don't understand why safety warning says not to mix new batteries with used batteries and when replacing, all batteries must be replaced at once. Not economical if you ask me.

If I have 2 batteries still good but another 2 are bad, why must I replace all of them? I am normally using battery tester and when I see one if batteries are low on charge, I replace only that battery.Not economical if you ask me.

Ranayna:
The new batteries will reverse charge the old almost empty ones.
That in turn can cause the affected batteries to leak.
When that happens your false economy is shot, because the leaking batteries may damage your device.

So yes, you really should change all batteries at the same time. If they are in series they will all be essentially empty anyway. They may still give a decent voltage with no or little load, but the useful capacity left will be low.

tunk:
As adviced above, replace all four.
You could set aside the "good" ones, and if you later ends
up with four indentical cells (same brand, type and charge),
you could try to use them.

tooki:

--- Quote from: Boris_yo on November 30, 2021, 03:40:52 pm ---Hello,

Found this saying on emergency flashlight. I don't understand why safety warning says not to mix new batteries with used batteries and when replacing, all batteries must be replaced at once. Not economical if you ask me.

If I have 2 batteries still good but another 2 are bad, why must I replace all of them? I am normally using battery tester and when I see one if batteries are low on charge, I replace only that battery.Not economical if you ask me.

--- End quote ---
There are good reasons for not mixing new and old. So always replace them together.

If you have some cells with a bit of residual charge, save them for infrared remote controls, which generally use very little power and are often perfectly happy with cells that are too low for other devices.

Boris_yo:

--- Quote from: Ranayna on November 30, 2021, 03:49:27 pm ---The new batteries will reverse charge the old almost empty ones.

If they are in series they will all be essentially empty anyway. They may still give a decent voltage with no or little load, but the useful capacity left will be low.

--- End quote ---

Didn't know reverse charge is a thing. I thought batteries can only be charged using charger...

What do you mean batteries in series? I use 4AA batteries in blood pressure monitor. Is that what you mean rather than using batteries from different devices together?

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