Many battery-powered electronic devices come with a warning like this:
Do not mix old and new batteries. Do not mix alkaline, standard (carbon-zinc), or rechargeable (nickel-cadmium) batteries.
One device I noted this warning on was an automatic card-shuffling device. It seemed entirely out of place, since the machine only used ONE nine-volt battery.
Of course, the advice is sound and should generally be followed. However, in this case I don’t think the warning necessary, as it would be practically impossible to use multiple batteries at once in this device.
According to Duracell, the reason to avoid mixing old and new batteries is that doing so will reduce overall performance and may cause battery leakage or rupture. If you’re curious, for a more detailed explanation take a visit here.