density can lead to a dangerous situation if the cell were to short against a conductor. 600 MA at 7.2 volts can warm up and explode in a hurry, and will make a fascinating tattoo of your pocketknife or keyring on your leg.
As good as nicads are, equipment manufacturers are always looking for the better mousetrap, and the mice seem to be flocking to the NiMH battery. The NiMH cell has made it possible for radio makers to create those really tiny radios that have as much if not more talk power than the older units, as they have close to twice the energy density of the Nicad. Also, NiMH cells are easier to make in flat or cubical configurations, further reducing the size and weight of the battery. NiMH cells are largely immune to memory effects, but this should not be taken as free reign to feed the battery a steady diet of topping off charges, as the MiMH cell doesn't take to that treatment any better than nicads do. NiMH cells do give up a couple advantages to the lowly Nicad-the battery has significantly lower life expectancy, in the neighborhood of 300 charge cycles compared to the nicads 500 to 1000, and the battery costs more to replace when it expires. NiMH cells are available as single cell replacements but are harder to find than nicads and of course cost more. NiMH cells also suffer from greater self discharge rates than do nicads. Given the trend to smaller handhold's, expect to see more NiMH batteries in the future, and improving battery technology should help put the NiMH on an even footing as technology marches on.
Proper treatment of the NiMH battery is very similar to that of the Nicad, but with more care given to prevent overcharging. NiMH batteries should not be fast charged in Nicad chargers, and should not be conditioned before charging-actually, nicads should be given this treatment very sparingly, and NiMH not at all. ICOM recommends using the battery normally until transmitting becomes impossible and then charging overnight, and I would expect other companies to have similar suggestions. Don't incinerate your dead NiMH batteries, don't short them and be extra careful if you feel the urge to disassemble a flat NiMH battery-not that you'd ever find replacement cells ! Third party NiMH battery packs are available, so you won't be held hostage to the maker when that NiHM dies after two years, but expect to pay more for a little flat battery that is molded to the contours of the radio than you would pay for what is essentially a square box. I personally find it hard to argue with the energy contained in a NiMH but am not convinced that it is worth the shorter life and higher cost compared to Nicad.
You don't think of lead acid batteries very often when thinking ham shack energy, after all, who wants to carry a car battery around with a handheld? However, a fair amount of hams use a large car or truck battery, or some combination of large lead acid batteries as their primary radio power source. I have considered this myself, as my present power supply, an infamous switching type power supply, makes significant noise in certain areas on certain bands, and the lead acid battery is pure DC and very quiet. Conventional "flooded" batteries such as car batteries also must be vented to the outside during the charging process to safely dissipate the explosive hydrogen gas that is generated. Sealed lead acid batteries can be safely charged indoors, won't tip over and spill evil smelling and corrosive acid on the good carpet, and are fairly economical sources of primary or emergency power. Lead acid batteries can be left on a "float" charge, or a light charging voltage, for indefinite periods of time without worries of overcharging, and actually fare better than being left to self discharge and form sulfate crystals which kill the cells. They do have a practical limit of 300 drain /charge cycles, but will last for years if you don't deep cycle them-think about the four year old Charlie Diehard under your hood, which has been rather indifferently housed and charged, and despite all the abuse still cranks over a couple hundred cubic inches of stiff engine, and you'll see why some hams use nothing more that lead acid batteries and float chargers. See if you can get your friend who works at the local phone company to bring you some old float cells, at least for backup power. It's a bit hard to run your radio on Coleman campstove fuel.
The ultimate in batteries is the lithium ion cell, or Lion for short. A Lion it is, too, powerful, exotic, and prone to turn on you and cause you grief and harm if mishandled. Drawing too much current from a Lion battery can actually cause it to burst into flame-very rare, but possible-and the charging cycle is very fussy. Charge it wrong and that expensive battery is an expensive paperweight. If you decide to get a Lion battery for your laptop, it would be a good idea to get the special charger and not try to use your old Nicad/NiMH charger, as the Lion requires very specific voltages and currents over a specific period of time and will not function correctly if charged overnight on the old Nicad charger. Treated correctly, Lions have the potential to last at least as long as a NiMH, but it has been my experience thus far that noone actually treats them correctly. Apparently laptop users consider it normal to replace their batteries every six months, and don't mind paying over $100 a pop for the privilege.
Lithium ion batteries have not yet made a deep penetration into the amateur radio marketplace. Some mobile packeteers and SSTV'rs may be using Lithium batteries in their portable computers without thinking about it. As of now, the only radio I could find that uses lithium ion batteries is the Yaesu VX-1R, a micro-mini handheld that carries a 500 milliwatt signal in a 125 gram package, with a charger that offers a one hour charge time. The race to place more power into smaller packages will no doubt lead more manufacturers to produce radios with lithium batteries, but until the prices for replacement and extra batteries come down some, I would expect to see some customer resistance to a battery that might cost more to replace than the radio. Look for further improvements in capacity and reductions in price, as is the case with all battery technology, and lithium becoming the new standard once the cost and charging issues have been addressed.
For as long as the first radio in the ham shack is a handheld, there will be rechargeable batteries, and like it or not, those batteries are a consumable item. Although some hams consider it perfectly normal to replace HT batteries every year, even heavy users should be able to get two or three years from each battery pack. No matter what kind of cell your rig is using, read the owners manual, use common sense and avoid the habitual use of dischargers, and your battery budget won't have to compete with such mundane expenses as food or rent.

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