I purchased two dozen AA NiMH batteries from All-Battery.com in an ebay auction on January 19, 2008 for home use. Twelve were to be for me and twelve for a friend. I purchased these specifically, because the price was good and they came in a package deal, including C and D adapters. The adapters seemed like a good idea for three reasons. 1) you need fewer types of batteries on hand, to always have charged AA, C and D cells, 2) most chargers are only for AAA and AA cells and 3) true high capacity C and D NiMH batteries are very expensive, and the cheaper ones are really just AA batteries in a larger shell. This was fine, because the particular uses for C and D cells that we have, Roomba virtual walls, are low drain and the more rapid turnover for the AA batteries would be acceptable.
Upon receiving the batteries, I charged all twelve of mine and tried them out. It turned out that they did not work in my Cateye LED bike light or in the controller for my radio-controlled helicopter, two main reasons I needed them. Comparing them with other AA batteries, it seems that the problem is that the positive terminal of the Tenergy batteries stands a little less proud from the body than the others, and the aforementioned devices happened to have plastic bosses around the positive terminals, so that the positive terminals of the batteries could not reach those of the devices. I contacted All-Battery.com about this, and they referred me to Tenergy for technical support. The Tenergy tech support person assured me that the batteries met the IEC specification and offered me a 50% refund. It was not clear if I had to send the batteries back and only 50% of my money would be refunded or they would refund 50% of the money without taking the batteries back, but I didn’t clarify this, as I thought I might add a little solder to the terminals to make them work – I did not get a chance to try that.
In the applications where the batteries did work, they seemed OK initially, but lost their charge very rapidly. My friend tried newly-charged ones in her Roomba virtual walls one day and they worked, but a few days later they did not. She also put them in her wireless computer keyboards, but they only lasted a couple of days, while other brands last several months. I put a newly charged pair in my etrex GPS one weekend and went on a hike, but the next weekend they were dead. It seemed that the batteries were running down very fast between charges, so I decided to do a test. I charged 12 batteries and measured the voltages. I found that the voltages were lower than the other NiMH batteries I had (Sony and Ultra). I also found that they discharged faster than other batteries and were very weak after one week. I unfortunately lost that data, but I wrote to All-Battery.com and Tenergy again, asking for a complete refund. They never answered my email, so I decided to do another test and write this review.
I charged 12 Tenergy cells and 2 Ultra cells. The Tenergy cells were bought January 19th 2008 and the Ultra cells were at least two years old but in excellent condition. I had bought them from Globalcomputer.com. I measured the no-load voltage of all cells immediately after charging and a couple of times over the next three weeks. The results for all 14 cells are plotted individually in the graph below. Even at time zero, all Tenergy cells had significantly lower voltage than the Ultra cells, by 68 mV on average, and the discharge rate was much greater, so that after three weeks, the Tenergy cells were on average 190 mV lower than the Ultra cells. After the three week measurements I tested pairs of batteries in a 2-cell Maglite. The Ultra cells lit the bulb brightly with no noticeable decrease during half an hour of continuous operation. On the other hand, all six pairs of tenergy cells produced a very dim light that extinguished completely within 30 seconds.
In conclusion, the Tenergy cells I received are defective in three ways. 1) they have a nonstandard positive terminal size that makes them unserviceable in some devices. 2) the fully charged voltage is lower than that of other NiMH cells and 3) The no-load discharge rate is much higher than that of other brands. I can’t see how any customer could be satisifed with this performance. I think that I may have gotten a bad batch or the comments from satisfied Tenergy customers I have seen on their site were from customers who had just tried their batteries after charging them. In either case, it is unacceptable.