Mixing Batteries

Bondy - the simplest answer is "no," do not mix battery types, intentionally, when you have a choice.

That being said, as with many things discussed here on CR4, there are many flavors of why and why not, as well as when and when not, and likely innumerable variations...

The chemistry matters, as noted above, but so does the cell construction and battery design, as well as the application (flashlight - low current resistive load, versus capacitive and switching loads of electronics) and other considerations.

The typical consumer cells: D, C, AA, AAA - are all the same standardized form factor, but their voltage and current delivery vary widely based on the above characteristics. In general, cells of any manufacturer can be safely mixed of those that have the same chemistry; all dry cells (standard cheap ones) or all alkalines, from any brand, should be just fine.

These dry and alkaline cells all use variations of similar constituents in their electrolytes that deliver 1.5 volts DC at a fairly low delivery current. That is roughly analogous to a row of cans 1.5 units tall, into the bottom of which you decide to poke a nail. When you pull out the nail, you get about the same flowrate, but the alkaline battery is a fatter can, so it can maintain the flow longer. Also, the alkaline chemistry is somewhat more stable so that can stays 'fresh' longer. Since the higher resistance prevents large currents from flowing, they occasionally 'fool' an electronic device into thinking that the battery is discharged, since the device attempts to draw current and it just doesn't get enough, so it shuts off - even though that battery can then be used just fine in a flashlight and will test 'OK'. The camera user typically stands there frustrated that the camera will 'turn on' and display a couple of bars of battery power (based on voltage) but every time she attempts to take a shot, the camera attempts to draw a surge of current for processing, doesn't get it, and shuts down instead while beeping 'low battery.' This is also why a "9V" dry or alkaline battery is 9V, as it is a stack of six small 1.5V cells.

When you get to rechargeable cells, however, the rules are a bit different. First, the nominal voltage is different, as the electrolyte potential limits each cell depending on the selection of materials. NiMH for instance delivers 1.2 volts DC per cell. There is no way to "charge" the cell to a higher voltage, that is set by chemistry. 1.2V is what you get from a fully charged cell, period. However, in our analogy above, this time the can may be a bit shorter, as it is only 1.2 units tall, but you don't have a nail-hole, you pull the plug on a much larger hole, allowing much faster flow when needed (low resistance). This is also why a "9V" NiMH battery is actually a manufacturer's choice of 7.2V, or 8.4V, or 9.6V, as it is a stack of six, seven, or eight small 1.2V cells. Most are 8.4V, as this has proven to maximize amp-hour capacity while keeping voltage high enough to keep devices from sensing low battery conditions (really annoying when a fully-charged 7.2V cell can't keep a smoke detector from beeping all night). Some electronic/capacitive applications, such as paintball markers, prefer the 9.6V assembly, where the users couldn't care less about long-term capacity as they will happily swap out fully charged batteries for the performance boost of higher voltage. You'd think more manufacturer's would provide this as a choice.

The exact tweaking of the materials and the design of the internal construction also determine how fat the can is, so those "amp hour" ratings mean something - the batteries with higher ratings store more energy. This is why NiMH and Lithium Ion batteries are far superior for electronics like phones and cameras and laptops; they provide a much better low resistance delivery of stored power even though at a slightly lower voltage. Even with only one 'bar' of battery power displayed on a device, the battery will still deliver the current needed for a surge of activity, until nearly/deeply discharged. The Ni-Cad's were and may remain an alternative for some niche applications, as they are very light weight, but for consumer articles they are generally outclassed by the newer materials in all cases I can think of, as the amp-hour capacity is higher for the same physical volume.

It may be, therefore, somewhat better to pair up batteries from the same amp-hour rating as well as chemistry, for equal performance. In a mis-matched pair (or pack), the battery with the lowest storage capacity will become a higher resistance point in the system and then add to the drain on the rest, while creating heat. How long this goes on and how extreme the difference is may determine whether or not enough heat results to do any damage to the cell, though I seriously doubt enough for rupture. Heck, it may even help recondition the cell somehow as long as it's not continuous.

Strangely enough, this also helps explain the occasional anecdotal advice for frustrated digital camera users to pair one alkaline with a NiMH when having battery-sucking problems to get that last all-important shot: I assume the alkaline will provide a slightly higher voltage to keep the electronic brain turned on, while the surge needed for flash recharging and photo compression processing gets sucked from the NiMH...for a few shots.