High Self Discharge Low Shelf Life and Hybrid Rechargeable Cells

OddMix.com - Power Technical Note - PWRN0810 - by Karl Nagy

Picture 1. Rayovac Hybrid type AA cells [7 KB]
Picture 1. Rayovac Hybrid type "AA" cells

It is widely known that rechargeable, secondary cell batteries are not as reliable and dependable as their primary cell counterparts. The problem is that unless the rechargeable battery is freshly charged they are out of power at the very time when they are most needed. Since it is not always possible to know the exact time of an impending emergency, most often people have to rely on the more expensive primary cells when using inexpensive secondary cells would be better. Not only disposable cells are expensive, but also they are a source of pollution on the environment.

Far the worst problem with using rechargeable cells is their high self-discharge rate. To make things worst, the cell's leakage or self-discharge rate is difficult or impossible to determine. And even if it could be done, the rate is seldom constant, but changes with the discharge rate, temperature, age and many additional factors. To obtain a maximum power from a set of identical cells they all have to be closely matched. That rule is equally applicable to any kind of cells, disposable or reusable. Cells have to be matched for ampere-hour capacity, but their self-discharge rate has to be as closely matched also, otherwise the leakier cell will be depleted before the others. The weakest, leakiest cell determines how long the battery can deliver power to its load.

Picture 2. Charger with Rayovac Hybrid battery cells [8 KB]
Picture 2. Charger with Rayovac Hybrid battery cells

Until recently the self-discharge rate for secondary cells used to be high, one to two percent of their ampere-hour capacity per day. A fully charged cell could be depleted in as little as a month or less. Even if a rechargeable cell could be recharged hundreds of time, that is not a very desirable condition. A good primary cell has almost negligible leakage rate resulting in a very long self-life. All of that is changing as one after another rechargeable cell manufacturer introduced less leaky, higher shelf life cells recently.

My favored one of the lot is the Rayovac NiMH [Nickel Metal Hydride] Hybrid cell - Picture 1. - with Electron Retention Technology that I had the opportunity to try recently. Both the AA and AAA batteries were in a two-year-old package, that were marked as "Already charged Ready to Use". The cells did worked flawlessly out of the package in my digital camera without the need of charging before use, just like the marking on the package implied.

My only problem with them is that because each cell would prevent the merchants to sell me at least 500 disposable cells, not too many stores are anxious to sell them. The Rayovac Hybrid batteries suppose to last up to 4x times longer than regular cells. A Hybrid cell should retain 80% of its charge after six months as opposed to only 40% of the old technology, regular rechargeable. And each HYBRID cell supposed to be equivalent to 1,500 alkaline cells Picture 2. No wonder they are hard to find!

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