I managed to charge all 4 batteries once and the first time I tried to recharge a battery the charger died. Either I received a defective unit or EBL makes junk.
Update 9/17/2017. EBL contacted me directly after having read my review, and replaced the defective charger with a new one. They also sent 4 more batteries, but that was by mistake. It's nice to know they stand behind their product and went the extra mile.

Owning a guitar with a Sustainiac pickup, these are a must. I no longer worry about how often I flip on the Sustainiac and rock it all the time, knowing that another pair of 9-volts are charged and ready to toss in whenever the current ones run dry. I'm giving this 4 instead of 5 stars however since they are slightly bulkier than a regular 9-volt. I can still get these into the compartments in the rear of the guitar, but its noticeably tighter and requires a bit of a push/pull.

After endless batteries wasted on smoke detectors i had had enough! Got these reusable ones and couldnt be happier. There brilliant siver color is also amazing, like silver surfer came to save me boat lots of cash by not having to purchase batteries every couple months. These are saving the universe one battery at a time!!

Now that I've used them a couple of weeks I can report that these 9v batteries last a lot longer than their Low Self-Discharge NI-MH equivalents such as the Tenergy and Imedion which are good batteries but store less energy - 250 to 300mAh capacity vs 600mAh. Please note that these batteries are not 9v but rather 8.4v like most rechargeable batteries except the Imedion which are labeled 8.4v but I measured their voltage a month after I charged them at 9.2v. Unfortunately, the Imedion don't last as long in the TENS and EMG units that I use them in, so these batteries are a plus for me, you get four, they come in a nice plastic storage case and the charger works well shutting off when they reach full charge.

Very poorly thought battery management circuit design and manufacturing process. Since the battery protection IC (HY2120-CB) does not have cell balancing feature, initial charge of each cell is quite important.
About half of them were well balanced but other half were not. The result of unbalanced cells is less capacity, only about 200mAh.
After balancing cells, the capacity increased to 620mAh.
According to the HY2120-CB datasheet, it could charge up to 8.56V (4.28V per cell) and discharge down to 5.80V (2.90V) if cells are perfectly balanced.