All about rechargeable 9V Batteries

The basics

1. "9V" is the physical size/shape, not the actual voltage.  An alkaline 9V is really 9 volts, but a rechargeable "9V" battery is initially 9.6V, 8.4V, 7.4V, or 7.2V, depending on the model in question.   In the rest of this discussion, "9V" refers to the 9V size, not the actual voltage.
2. Various "9V" voltages.
1. 9.6V (NiMH).  It's rare, but will always provide good runtime and performance.  Achieved by using eight 1.2V cells.
2. 8.4V (NiMH).  These will often be good too, but some power-hungry devices might do worse with 8.4V than with 9.6V. Achieved by using seven 1.2V cells.
3. 7.4V (Li-ion).  Usually work well even with the lower voltage because lithiums have such a high capacity.  Achieved by using two 3.7V cells.
4. 7.2 (Li-ion).  Same as above, using two 3.6V cells.  Li-Ion cells are generally 3.6V or 3.7V.
5. 7.2V (NiMH).  I'm skeptical would ever provide good performance, but I haven't tested them so I can't say for sure.
3. Choose between NiMH and Li-ion.  9V rechargeable batteries come in two flavors: NiMH and Li-Ion.  Li-ions have about twice the capacity as NiMH, but of course they're more expensive.  Your choices are listed in the table.  For musicians' effects pedals, see further down this page for a special product for that purpose.
   
4. 9V rechargeables die more often than AA/AAA.  Be warned that 9V-size rechargeables are actually made up of 2, 6, 7, or 8 separate cells, and if one cell dies then the whole battery dies, so 9V NiMH batteries are much less hardy than AA and AAA rechargeables.
5. Must use a special charger. 
   
1. Most chargers don't handle 9V-size batteries.
2. Every combo charger I've seen which handles multiple NiMH sizes (AA, C, D, 9V) doesn't use negative delta-V to prevent overcharging.  Must use a separate 9V-only charger to prevent over- or under-charging.
3. I can't find any 9V charger that handles both NiMH and Li-ion.  Must use a separate 9V NiMH charger or a 9V Lithium charger depending on what batteries you have.
4. The EBL-802 claims to charge both NiMH and Li-Ion (in both the Amazon description and the manufacturer's website), but the back of the product says only "NiMH" and makes no mention of Li-Ion.  Also, AAA and AA must be charged in pairs (2 or 4 pieces), though 9V can be charged 1 or 2 pieces at a time.
   
   

Smoke Alarm Batteries

Types of 9V battery chemistries compared

• Lithium (disposable): 10 hours
• Lithium-ion (rechargeable):  7-7.5 hours
• Alkaline (disposable):  5 hours
• NiMH (rechargeable):  4 hours

The energy of 9V alkalines seems to be 2.8-3.7 Wh, while "Heavy Duty" 9V's are 0.4-0.9 Wh. (ZBattery)

Musicians:  Use the Pedal Juice battery

Musicians might be interested in the Pedal Juice battery, where a single rechargeable battery pack can supply 9V of clean power to all your effects pedals via a daisy-chain cord.  It beats using individual 9V batteries, because there's only one battery to charge, and it beats using AC, because the battery power is clean and you don't have to run electrical cords to outlets.  It's pricey at $131, but cheaper over the long term if you've got lots of pedals that you use disposable batteries in often.

Make your own 9V battery pack

It's very easy to get an 8-cell battery holder, load it with 8 NiMh batteries, and attach a 9V battery connector to it.  Voilà, you made a 9V battery.  It'll provide way more run-time than a 9V-size battery, too.

When is a 9V battery dead?

Depends on the device you're powering, since different devices require different minimum voltage.  I imagine that no device will run on less than 6.0V.  Smoke alarms typically start chirping between 6.0-7.6V, depending on the manufacturer.  See my smoke alarm batteries page for more on that.

Don't try a 9V battery in a 3.7-volt device

While checking my server report, I found that someone found this site on a search for "can 3.7 volt device run off 9v battery?"  So in case anyone else is wondering about the answer to that question, the answer is that some devices will definitely be sensitive to that much voltage and the 9V will fry them.  Some will be hardier.  Electronics are more dangerous, motors less so.  Me, I wouldn't try it.  It would be hard to physically make the connection anyway, since a 9V battery is a different size than a 3.7-volt battery.