Is it okay to mix old and new batteries?

Most batteries would come with the same standard advice in full caps DO NOT MIX NEW AND OLD BATTERIES. Now, we all know that mixing new and old batteries will mean that the old battery will be the first to go, dragging down the performance of the entire group of batteries hence, we mixing is a bad idea. But is it really a bad idea all the time?

But wait. Why would you want to mix old batteries with new ones anyway? Chances are, some batteries are "half used" and it is difficult to say which ones should be thrown away. This is especially so for "drained" batteries that were used in digital cameras. These batteries often have enough remaining capacity to serve low powered devices such as remote controls, sensor lights and calculators for a very long time.

If you have a at least cheapo multimeter, you would be able to find out the voltage of the battery. Although this is not exactly a scientific way to tell the remaining capacity of your battery, it at least give a very rough estimate about when you should actually throw the battery away.

A typical AA/AAA battery should read 1.5V without load. Without load means the battery is not being used at the moment. Normally, the voltage reading of a battery will sag if it is being used. This is more prominent under more demanding loads.

Ok, if you want to mix batteries, read on. But be reminded that this advice is only applicable to non-rechargeable batteries! Mixing full and near-empty rechargeable batteries will certainly damage the near-empty ones as they go beyond empty.

1st step: Check the voltage under no load!
- Put the voltmeter to read "DC voltage"
- Touch the terminals with the probes.
- Is it between 1-1.5V? If it is, re-use it for less demanding devices!

 Information is key...

Step 2:

- Mix those batteries!

Step 3:

When the device is once again low on power, re-check those batteries and see which ones are low. I found that for a 3-battery device, usually only 1 battery is completely dead and needs replacing. Once that dead battery is replaced, it works well again! By doing this, I saved many batteries while keeping my sensor light bright and nice.

Why oh why manufacturers make devices that uses odd number of batteries?

Disclaimer: Alkaline and carbon zinc batteries may leak after a while, regardless of whether they are empty or not. However, driving them till empty increases the chance of leakage, which is corrosive to metal contacts. Do check your battery powered devices once in a while.

I discourage the use of non-rechargeable batteries because they are extremely low on value since they only work once. Rechargeable batteries are slightly more expensive, lasts at least 500 cycles and the electricity used to recharge them is negligible compared to the price of new non-rechargeable batteries. However, they can be rather cheap when bought it bulk for devices that you don't want to keep on an eye on, such as remote controls. These low powered devices have the capability to operate even at very low voltages. This means that you'll have no clue that the battery is almost dead. Rechargeable batteries should never be brought below too low a voltage as it will damage them.