This battery has been the perfect upgrade for a Powerex 300 mAh 9V rechargeable battery. That battery would last 3 weeks in a wireless microphone when new. After some time, it would last closer to 2 weeks. After two years of use, I was searching for a replacement that would work for at least one month between charges. In order to accomplish that, I knew that I had to find a >=500 mAh battery. After some online search, I found that the Li-Ion 9V batteries would meet that requirement and I settle on this one because it had good reviews and a good price.
I have been using this battery for 3 years and 4 months. It takes between 45 and 50 days of normal use to get discharged, when the previous had to be recharged in 21 days at its prime. Also, it is noticeable lighter than the NiMH battery it replaced. Another advantage is that Li-Ion batteries have a longer life than NiMH. In my experience, with different types and brands of NiMH batteries, they provide 80 full charge cycles at best and about 50-60 on average, despite the claim that they can reach up to 300 cycles. HA, HA. This Li-Ion battery is of the same technology as a cellular telephone battery, and will have peak performance through several hundred charge cycles, just like a cell phone battery.
This battery has automatic shut down protection, which means that it can not be discharged to the point where the cells are damaged from depletion. The battery will stop delivering current before this happens.
The full charged voltage for this battery seems to be 8.4V for two 4.2V cells (4.2V x 2 = 8.4V). If that is the case, no matter how long you leave it in a charger, it will not charge to higher than 8.4V. That does not mean that the time between charges will be shorter than it would be for another battery that would charge to 9V or higher. Battery or cell voltage does not determine time between charges. The size (not the voltage) of the cells is what determines time between charges. That is measured in amperes per hour (Ah) or milliamperes per hour (mAh). For example, a car battery and a typical computer UPS battery are both rated at 12V. But, the car battery is bigger and stores more current because it is rated at 500 or more Ah, whereas the typical computer UPS battery is rated at just 7 Ah. Since, they are the same voltage, you could use a car battery in a computer UPS unit and the UPS unit would work as it should with the difference that it would provide longer than 30 hours of back up time instead of the typical 20-30 minutes.
You can find a very cheap and effective Li-Ion charger on eBay with the search words "9V Li-ion charger." The black universal chargers that have the shape of a ship (boat) that have connectors for two 9V batteries and one connector for all sizes of cylindrical batteries (i.e. AA, AAA) are Li-Ion chargers by specification and are perfect for this battery. I have two of those. It takes about 4 hours for them to fully charge my 9V MaximalPower battery. Full charge detection has been flawless on both of them with this battery.
Update on 2013-06-14: This is the charger the I wrote about above:
I saw it, just when I had finished my review. So, I knew that I had to come back and post the Amazon.com link for this charger. With NiMH batteries, this charger (I have two of them) has not been very accurate in determining the end of charge (light changing from red to green). With a 9V NiMH battery, the red light would never turn to green. However, with Li-Ion batteries, end of charge detection works as it should.
Update on 2015-06-10: The charger which link I posted above is now unavailable. Now, you can easily find a suitable charger at a reasonable price online like the one from the link above or this one: