For alkaline batteries, mixing old and new batteries will result in a situation called "Deep Discharging" od the old batteries which results in battery leaking and outgasing.
For rechargable batteries, mixing different capacities will result in deep discharging of the lower capacities as they run out before the higher capacities.
There is a risk of damaging the lower capacity batteries, as well as the equipment should the battery leak due to this.
The risk is not very high because as the lower capacity battery run out, the equipment will stop working, and if you recharge the batteries immediatly.
However, by doing that, you essently end up with having only overall effective capacity of the lowest capacity battery you have in the chain. Mixing a 1200mAH with a 1800mAH does not gove you the average capacity of 1500mAH. Instead, you only get effecticely 1200mAH.
When the 1200mAH runs out, you have to charge it as well as the 1800mAH that has not been used up.
The risk is exceptionally high if you leave the mixed batteries in the equipment for long period of time. Even in the off state, most modern electronic ewuipment draws a very low current from the batteries to sustain minimal functionality of maintaining internal RAM contents, and maintaining the capability to detect when the user wants to turn it back on. This current, even though very small generally, will contiinue to deischarge the batteries and causes possible damage to the one that has lower capacity.
So I would advice against mixing different capacities.
For that matter, it is also not advicable to use a fully charge battery with a half-depleted battery, even is they have the came full-charge capacity.
- Roy
