Cons:

• Low voltage of 1.2V means that cheap (unregulated) flashlights run dimmer, and devices needing 4+ batteries might run through batteries quickly, or not work at all.
• Many brands self-discharge to empty after just a few months of sitting around if you don't use them.  Make sure buy the Low Self-Discharge (LSD) version if you want long shelf-life.

Intro.  Until the late 1990's, NiCd's were the only option for rechargeable batteries in household sizes, but their capacity was terrible, and they contain toxic cadmium, which means they were supposed to be disposed of as hazardous waste, not in household trash.  Around the turn of the century we were saved from this tyranny when NiMH's became widely available, offering triple the capacity, and with non-hazardous materials, for about the same price.  As a result, NiCD's have all but disappeared.


Where to Buy.  I like Amazon for most goods, because the prices are low and they stock everything.  If you need them today then many grocery stores stock them (as well as Radio Shack), but the prices are often higher and the capacities lower then what you can get from more careful shopping.  eneloops are good, and I have a separate page where I cover my recommended brands (as well as which to avoid).

High-Drain Performance.  Devices that need lots of power quickly, like digital cameras, are called high-drain.  That's as opposed to devices that just sip the juice slowly, like clocks.  NiMH work great in high-drain devices.  In fact, many digital camera manufacturers recommend them, and even design their cameras for the lower voltage of the NiMH's.  An example of a battery that doesn't work well in high-drain devices is a standard alkaline (though there are premium alkalines like Duracell Ultra that work fine in cameras).

Capacity.  AA capacities range from 1200 to 2700 mAh.  Beware of obscure brands that promise higher capacity—they usually deliver just a fraction of their label rating.  (See my list of good and bad brands.)  Also be sure to get a good charger because some cheaper ones don't fill the batteries up completely. NiMH's purportedly lose some of their capacity permanently if not used for long periods of time. (Battery University)

Self-Discharge.  Normal NiMH's have the highest self-discharge rate of any kind of battery (meaning they lose charge just by sitting around, unused), but there are Low Self-Discharge (LSD) versions available (like Sanyo eneloop).  The tradeoff is that the LSD versions have a little lower capacity.  So when buying NiMH's, you have to choose between longer shelf life or higher capacity.  If you burn through batteries quickly, get the regular NiMH's so you can enjoy the larger capacity.  But if you go months before using up the battery's capacity, go for LSD instead.  The table at right shows the relationship between self-discharge and capacity.

    One nice thing about the LSD versions is that they come pre-charged.  Regular NiMH's must be charged before use.

Voltage.  NiMH's are rated at 1.2V, which is lower than the 1.5V that alkalines put out.  This is generally not a problem, but it does mean that flashlights will be dimmer, and devices that need 4 or more batteries might burn through the batteries very quickly or not work at all.  On the other hand, some devices (like many digital cameras) are designed to work with the lower 1.2V, so the reduced voltage is definitely not a problem there.  If NiMH's don't supply enough voltage for your device, consider NiZn's, or mixing NiMH and NiZn together, after seeing the caution about mixing NiMH and NiZn in the same device.

Voltage Drop.  Like most other rechargeables, NiMH batteries maintain most of their voltage over the whole charge and then suddenly plummet.  This contrasts to alkalines, which lose their capacity steadily. For this reason many electronic devices that tell you how much battery life is left have a hard time reporting an accurate level for NiMH's. The voltage is very similar for both a fully-charged battery and a nearly-spent battery. Some devices (like my GPS wristwatch) let you specify in the setup menu whether you're using NiMH or alkalines, so they can try to be more accurate with the battery-remaining indicator.

Charging.  Overcharging can reduce cycle life (the number of times the battery can be charged).  Smart chargers know when the battery is full and stop charging. Dumb chargersrun on a timer and will almost always overcharge or fail to fill up the battery completely, and they usually really fail to fill up C and D sizes.  Charging NiMH with old NiCd chargers is not recommended.  (Modern chargers handle both NiMh's & NiCd's.) Note that even some smart chargers are better than others.

    You'll get more cycles by charging your NiMH's when they're only partially depleted rather than fully depleted.  (Battery University)  But since you can typically charge NiMH's hundreds of times, it's probably not worth worrying about, and you can just charge whenever you like.  Even Cadex, who makes battery test equipment and therefore knows a lot about batteries, says that it's hard to formulate more specific advice for charging.  They do recommend draining down to 1.0V once every 3 months.

    An ideal charge rate is probably around 4-6 hours.  Faster than that will work, but isn't optimal.  For slower charges, besides wasting time, there's the possibility that a smart charger will miss the cutoff signal that tells it to stop charging.

    Energizer recommends storing NiMH batteries in a charged state, and to exercise them (alternatively drain and charge) a few times to restore their capacity.(PDF, #20)  Somechargers I recommend will exercise the batteries for you automatically.

    See the Charging Tips page for more about charging.

Cycle Life.  NiMH's are good for hundreds of charge cycles in theory, but overcharging and repeatedly running the batteries down all the way can reduce cycle life.  To avoid overcharging, use a smart charger that stops charging when the battery's had enough. (See the chargers page for recommendations.)  You'll also get more cycles if you charge your batteries before they're fully run down.

     The original (2005) Sanyo Eneloop LSD's were rated for 1000 cycles, and the newest ones are rated for 1500 and 1800 cycles, though it would take years and years for most users to get to even 1000 cycles.

Recycling. When your battery no longer holds a charge or its capacity is no longer useful, you can easily recycle it at over 30,000 locations in U.S. & Canada such as Sears, Office Depot, Home Depot, Target, Wal-Mart, Best Buy, and others.  Find the nearest location to you from RBRC or Earth911.

NiZn (Nickel-Zinc) -- A good rechargeable, better & worse than NiMh in some ways

• Rechargeable
• Works great in high-drain devices
• Lasts longer in some high-drain devices than NiMH's
• Higher voltage (1.65V+) makes lights burn brighter (except some LED's which regulate the voltage)

Cons:

• The high voltage (1.65V) can burn out lights quicker, fry some electronics with no voltage regulator, and just not work in some electronics that do have voltage regulators
• High self-discharge rate (they lose ~13% of their initial charge per month just sitting around)
• Capacity plummets as the cells are cycled (used & recharged)
• Requires a special, proprietary charger.
• Possible reliability problems (high failure rate: cells die quickly or self-discharge even faster than normal)
• They're ever-so-slightly larger than normal, so they might not fit in those rare devices in which the batteries are already a tight fit.
• Semi-discontinued (see below; for now, Amazon has them, and the charger)
• Not available in any sizes besides AA and AAA

Intro.  NiZn's were introduced by PowerGenix in 2009 as an alternative to NiMH's.  Their gimmick is that they have a higher voltage, so they're a solution for devices in which the 1.2V of NiMH cells is too low.  The higher voltage also means that camera flashes power up faster and flashlights burn brighter.  But the higher voltage could be both a blessing and a curse, with the higher voltage burning out lights and sensitive electronics, as we'll see below.  PowerGenix made them in only the AA size, and then discontinued them, but off-brands from China are available on eBay.

Where to Buy.  PowerGenix made NiZn for only a year or two, then stopped.  In June 2012, they informed me that they're talking to other companies about having them take over production, but I'll believe it when I see it.  In the meantime, the unsold stock is still available at Amazon as I write this, and when that's gone, lots of Chinese retailers sell no-name and off-brand NiZn's on eBay.  (Yes, quality could be an issue, but if you want the extra voltage from NiZn's, your options are limited.  And in fact, even the PowerGenix brand wasn't all that reliable.  More on that in a minute.)  Remember that you'll need a special charger, and that's the same deal as the batteries:  Buy the PowerGenix one before they're all gone, or get a no-name Chinese brand from eBay.

Reliability.  PowerGenix NiZn's suffer from reliability problems.  See below for my poor experience with capacity. Also, NLee reports that after buying four cells and putting them through 20-30 deep cycles (0.9V), two failed (reduced voltage and rapid self-discharging), and the other two suffer from reduced capacity (80% of original).  Many customers on Amazon report their batteries dying prematurely too.  It appears that over-discharging NiZn's, which can easily damage them.  (NiMH's are more tolerant of an over-discharge.)

High-Drain Performance.  High-drain devices are those which need lots of power quickly, like digital cameras.  That's as opposed to devices that just sip the juice slowly, like clocks.  NiZn work great in high-drain devices, if the voltage isn't an issue.  (See the Voltage section below.)

Capacity and Run Time.  There are many reports of reliability problems, including my own experience, which kind of make any published specs about capacity moot.  I used 9 for about a year (probably fewer than 10 cycles) in electric toothbrushes and electronic door locks.  Seven of the nine (Star Trek Voyager reference unintentional) dropped to only 45-150mAh in capacity, and the remaining two were 996 and 1298mAh.  They should have been 2500mWh ÷ 1.65V = 1515mAh.  NLee the Engineer says that after just a year, 2 of the 4 he purchased suffered reduced capacity down to 80% of the original, and the other 2 simply failed.  Many others have made similar complaints.  The following discussion assumes that NiZn's don't suffer reduced capacity early, although that's probably not the case.

     NiZn's give either longer, shorter, or fairly equal run time, depending on what device you use them in.  engadget said that they got 300-400 flashes from their camera flash unit with NiZn's, vs. only 200-300 with NiMH's, and Tom's Guide said their NiZn's ran a CD player for three times longer than NiMH's.  However, devices which don't limit the input voltage (like most flashlights and electric toothbrushes, for example) will run out faster with NiZn's, because while the device was running the light was burning brighter or the motor was spinning faster.  NiZn's will make camera flashes recycle much faster, though burning through lots of flash shots quickly can fry the flash.  Also, the NiZn will likely not give as many total flashes as NiZn, so you get super-fast flash recycle times at first, but then it gets worse than NiMH.  (Strobist quotes a user that said NiZn's got worse after 50-75 shots, and too slow to use after 200 shots, compared to 400 shots for NiMH.)

      Usually battery capacity varies from brand to brand, but in this case there's only one brand so the AA and AAA capacities are 1500 and 700 mAh respectively.  That's about half of the best NiMH's.  PowerGenix therefore lists the spec on its battery in mWh (total energy) rather than mAh, because total energy between battery types is more similar, and PowerGenix says that's a more apples-to-apples comparison.  That's debatable, from either side.  It's true that the total energy is the same, but again, if the device being used doesn't limit the voltage, the device will use the extra voltage and the NiZn will spend its energy faster, so the NiZn's will provide less runtime.  (And if the device does limit the voltage, then there's no advantage to using NiZn's in the first place, because the only reason you'd use them instead of NiMH's is if you needed the extra voltage to begin with.)

Voltage.  NiZn's have the highest voltage of any rechargeable battery.  The nominal voltage is 1.65, which is even higher than the 1.5V of alkalines.  And fresh out of the charger, the initial voltage is as high as 1.85V. (PowerGenix, PDF)  The higher voltage can be both a blessing and a curse.  The upside is that flashlights burn brighter, and battery life will generally be longer in high-drain devices.  (Some LED lights limit the voltage, so in that case NiZn's won't be brighter than 1.5 alkalines, but they'll still be brighter than 1.2V NiMH's.)

     But there are downsides to the extra voltage.  For lights, the brighter light means that the bulbs will burn out faster, sometimes immediately. (Tom's Guide)  For cameras, a rapid flash cycle can fry the flash. (Amazon review)  For electronics, first understand that some devices have a voltage regulator (which limits the max voltage coming from the batteries) or a voltage protector (which shuts off the device if the battery input is too high).  If your device doesn't have one of these, and the device is very sensitive to voltage, then the batteries might fry it.  It's hard to know whether a particular device is a "fry", "auto shut-off", or "no problem" variety. (Good luck.)  The fewer batteries your device takes, the less likely you are to have a problem.  A one-batery device is the least dangerous, two-batteries are a little more so, four batteries even more, and with 8+ batteries you're just asking for trouble.  Powergenix mostly ignores this problem in their marketing materials, so shame on them.

    If you're worried that NiZn's could be too hot for your device, then NiMH would be a better bet.  If NiMH doesn't supply enough voltage for your device, you can mix NiMH and NiZn together, after seeing the caution about mixing NiMH and NiZn in the same device.

Self-Discharge.  Powergenix claims a self-discharge rate of 8% per month, but NLee the Engineer's tests showed 13% per month, and I'm inclined to trust him.

NiZn's have a pretty high self-discharge rate, about 13% per month. (NLee the Engineer)  This means they lose their charge by just sitting around, unused.  Unlike NiMH's, there is no low self-discharge version available.  If you use up and recharge your batteries quickly this won't matter to you.  But if you need longer shelf life, you'll want to consider a LSD NiMH instead.

Voltage Drop.  Like most other rechargeables, NiZn batteries maintain most of their voltage over the whole charge and then suddenly plummet.  This contrasts to alkalines, which lose their capacity steadily.  For this reason many electronic devices that tell you how much battery life is left have a hard time reporting an accurate level for rechargeables, but this is especially true for NiZn's, because their voltage is so high.

Charging.  You need a special NiZn charger for these cells.  NiMH chargers will not work!  (See below for those who insist trying anyway.)  PowerGenix makes two chargers:  the white charger is smart and manages each cell separately, but there's only one status LED and it doesn't light up until all the batteries are done.  (If one cell is bad, the red charge light will blink.  Then you get to play musical batteries figuring out which one is bad.)  The black charger requires charging in pairs, and doesn't manage the batteries separately.  Never buy a charger like that.  There are also no-name or off-brand NiZn chargers available from China on eBay.

    The white charger is labeled a "1-hour charger", but what that really means is that the batteries mostly full after an hour, but not completely.  Powergenix says it takes about 1.5 hours to charge 1-2 AA's, and 2.5 hours to charge 3-4 AA's.

    Unlike NiMH's, NiZn's tolerate fast-charging well.  The PowerGenix spec sheet suggests a rate of between C/2 to C is okay (30 minutes to 1 hour).

    Charging NiZn's in a NiMH charger.  It's theoretically possible to charge NiZn's with a NiMH charger, but there are at least three problems.  First, even a smart NiMH charger probably won't know when to stop charging NiZN's, and will overcharge, damaging the battery.  Overcharging is harder on NiZn's than NiMHs.  Second, how will you know when to stop charging?  Armed with PowerGenix's recharge profile chart (PDF) and the battery and charger specs you could make a guess.  (Good luck.)   Finally, the charging profile of a NiZn is different from a NiMH.  A NiZN should get less and less charging current as it approaches fullness, but NiMH chargers don't do that.  The top-of-the-line La Crosse BC-series NiMH chargers put a minimum of 200mA into the batteries, for example, while the NiZn's should get only about 100mA near the end of the charge.  I don't know the penalty for feeding more current than is recommended, but a shorter cycle life is my guess.  Again here's PowerGenix's recharge profile chart (PDF).  All that said, in a pinch, I've used my La Crosse BC-700 to slightly recharge NiZn's while traveling when I forgot to pack my NiZn charger, but I've terminated the charge well before the cells could be overcharged.  When you put the cells in the BC charger they'll read "FULL" if their voltage is over ~1.3 volts, so I overrode that by pressing the cell button and then the Mode button to switch to charging mode.  I monitored the voltage and took them out when they hit 1.8V, although you should be safe by going up to 1.9V.  Even so, at that point your batteries will likely be only about half full.  Anyway, all this is at your own risk, of course.

    Discharged Voltage.  The PowerGenix site provides absolutely no guidance as to how far NiZn's should be drained before charging to maximize capacity and cycle life.  (Shame on them.)  The typical discharge level for rechargeable batteries is 1.0 to 1.1V, and 1.1V is when I try to recharge my batteries (both NiMH and NiZn).  The charger won't recognize them at <0.5V, but even though the charger will recognize a 0.6V cell, its capacity or reliability might be greatly reduced if you drain your cells to that level.

    Problems with the charger.  NLee says that the charger stops charging when the voltage hits 1.9V, but if the battery is damaged and can't reach 1.9V, the charger will just keep charging and ruin the battery.  That's not my experience:  I had a damaged cell that showed as full at only 1.16V.

    NLee also says that batteries left in an unplugged charger will drain 10-100 times faster than batteries left in other chargers, which means that they'll be discharged after only ten days.

Cycle Life.  PowerGenix claims 100-500 cycles for their NiZn's, compared to 100-800 for NiMH and NiCd, but they don't specify the depth of discharge.  NLee the Engineer says he found technical data on Powergenix's site that says that their NiZn's get only 200 deep-charge cycles, but I couldn't find any such data (probably no longer there).  NLee's testssuggest a 50% capacity drop after only 100 cycles.  I'm currently running my own tests, and it does look like cycle life is indeed pretty lousy.

Recycling.  NiZn's are recycled in the same programs that take NiMH's and NiCd's.  When your battery no longer holds a charge or its capacity is no longer useful, you can easily recycle it at over 30,000 locations in U.S. & Canada such as Sears, Office Depot, Home Depot, Target, Wal-Mart, Best Buy, and others.  Find the nearest location to you fromRBRC or Earth911.

Li-ion (Lithium Ion) — not available in standard voltage, except for 9V

Pros:

• Rechargeable
• Works great in high-drain devices

Cons:

• Not available in standard voltage, except for 9V size (AAA, AA, C, and D-size Li-ion put out 3.7V instead of 1.5V)
• Accidentally putting a 3.7V Li-ion in a 1.5V device could easily fry it.
• Requires a special charger

Voltage.  Normal AAA, AA, C, and D-size batteries usually put out 1.2-1.5V, but Li-ion's put out 3.7V.  If you put them in a 1.5V device, you could easily fry it.  The reason these high-voltage Li-Ion's exist is for super-rare devices that are designed for them, or for hobbyists building their own devices who want a lot of voltage in a small package.  Some flashlights/headlamps are designed for either standard 1.5V or 3.7 Li-ion's, and they're typically brighter with Li-ion's.  Li-ion 9V batteries are either 8.4V or 9.6V, so they're direct replacements for any device that uses a standard 9V.  See more on 9V rechargeable batteries.

Naming convention.  To avoid confusion with standard batteries (since putting a high-voltage Li-ion in a device that expects a standard battery could fry it), Li-ion batteries typically usually use a different naming system rather than the standard "AAA", "AA", etc. names:

1. AAA: "10440"
2. AA: "14500"
3. C: "25500"
4. D: "32600"

Charging.  Li-ion batteries require a special charger.  A NiMH charger will not work.

Other sizes.  Li-ion battery packs are popular in laptop computers and digital cameras.

NiCd (Nickel-Cadmium) — low capacity and obsolete

Pros:

• Rechargeable
• Work great in high-drain devices

Cons:

• Pitiful capacity — lowest of any rechargeable battery
• Contain toxic cadmium. Can't be disposed of in household trash.  (You're supposed to take them to a Hazardous Materials collection facility.)
• Low voltage of 1.2V means that flashlights run dimmer, and devices needing 4+ batteries might run through batteries quickly, or not work at all.
• High self-discharge rate (they lose 10% of their charge in the first 24 hours, and then 10% of their initial charge per month just sitting around)

Intro.  NiCads are obsolete rechargeables. They've been replaced by Nickel-Metal Hydrides (NiMH).  NiCd's don't have any advantages over NiMH, and they have lousy capacity and are toxic to boot.  The only reason to use NiCd's is if you already have some—though even then, if you upgrade to a modern battery you won't have to charge them so often.  NiCds are hard to find now, but why would you want to?

High-Drain Performance.  NiCd's deliver the juice fast enough to power high-drain devices like digital cameras—but not for long, since their capacity is so weak.

Capacity.  Different brands have different capacities, but with NiCd it's always low anyway (which is why you should be using NiMH instead).  The AA size has 600-1000 mAh (compared to 1200-2900 for a NiMH), and the D size has 1800-5000mAh (vs. 2200-12,000 for NiMH).  Note that many cheap chargers fill up the D size to only a AA capacity, giving really awful run time.  (See our recomended chargers.)

      Memory Effect.  The theory of the NiCd memory effect is that if you repeatedly partially discharge a battery to the same level before charging it, it will "remember" the discharge level and then always fail at that point during use, never allowing the full capacity of the battery to be used.  Many observers don't believe that this effect is real, and most of those who do admit that it's hard to reproduce and can be fixed by cycling with a good battery charger anyway.  Now, NiCd's can suffer from decreased capacity due to repeated deep-discharging or overcharging, and many people mistakenly blame the resulting reduced capacity on the memory effect.  (Wikipedia, Dan's Data, Repair FAQ)  But the real problem wasn't a memory effect, it was deep-discharging or overcharging.  NiCd's with reduced capacity can often be resurrected by exercising them (discharging down to 1.0V), or conditioning them in a charger that has a conditioning mode. (Battery University)

Self-Discharge.  NiCd's discharge quickly (meaning they lose charge just by sitting around, unused).  They typically lose 10% of their initial charge in the first month, and then 10% of the initial charge each successive month.

Voltage.  At 1.2V, no other battery has less voltage than NiCd's. (For reference, alkalines are 1.5V.)  This is generally not a problem, but it does mean that flashlights will be dimmer, and devices that need 4 or more batteries might not work at all.

Voltage Drop.  Like most other rechargeables, NiCd batteries maintain most of their voltage over the whole charge and then suddenly plummet.  This contrasts to alkalines, which lose their capacity steadily. (See the chart under NiMH.) For this reason many electronic devices that tell you how much battery life is left have a hard time reporting an accurate level for NiCd's. The voltage is very similar for both a fully-charged battery and a nearly-spent battery.

Charging.  Overcharging can reduce cycle life (the number of times the battery can be charged).  Smart chargers know when the battery is full and stop charging. Dumb chargersrun on a timer and will almost always overcharge or fail to fill up the battery completely, and they usually really fail to fill up C and D sizes.  Note that even some smart chargers are better than others.  See the chargers page for charger recommendations.

    You can charge whenever you like, but constantly draining them completely before charging shortens their life.  Occasional draining down to 1.0V is okay, and recommended. Very slow charging can damage the batteries.  (Ken Nishimura, an electrical engineer with a PhD from UC Berkeley)

Cycle Life. NiCd's are good for hundreds of charge cycles in theory, but overcharging and repeatedly running the batteries down all the way can reduce cycle life.  To avoid overcharging, use a smart charger that stops charging when the battery's had enough. (See the chargers page for recommendations.)  You'll also get more cycles if you charge your batteries before they're fully run down.

Recycling. When your battery no longer holds a charge or its capacity is no longer useful, you can easily recycle it at over 30,000 locations in U.S. & Canada such as Sears, Office Depot, Home Depot, Target, Wal-Mart, Best Buy, and others.  Find the nearest location to you from RBRC or Earth911.

9V size is especially puny.  NiCds are already very weak compared to NiMH, but the 9V size of NiCds is especially weak.  NiCd "9V" voltage is actually only 7.2V (or 8.4V if you're lucky), while NiMH voltage is typically 9.6V or sometimes 8.4V.  See more on 9V-size rechargeable batteries.

More:  Radio Shack's technical info in NiCds 

Rechargeable Alkalines (RAM) -- higher voltage + long shelf life

Pros:

• Rechargeable
• Slow discharge rate (long shelf life)

Cons:

• Short cycle life (can't be charged as many times as a real rechargeable like NiMH or NiZn)
• Tiny initial capacity in some brands
• Capacity (and sometimes voltage) is reduced on every cycle
• Doesn't work in high-drain devices
• Requires a special charger, and charges much slower
• Way more prone to leaking than any other kind of battery

Summary.  These are often called RAM's, for Rechargeable Alkaline Manganese.  When they came out in 1992 they were competing against rechargeable NiCd's, and claimed the best of both worlds—reusable like a NiCd, but with the higher voltage and long shelf life of an alkaline.  Those advantages have been narrowed now that LSD NiMH's offer long shelf life and NiZN's offer higher voltage, but RAM's remain the only battery that offers both long shelf life and higher voltage in the same package.  Of course, there are some downsides: 

• RAM's can be deeply-discharged and then recharged only 5-100 times, vs. many 100's of times for NiMH or NiZN.
• The battery capacity is reduced significantly on every cycle.  Ouch!
• In some brands, the voltage is reduced on every cycle too.  There goes one of the whole reasons for using RAM vs. NiMH in the first place.
• RAM's don't work well in high-drain devices like digital cameras.  (But they're great in things like clocks, flashlights, and remote controls.)
• RAM's, like regular alkalines, are prone to leaking (unlike real rechargeables like NiMH and NiZn which are virtually leak-proof).

Where to Buy.  Most manufacturers have discontinued RAMs, since they've been pretty much made obsolete by NiMH and NiZN.  They're hard to find, and I've never seen them in stores, even when there were more manufacturers.  The only current manufacturers of RAM's that I was able to find are Pure Energy andAccucell.  (Sundance Solar sells the Pure Energy brand, and careful, because confusingly "Pure Energy" also comes in a NiMH flavor.)  Rayovac discontinued their Renewal brand RAM's in 2004, and the Juice and Lenmar brands appear to have disappeared circa 2011.

High-Drain Performance.  RAM's don't work in devices that need lots of juice quickly, like digital cameras.  For high-drain devices use NiMH or NiZn.

Capacity.  Initial capacity of RAMs is often dramatically lower than a real alkaline, or even a NiMH for that matter.  Also, the capacity of a RAM decreases after every use/recharge cycle.  For example, Pure Energy brand RAM's lose half their capacity after 25 deep-discharge cycles. (PDF)  Lenmar's are worse, becoming useless after 25 deep-discharge cycles.  Considering that the capacity drops on each recharge cycle, a Lenmar effectively provides the same amount of energy over 25 cycles as 10 regular alkaline batteries would have. (Lenmar)

     RAM's retain much more capacity and can be recharged many more times if they're recharged before being depleted—the sooner the better.  They may be able to be charged 100's of times that way, and will offer much more lifetime capacity vs. deep discharging.  Of course, it's a hassle to constantly recharge your batteries when they're only partially drained.

Self-Discharge.  RAM's have the lowest self-discharge rate of any rechargeable battery—less than 1% of their initial charge per month.  That's way less than even the low self-discharge flavor of NiMH's.  This makes RAM's good for infrequently-used devices, or devices which take months to use up the battery (e.g., clocks).  Though I'd still prefer LSD NiMHs, and avoid the risk of leaks.

Voltage.  At 1.5V, the RAM's voltage is just about ideal compared to other rechargeables—it's higher than the 1.2V of NiMH's (so lights run brighter), but lower than the 1.65V of NiZn's (so you don't risk blowing bulbs or frying electronics).  However, just like the capacity drops on every charge, voltage can drop as well.

Voltage Drop.  RAM's lose their voltage gradually over their use like regular alkalines (not suddenly like NiMH's and NiZN's).  See the chart under NiMH for a picture.

Charging.  You need a special charger for RAM's, a regular NiMH charger won't work.  Capacity is retained (and cycle life is greatly increased) by charging RAM's before they're fully depleted—the sooner the better.  RAM's will offer much more lifetime capacity this way.  RAM's take a lot more time to charge than other rechargeable batteries.  Here's my page of RAM chargers.

Cycle Life.  RAM's offer the fewest deep recharge cycles of any rechargeable battery, but if you're not charging your batteries every day RAM's could be a good fit for you.  (If youare charging your batteries every day, look at NiMH or NiZn.)  The worst RAM's likely get at least 20 cycles.  Pure Energy claims at least 50 deep recharge cycles, thanks to a combination of improved battery chemistry and their optimized battery charger.  For shallow discharges, recharge cycles could go into the hundreds, and at least in Pure Energy's case, into the thousands.  (See Pure Energy's PDF report.)

Recycling.  It's a lot harder to find places to recycle RAM's than real rechargeables like NiMH and NiZN.  See the Alkalines > Recycling section for more.

Why there aren't 9V rechargeable alkalines.  From the Rayovac website: "There are two reasons: (1) 9V batteries actually have six small 1.5 V-cells inside them. Reusable alkalines need to be charged individually for reliable performance. Since you can't access each cell inside the 9V battery individually, they could not be reliably recharged. (2) Most products designed to run from 9-Volt batteries last for many months. Recharging these batteries provides limited benefit in terms of cost savings (the cost of replacing batteries is not significant anyway)."

source: http://michaelbluejay.com/batteries/rechargeable.html