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Why Discharge? — Over-discharging and its effectAll of the Why Discharge? topics can be found together in our Why Discharge? white paper. (PDF, 35KB) While discharging until the cells in a pack are "empty" (to approximately 0.9V/cell) is good, discharging them any further is not better. In fact, it can destroy cells in your pack. Not all of the cells in your pack are identical. As the pack ages the cells become more and more unbalanced (different voltage or charge levels between the cells) and some cells can "empty" before others during a discharge if the pack's voltage is brought too low. If these already-discharged cells are discharged even more, they are subjected to what is essentially a reversed-polarity charge. This causes excess gas to be produced which builds up pressure in the cell, possibly forcing the gas to escape through the cell's safety vents. This causes a loss of water and damages the cell. Discharging must be stopped before this reversal (and the damage it causes) can occur. Accepted cutoff values for NiCd/NiMH packs range from 0.8V/cell to 1.1V/cell, depending on the discharge current level (the higher the discharge current level, the lower the cutoff should be). All of CamLight Systems' Pack Dischargers and Auto-Cutoff Modules have per-cell cutoffs in this range. We recommend 0.9V/cell as a standard. It is low enough to make sure all of the useful charge in a pack is removed without being so low as to risk cell-reversal due to over-discharging. The better the cells are matched in a pack (i.e., the pack is balanced), and the longer they stay that way, the less you have to worry about cell reversal during a discharge. The cells will each discharge at the same rate and their voltages will stay matched. This helps prevents cell reversal since no cell is being brought down to zero volts ("emptied") before the others and damaged as a result. You can accidentally over-discharge such a pack with much less fear of damaging one or more cells. All packs will become more and more unbalanced as they age. Moderate to high current charging and discharging to approximately 0.9V/cell can help keep your packs balanced as long as possible. If your pack configuration and size allows it, occasionally discharging each cell (individually) to 0.0V and then charging the cells together as a pack is the best way to maintain this balance. In between these zero-volt discharges, keep discharging your packs to approximately 0.9V/cell. Home Page · Pack Dischargers · Auto-Cutoff Modules · Load Modules · Accessories & Parts · Custom ProductsCopyright © 2003-2008 John Muchow. All rights reserved. |