Which cells would be better: Lithium, or alkaline?
The application would be for a back-up battery pack that would only be used if the Ni-Cads were depleted and a/c wasn’t available.
Lithiums (non-rechargable) have a lower self-discharge rate than alkalines–on the order of 10 years instead of 3. That tends to make them good for back-up or emergency applications. They are also very lightweight, in case that’s important.
On the other hand, they are more expensive per unit energy. If you expect to go into back-up mode frequently, and a 3-year replacement cycle isn’t a problem, alkalines may be a better choice.
I’m assuming, since this is a backup, that the primary characteristic you’re looking for is shelf life.
[del]From some quick googling, it seems like non-rechargable lithium batteries have a similar shelf life as alkaline: about five years.[/del] If you’re expecting the main batteries to be replaced relatively quickly, it probably doesn’t matter which type; if not, Lithium lasts a little longer. Also, if you’re looking at cold-temperature usage, Lithium batteries work much better at sub-freezing temps than alkaline.
Edit: I’m willing to believe that Dr. Strangelove has better info than me.
I should add that the numbers can vary wildly–I’ve seen anywhere from 2 to 5 years for alkalines, and 8 to 16 years for lithium. Here’s a random example of one advertised with a 16-year shelf life.
The cold-temperature performance is definitely a good point.
Oh, another one: lithiums have a much higher capacity in high-current applications. An alkaline AA driving a 1 A load can lose something like 2/3 of its capacity, while the lithium doesn’t care. The difference is even greater with cold temperatures.
Yes, but in low current drain situations / normal temperature the difference goes away
Mostly. But the difference is more like 10% than 3x. The OP hasn’t specified the application, but if the primary power source is Ni-Cd, there’s a good chance that this is a high current application.
Two-way radio. It runs on a 9.6v Ni-Cd battery pack, or 8 alkaline batteries. Puts out 5w. I’ll use it for listening, not transmitting. A couple of times a year I’ll probably have it on for 6 or 8 hours. I still have the instructions, but they’re not handy at the moment.
I see. I would go for the lithiums for shelf-life reasons. I assume this radio is mostly used under emergency conditions (severe storms, etc.)? If so, the last thing you want is to find a drawer full of leaky alkalines when trying to get your radio working. You might even forget the Ni-Cd entirely if you really only use this device for <20 hrs a year.