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Single Cell Powered Lithium Ion Rechargeable Power Tools

OddMix.com - Power Technical Note - TN0903 - by Karl Nagy

Picture 1 depicts a new single cell powered, highly useable and popular tool that was next to impossible to obtain before the past Christmas holiday season. Every time the stores restocked this handy little item, it sold out again quite rapidly. The euphoria is easy to understand, especially for those who are used to other higher power drills and screwdrivers. The higher the battery voltage so is the number of cells used in them and their available power. Also higher is the chance that the complicated battery pack would fail. Drive in just that last fastener after the motor started to slow down and the damage is done for good.

Picture 1. Single Cell Powered Ryobi Power Screwdriver [6 KB]
Picture 1. Single Cell Powered Ryobi Power Screwdriver

No power pack made of more then a single cell can survive for long as any multi-cell battery is only as strong as its weakest cell. In even the best and more carefully selected battery packs, there is always an imbalance of cell capacity. Even if the cells were precisely balanced initially, they develop differently when cycled under load. Far the worst is the notorious Ni-MH uneven cell leakage or self-discharge rate problem. Not many cell makers are able to select identically leaking cells. Perhaps they are not even trying hard. After all their business is making and selling batteries.

That is why single cell equipped tools are unbeatable. Multi-cell Lithium power packs almost always employ complicated electronics that monitors the condition of each cell in the pack. All notepad and lap top portable computers are equipped with multiple cells, most often in a two plus two cell configuration. Two sets of two Lithium Ion cells connected parallel are then used together in series for higher voltage. The double (paralleled cells) are treated as one larger capacity cell by the integrated monitoring electronics.

The electronics is also able to monitor how far the power within the cells is depleted and makes this information available to the user. Many battery packs have special names to reflect on these built in guards or intelligence. Sony named its batteries Info-Pack. Other similar names are also common. Many of us experienced failures in the protective circuits that made otherwise healthy cell power inaccessible.

As many millions of cell phones successfully demonstrated, single cells power supplies are almost indestructible. Certainly, single cells can be depleted near zero volts and they are still not experiencing cell reversal. If the charger circuit is well made and precisely regulated, cells always would last close to their predicted life of hundreds of full and thousands of partial charge-discharge cycles.


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