Nicad batteries don't respond well to being ignored. For best results, you must put the batteries through a full charge/discharge cycle at least once a month.
On the first of each month (or any other day you can remember), charge up the batteries until they're full, then turn the radios on and leave them on until the batteries run out, and then charge them back up to full. Do that once a month, every month, and the batteries may last longer than five years.
If you buy brand new in the box spare batteries, don't charge them up until they're actually being called upon to enter active service. Their lifespan doesn't really begin until their first charge, which you can think of as their birthday. I've charged up batteries that had been sitting on the shelf, new old stock, for eight years, and put them in service and they performed as well as a brand new battery.
By the way, two way radios, Motorola radios in particular, are my specialty. I know what I'm talking about here, but I mostly deal in the ultra-expensive Astro series digital products, and not the more affordable (but good) Radius products.
CJ