Amazon.com: Customer Reviews: Maximal Power Combo 9V MaximalPower 9V Battery and Charger Pack of 2 9V li-ion Rechargebale Batteries
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on January 28, 2013
01/28/2013 - Used in cordless/hands-free Mics in church. Very good capacity - lasts 4+ hours and charges. works well so far and i plan on buying more if we get more of the same mics.

05/04/2013 - Update:
After twice weekly use, the batteries DO NOT last more than 20-30 mins now (always returned to charger after each use). I know that Lithium-ion batteries should last 1000+ recharges with 80% depth of discharge, and GOOD Ni-Cd batteries recharge 100s of times, so there is something suspect with these. ..

08/21/2013 - Update:
These batteries have a larger mAh capacity because they are "lower" voltage. If you don't need 9volts, like the included LED flashlight which ran for 53 hours(!) with usable light, these will work very well. If you need them for a "higher" voltage application these will have limited use. For me, these were used in a wireless microphone system that was near the base for the first month then I started using them farther from the base and they did not perform well. Below are the results of my tests:
Battery

9volt date voltage charged
#1 7/10/2013 8.26 3months ago
#2 7/10/2013 8.27 3months ago

#1 7/10/2013 8.31 10min charge to full
#2 7/10/2013 8.31 10min charge to full

#1 7/10/2013 8 after using included LED light for 1 hour

#1 7/12/2013 5.26 after 53 hours of LED
#2 7/12/2013 8.34 after 53 hours of charge

#1 8/21/2013 8.35 after 40days on charger
#2 8/21/2013 8.29 after 40days standby
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on December 10, 2012
Bought these for a TENS/IF/MIC/EMS system, it eats 9v batteries. This is based on first cycle on the charges so take it for that. I am getting roughly twice the duration as I was using Duracell 9v batteries and substantial increase in amperage. Charge time is roughly 8 hours. The charger has fold in prongs so its easy to pack and travel with. If the batteries last through as many charge cycles as they should being li-ion, I will be very happy.
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on August 13, 2013
It lasts long as alcaline in my effects pedals for guitar. I've re-charged it 10 times so far and ir really works as advertised. Love it.
Also, do the homework, can be found much cheaper on Ebay incl. shipment, so I don't know why it is priced so high at Amazon. Anyway high recommendation for this product.
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on November 21, 2012
I ordered this charger with Maximal Power 9-Volt Li-ion 550mAh Rechargeable Battery. At first it seemed as if the battery (or tester) was defective, but on further troubleshooting I found out that the battery works fine and the powered product is defective. The instructions say to charge for 10 hours on the first charge, but the 'charged' light comes on after 2 hours which seems a bit confusing, since you'd naturally think that it stopped charging.
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on April 15, 2013
Review updated on 2015-06-10.

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:

Universal Lithium Ion (Li-on) Rechargeable & Nickel Metal Hydride (NiMH) Battery Charger - Single Slot

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:

http://www.amazon.com/FlePow-Battery-Rechargeable-Lithium-ion-Batteries/dp/B00NEHKENO/ref=sr_1_6?ie=UTF8&qid=1433959117&sr=8-6&keywords=9v+li-ion+charger.

There are many options on chargers online, at very good prices. Paying more than $10 for a charger including shipping, is paying too much.
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on June 16, 2013
I am a gigging musician and my wireless in ear monitor receiver uses a 9V battery. I got tired of spending so much money on alkaline batteries, so I invested in these rechargeable batteries. They have worked great. they last more than the length of our full show and have paid for themselves many times over. My singer also purchased them for his wireless microphone. I highly recommend them for anyone who wanted a good quality battery and it tired of spending money on ones that you end up throwing away when you are finished with them.
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on January 16, 2013
I bought my son a metal detector that runs on two 9 volt batteries. Looking at going through alkaline batteries at the rate of 2 every eight hours of use, this seemed to be a no brainer. Within three to four uses, this will pay for itself. Also, these batteries are twice the capacity of alkaline batteries, so they will last longer when used.

I found no issues with the batteries, and the charger is compact, with a red/green led above each battery, indicating charging status. I have other chargers, but none that charge 9 volts, so this was the best deal I found on Amazon. I don't live in a large area, and there were limited options with regard to rechargable 9 volt batteries and chargers, so buying from Amazon was my only real choice. Although we've only charged the batteries twice, we have found no problems with the batteries or the charger, and would recommend to anyone needing to buy 9 volt batteries more than once per year, as this would be a significant cost savings.
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on October 23, 2012
i decided to try these out because my six *p*w*r 520 mah batteries (~$23 each) started to go. those batteries were good for three years plus. i use 9v batteries for my church's handheld shure mics (those mics are 10+ years old). used alkalines, ni-cad (very briefly), ni-mh, and now li-ions/polys. li-ions are the best by far next to good ol' alkalines. these maximal 9v have held up so far the past month without any failing when run for 4+ hours. sorry, can't be more specific because i let the singers and other sound tech's change them out so i don't know how long a fully charged one actually lasts. i'm using the *p*w*r 4-bay fast charger because i didn't want to purchase another charger unless i had to. i haven't seen and problems using the maximal 9v batteries and the charger...so far so good. i have the charger on a 7-day timer set at different intervals throughout the week; the maximum time the batteries are charged is no more than 4 hours per day...about 10 hours if the battery is left in the charger the entire week. time will tell if they last over 3 years. great price and good lasting charge.
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on January 16, 2014
I had decided to use rechargeable Lithium ion 9v batteries in my wireless microphone setup in the future to avoid worrying about running out of 'gas' during a presentation.
I purchased two samples of three different brands and ran three runs of each, depleting them to exhaustion each time. As you might expect, the facts were different from the claims about storage capacity.
If you're interested in trying rechargeables, based on my tests, I can recommend the EBL brand slightly over the Maximal. It's rated at 600mah, but tested out at 618mah. Maximal Energy at checked out at 482 vs 550 claimed. The others one I tested, Tenergy tested at 330maH vs 500maH claimed. I also included some Duracell alkalines which were in between the EBL and Maximal, but at several bucks a pop, are an expensive way to go.
The last time I checked, you can now buy a four-pack of EBLs for $30, including charger!
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on April 28, 2013
This is an excellent battery, very well built (at least it looks so) and I'm using 2 of these, one for a Nady 4 channel stereo mini mixer, and another for my danelectro electric guitar mini amp. Both with great results.

Beware you will need a Li-Ion charger, because this kind of batteries need pulses of current for charging correctly. I own maximal power 9 volt battery charger which I also recommend and it comes bundled with 2 of these bateries.
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