If you already have NiMH and it's doing well, yeah... don't worry about it. The problems with NiMH are less than the problems with NiCD. And most NiCD and NiMH problems can be traced directly back to "dumb" chargers that charge at C/10 or so rates and continue to charge after the battery is heating up and often begins to mildly out-gas due to internal heat. Thus, losing some capacity.
If you're regularly charging your NiMH and it's holding up longer than a few transmissions (the hard part for those of us who use handhelds only for "emergency" purposes -- take it out once in a while when you're in the local pattern and use it just to see if it's really working, and then take it home and charge it back up), you're probably fine.
You used to see a lot of NiMH in things like cordless phones, for example. They'd sit on the charger base and cook for days on end, and even though you hardly used the phone at all and "always kept it on the charger", the batteries would show serious signs of degradation after a year or so... compared to when they were new.
Maha and others manufacturers make "smart" chargers that also can measure a battery's actual capacity through multiple charge/discharge cycles, but they're overkill for most of us pilots. They're available if you're really picky about seeing how your batteries are doing.
I have one, as do most ham radio operators these days, as they're about $100 and cheap, but do a great job of full-discharge/recharge cycling of these older battery chemistries.
Lithium Ion are completely different in that discharge/recharge cycles actually lower their lifespan, while a good solid discharge/recharge cycle or two can often breathe a little new life back into an "abused" NiCD or NiMH.
NiCD and NiMH also suffer from a different problem: The chemistry means that an AA sized cell doesn't put out quite the voltage that an alkaline AA cell does. Most radio manufacturers know this, but don't really make any change in the radio to compensate, and since voltage will drop in the cells at each high-current use transmission, the radio's internal voltage regulator will typically drop out a number of minutes sooner (say, a couple of transmissions, depending on length of transmission) on the NiMH pack than on the AA pack loaded with good quality alkaline cells. All things being equal, anyway. Which is rare in the real world.
So if you already own NiCD or NiMH tech, and it's working and you keep the packs charged and "exercise" them a bit from time to time, they can last a very long time. NASA has NiCD charge/discharge down to a real science and has some nice papers on the subject... since they're used in most satellite and other space applications... so treating a NiCD "right" they're great tech... the problem arises in radios that are sold with cheap wall-wort chargers that aren't current limited or controlled, that treat the batteries poorly.
And of course, all three types of chemistry have also been certified and flown on various commercial aircraft as backup battery systems for years. Again, well-designed charging and monitoring systems... work wonders.
The comment about picking Lithium Ion over the others was directed at those looking at new radios today... it's just the "best" tech for high-current draw mobile radio applications. Just like modern laptops or iPods or whatever... they're lighter and last a long time with very reproducible results. The NiMH and NiCD can be "squirrelly" when used lightly (or not at all) and left in a flight bag for a long time.
Aviation HT batteries of NiCD and NiMH chemistry are similar to airplanes themselves.... keep 'em flying or they become hangar queens fast. Lithium Ion, just top 'em off once in a while if you're not using them, and they're generally ready to go anytime.
This thread has got me interested in looking at the aviation HT market again. I'll report back on other features that are desirable, if I get some time to do some analysis on what's available.
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