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Why Discharge? — How does proper discharging help?


All of the Why Discharge? topics can be found together in our Why Discharge? white paper. (PDF, 35KB)


Regularly scheduled discharging of your packs down to a known cutoff voltage (typically around 0.9V/cell) helps to "condition" the cells in the pack, i.e, it helps to create the smaller cadmium crystals that increase the cell's voltage under load. It also reduces the presence of other alloys with lower electro-chemical potential, i.e., lower voltage levels.

Under normal use, and moderate to high charging currents, the cadmium that is deposited during a charge is deposited as very small crystals with a large surface area. This larger surface area is important because it exposes more of the cadmium to the electrolyte and decreases the cell's internal resistance, thereby increasing the cell's voltage under load.

Unfortunately, over time these small crystals want to join together to produce larger crystals. These larger crystals decrease the surface area of the cadmium and are harder to dissolve while discharging. This leads to higher internal resistance and voltage depression under load. Three things help these large crystals to form:
  1. Slow charging - While good for initially forming new cells, it shouldn't be your standard method of charging. Slow charging encourages slow crystal growth and this aids in the formation of large crystals.
  2. High temperature - Crystal growth speeds up as the temperature increases.
  3. Time - More time between full discharges means that the crystals have more time to combine and grow larger.
These larger crystals can actually grow enough to puncture the separator inside the cell (shorting it out internally) and cause the cell to self-discharge much faster than normal.

Using your battery packs without discharging them completely converts only some of those larger crystals back to the desirable smaller size when you charge them. The remaining larger crystals now have even more time to join together and increase the internal resistance of your cells. A proper (complete) discharge and recharge converts all of the cell's cadmium to smaller crystals, reducing the internal resistance and allowing you to maximize the current you can draw from the pack. It also helps to increase the voltage of the pack while under load.


How else can a discharger help me?
Using a discharger with known cutoff voltage levels will consistently bring your packs down to the same voltage level every time. This makes it easy to:
  • Track and log the capacity of your packs to better know when to replace them.
  • Recondition "tired" packs to bring back most, if not all, of their original capacity and voltage under load by converting the cadmium back to smaller crystals.
If the discharger has adjustable loads, you can easily and accurately:
  • Test the efficiency of the cells at different current levels making it easy to know how hard you can push them.
  • Measure the temperature of the pack at various discharge current levels to test different shrink-wrapping, mounting and cooling strategies.
  • Test the useful run times of your packs at various discharge current levels.
  • Compare different chemistries (NiCd vs. NiMH) and cell capacities at various discharge current levels to pick the best pack for each of your applications.
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