Mixing New and Used Battery

[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]

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.

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]

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.

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?

[Boris_yo]

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.

 But since remote controls use 2 batteries, I must still make sure batteries of same brand and type are used and both are at the same charge?

[tooki]

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.

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?

A battery gets charged any time a voltage higher than the battery’s own voltage is applied to it. It’s something you want to avoid happening by accident, and one way that can happen is when you put batteries with very different charge levels into a device.

Google “series and parallel circuits” to understand what “in series” means. It’s a core principle in electronics.

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.

 But since remote controls use 2 batteries, I must still make sure batteries of same brand and type are used and both are at the same charge?

I just mean that if you, for example, take 4 batteries out of your blood pressure monitor (which uses motors and thus will require comparatively “strong” batteries) because it won’t run any more, you may be able to use 2 of them in your TV remote for a while. (Put them into a separate small bag, clearly labeled “for remote controls” and only ever take them from the same batch. Do not mix and match batches.)

Just get out of your head the entire idea of mixing batteries from different batches/sources, period. You can’t accurately characterize the used batteries well enough to determine which could theoretically go together, and the monetary savings from doing so are pointlessly small.