Safety
A good safety feature of a custom-built charger is to use a resettable fuse in series with the cell, particularly of the bimetallic strip type. This fuse will open if either the current or the temperature goes too high.
Modern NiMH cells contain catalysts to immediately deal with gases developed as a result of over-charging without being harmed (2 H2 + O2 ---catalyst → 2 H2O). However, this only works with overcharging currents of up to 0.1C (nominal capacity divided by 10 hours). As a result of this reaction, the batteries will heat up considerably, marking the end of the charging process. Some quick chargers have a fan to keep the batteries cool.
A method for very rapid charging called in-cell charge control involves an internal pressure switch in the cell, which disconnects the charging current in the event of overpressure.
There is an inherent risk with NiMH chemistry that overcharging will cause a buildup of hydrogen, causing the cell to rupture. Therefore, cells have a vent. Hydrogen will be emitted from the vent in the event of serious overcharging.
Read more about this topic: Low Self-discharge Ni MH Battery, Charging
Famous quotes containing the word safety:
“Perhaps in a book review it is not out of place to note that the safety of the state depends on cultivating the imagination.”
—Stephen Vizinczey (b. 1933)
“I nightly offer up my prayers to the throne of grace for the health and safety of you all, and that we ought all to rely with confidence on the promises of our dear redeemer, and give him our hearts. This is all he requires and all that we can do, and if we sincerely do this, we are sure of salvation through his atonement.”
—Andrew Jackson (1767–1845)
“For hours, in fall days, I watched the ducks cunningly tack and veer and hold the middle of the pond, far from the sportsman;... but what beside safety they got by sailing in the middle of Walden I do not know, unless they love its water for the same reason that I do.”
—Henry David Thoreau (1817–1862)