Edition 51
Technical Trends - The Cell is the Real Power
by Bryant Underwood, Public Safety Sourcing, Cassidian Communications

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In Reverse Logistics there is always a need to deal with batteries. Not only are they often the source of the unit failure but they need special handling for disposal or recycling. For the folks out there repairing notebook computers, you know the handling and troubleshooting of battery issues can be expensive and complex. Have you every wondered why tablet computers and cellphones have fewer issues than notebook computers? Let me explain how the power in a battery is all about understanding the cell.

The name ‘battery’ technically refers to a specific device comprised of multiple cells. This is usually needed to get a higher voltage than what can be provided from a single cell. This term, Battery more correctly described what was called a ‘battery of cells’. Over time this got shortened and today we just refer to anything that supplies power from a chemical reaction as a battery. The key to understanding how complex battery management can be and how easily just charging the battery can cause damage, is all about understanding the problem with using a battery instead of a single cell.

Before we get started in too much detail you need to understand a couple of things about chemical cells that supply current. These cells use a variety of very reactive chemicals to provide a stream of electrons (discharging) and to receive a stream of electrons (charging). In each of these steps a host of chemical reactions occur. The chemical state of charge is inferred by the charging equipment through sensing of temperature, voltage inflection points and time. Since the battery charger never really knows the actual chemical state in each cell, like specific gravity, the charging system can make serious errors. If any part of the discharging or especially, the charging process goes wrong the results can be catastrophic.

The main mechanism that allows this to happen is from too deeply discharging a battery. The schematic below depicts a battery composed of six, lead acid cells and a load.

As the battery voltage begins to get depleted there will be one cell that ‘dies’ first, meaning it is chemically ready to receive a charge. That voltage level for a lead-acid cell is ~1.75V under load. That means there are 5 other cells that are still above 1.75V and supplying current. For our illustration lets assume cell 3 is the first cell to fully deplete.

If the drain on the battery is not halted and the battery placed back on charge, the damage cycle will begin. To understand how damage occurs, take a look at the following schematic. The circuit is exactly the same as the one above, except it is redrawn from the cell 3’s point of view.

From this view notice what we now have is a 5 cell battery with 9 volts of charge, connected backwards, through a very low value resistor to a dead cell. Dead cells can be very fussy in charging, but they really hate to be charged backwards. When that happens, plate erosion and sulfation (crystalline electrolyte) will occur. When this first happens the results are not too bad, but typically if someone had the need to deep discharge a battery once, it will happen again. After the first deep-discharge the battery capacity is now reduced and creates an even greater need to over-discharge again and again. This is a cascade of damage that will destroy the entire battery in short order. These examples use lead-acid chemistry, but the effect and outcome are the same for any battery.

So why are cellphones and tablets more immune to this failure mode than notebooks? Most cellphones and tablets do not use batteries, but use a pack comprised of a single Li-ion cell. Li-ion cell chemistry produces a much higher voltage than other battery chemistries that gives the engineers more options. Rather than use series connected cells in a full battery pack to get the higher voltages, these devices are more energy efficient and use buck/boost converters to get the other voltages needed for operation. Since there is only one cell, it is much easier for the devices circuitry to infer a valid chemical charge state. That improved accuracy and charge management allows the battery/cell to last much longer compared to if it was in a single pack.

The only downside to this approach is the energy lost from the efficiency of the Dc-Dc converters. That efficiency cost is minor for lower current demands devices like phones and tablets. For full blown notebook commuters, the demands of the hard-drives and other systems make the ‘single-cell’ design less desirable. Next time when you get the call from a Customer annoyed their $200 battery died, you are now armed with what you need to explain why they need to instead, buy the $280 battery with higher capacity. Its’ all about the protecting the cells.
RLM
Bryant Underwood manages Public Safety Sourcing for Cassidian Communications, an EADS North America Company in Frisco Texas.

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