So, NiMH batteries are around a decade old now, and while there have been improvements in # of recharge cycles, capacity has been pretty much flat.
All the modern stuff seems to be lithium, but this operates at voltages that don’t play nicely with most 1.2-1.5 electronics.
What technology (if any) is close to replacing NiMH?
NiMH is much older than a decade by now. In fact, it’s older than me! I think NiZn was probably supposed to be the successor, but it failed. Some people even tried rechargeable Alkaline, but that isn’t so great either. Both of those chemistries are still around btw, but not useful to replace NiMH. I’m not a battery guru, but I’ve not heard of anything coming close to replacing NiMH.
Yes, Eneloops were the last innovation of NiMH, and I think they made them pretty-much an ideal replacement for alkalines. It doesn’t appear capacity is going to improve much, so I think we’ve reached the best NiMH are going to get. Maybe a bit more power (wattage), but that’s really not needed given the current capacity.
There’s Energizer lithium primaries if you want more capacity. Better in almost everything, except power, compared to NiMH Eneloops. Well, they’re disposable, so maybe not better in that regard either.
In rechargeables, the next step-up is lithium-ion cells. It’s not really a direct comparable, as you note, given the difference in voltages. But if you want more power and higher energy density, lithium-ion is the next step up.
In ~1.5v cells, I don’t think there’s any huge improvements on the horizon. Good news for Duracell and Energizer shareholders.
I wonder if there could be a hybrid or a sort of in-between cell chemistry that has the higher voltage of NiZn and the higher capacity and low self-discharge of NiMH. Maybe there’s something they haven’t tried because Li-Ion made it not worth their while to push Nickel battery technology.
NimH is a very mature technology now, and an excellent, safe and inexpensive one.
The major technical improvement was ultra-low self discharge, (Eneloop and others.) making them a “fit and forget” cell
Capacity is comparable with Liion, e.g. compare an AA 2500 mAh 1.2V cell with a 14500 700 mAh 3.8V cell, The AA NimH has more capacity (3 Wh c.f. 2.66)
And substitutable with e.g. alkaline primary cells, or lithium primaries (no, they don’t have any more capacity, just better behaved for things that take peak currents, like digicams, otherwise much the same, see HKJ for facts and data).
At really low temperatures Li primary rule, NimH second, Liion and alkaline fail badly, and I do use torches in these conditions and speak from experience. My headtorch has an external battery pack which is tucked inside my jacket to keep it warm, and this matters.
Ya I can remember over 20 years ago getting my first nihm battery charger for my aa cells as a kid. Much older then a decade.
About a decade ago in 2009 in the barracks. I had a CD player and bought a energizer fast charger and 4 cells that came with it. To listen to music because there was nothing else to do. It literally caught on fire after there 3rd use. The batteries didn’t but the something inside the unit did. The batteries ended up being fine. I was in my rack and smelt burning plastic. Luckily the floors are solid concrete. Nothing around that could catch fire. I’ve never had anything energizer that plugs into a wall again. It was one of those 20 minute chargers or whatever. Before I knew anything about batteries.
Crazy thing was I use to recharge alkaline cells in it as well. I was like 9 years old. Sometimes they would leak out and other times they would recharge and work.
I still have an ancient charger that can re-charge alkalines, using a subtle pulsing mode. It still works, as long as the cells aren’t fully discharged, but hardly worth the effort these days.
Their chemistry is simple and amenable to re-charging, but realities mean things can go wrong (leaks, drying up etc.) so don’t bother trying. NimH changed all that.
It’s crazy to me that modern Li-ion batteries, at least at the 14500 size don’t have much more capacity than the NiMH batteries.
I’ve experimented with recharging alkalines, and it’s definitely not worth it. They will leak badly within a few days after charging. You can top them up with about a 20% charge, as long as you do it while they’re half-full or higher. And you can do it a few times, but they’ll leak before you do it more than 4 or 5 times.
You have to keep the charge rate low, 200mA is okay. Otherwise, they won’t take the charge. Stop at 1.6v. For safety, do it somewhere that can take a mess in case they rupture during charging. I’ve never had that happen, though.
It’s fun to experiment. But it’s definitely not worth it economically, or for safety reasons.
Well, exceptions abound, but comparing good quality NiMH cells against similar quality Li-Ion cells, it seems that NiMH has twice the capacity in mAh, but Li-Ion has three times the voltage, so Li-Ion comes out just slightly ahead.
There aren’t any really good 14500 cells. 18650 (and maybe 21700) is where the sweet spot is now.
I wish a major company would develop the 26650 format. It seems plb and others are now. We can get 5500 and close to 6000mah with a low discharge rate. And it can do 20 amps+ Like the new shockli battery. But they don’t have access to the patents and technology like the big giants. I guess they could try to reverse engineer it. Like run a mass spectrum on the electrolytes to see the exact ratios and such. And the anode and cathode materials to see their make ups
Isn’t it just. For example you can buy a D cell in NimH with 10,000 mAh capacity, which is maybe equivalent to a decent 18650 Li ion. But far larger and heavier, and more expensive. Though you could strap x4 2500 mAh AAs together into the same diameter and shorter, for less money.
In the AA/14500 size though, which covers a lot of useful devices, they are comparable. It seems nobody is interested in developing 14500, I think it is a lost cause. But the AA NimH will continue to be the best, though I doubt much more improvement will come, it is very mature. (Watch out for “fat” AAs though, some won’t fit devices built to spec.)
Summary: in AA/14500, NimH has the advantage except for the highest currents, and is an affordable, safe and readily available cell, in alkaline or even lithium primary versions. A torch that can only work with the voltage from a 14500 is a bit of a dead-end (though I have two).
I have a lot of use for AA torches.
Coupled with the versatility and performance characteristics of the simple AA NimH, AA torches IMO are the most underrated of flashlight platforms.
What they can do plus all the advantages of their compactness still kinda astonishes me to this day. I mean we’re talking about something with only 1.2V behind it pushing over 200 lumens in some cases for a good duration.
Improvements in the NiMH chemistry are just icing on the cake and not really required when we already have those glorious eneloops. 
Maybe part of the reason that NiMH (along with alkaline, NiZn, etc) is not seeing much development is because they aren't a very optimal voltage.
LEDs require, what? 3 volts or so? If you use Li-Ion, its not too hard to make drivers with around 90% efficiency. And lots of other electronics runs around 2-5v.
But sub 2 volt chemistries require boost drivers, which are not as efficient, and if you want to avoid them, you need to use multiple cells in parallel. Both of these options can be less appealing than using a single Li-Ion cell.
Please tell me if I'm wildly off track or something here. :)
I think the problem just comes from size. Each battery chemistry has an optimal cell size for max energy density and small cells just aren’t as optimal. Pretty sure that’s why 21700 was picked up by Tesla because the energy density is more optimal vs 18650s.
I thought the 21700 was for “packing” density, along with energy density to match.
Not everyone uses batteries for flashlights 
Obviously plenty of 1.5v applications out there right now.
I suspect economy of manufacture as well, i.e. much of the cost of making a cell must be in the casing materials and processing to assemble it, which is probably little different between 18650 and the “21-70” as Tesla name it. So the larger capacity cell should inherently be cheaper, per Watt-hour of capacity.
Just as e.g. AAAs cost nearly as much as AAs, per unit, despite having 1/3 the capacity.