What is the difference between Nickel Metal Hydride (NIMH) rechargeable batteries and rechargeable alkaline or rechargeable Nickel Cadmium (NiCad)?
Nickel Metal Hydride (NiMH) rechargeable batteries are different for a number of reasons. NiMH rechargeable batteries are far superior to rechargeable alkaline and NiCad chemistries because of the amount of capacity, lighter weight, recharge cycles, no memory effect, not to mention the enormous cost savings over time.
Rechargeable alkaline batteries can only be charged up to 50 times. What’s more, rechargeable alkaline batteries must be completely discharged before they can be charged again.
Because of its chemistry, rechargeable nickel cadmium (NiCad) has a limitation to the amount of capacity it can hold. In addition, NiCad batteries are highly susceptible to ‘memory effect’. Also NiCad batteries are considered toxic to the enviromnment, and must be disposed of properly.
What does ‘memory effect’ mean?
NiCad batteries are much more susceptible to this problem. ‘Memory effect’ occurs when a rechargeable battery is charged consistently at levels above recommended discharge. If 50% of a rechargeable battery’s capacity is used and then the battery is charged – and the battery is constantly used and charged at this level – the other 50% will unfortunately never be used. And so, the battery will only be using half of its power rating. For example, a 1000mAh battery will only be operating at 500mAh. (Because of the advanced NiMH technology, all top NiMH rechargeable batteries do not suffer from 'Memory Effect' )
What is the best state to store my Ni-MH rechargeable batteries. Charged or un-charged?
To get the best use and longest life from your batteries, you should always store Ni-MH rechargeable batteries in a fully charged state.
It is also recommended that you charge your Ni-MH rechargeable batteries at least every 30 days or so during storage.
This will assure that your batteries are always in top peak condition and do not become incapable of accepting a charge.
Ni-Cd batteries on the other hand should be stored fully discharged.
How do NiMH Rechargeable Batteries work?
Similar to alkaline batteries, NiMH rechargeable batteries batteries have four basic components - a positive electrode (cathode during discharge), a negative electrode (anode during discharge), a separator, and electrolyte. But that's where the similarities end. The chemicals reactions that occur inside a NiMH rechargeable battery are reversible, allowing them to be recharged again and again.
Charging The NiMH Rechargeable Battery
Before a NiMH rechargeable battery can be used, it must be charged. NiMH batteries are built in a discharged state.
Just like an alkaline battery, NiMH rechargeable batteries provide electrical power from a chemical reaction.When a chemical reaction happens in a battery, electrons are either produced or consumed. This kind of reaction is called an electrochemical reaction. To make these electrons flow in a useful direction, NiMH rechargeable batteries include components called separators and current collectors.
When a NiMH rechargeable battery is charged, the battery charger pumps electrons out of the positive electrode. This causes the nickel in this electrode to oxidize, or change from Ni+2 to Ni+3. During this oxidation, hydrogen atoms leave the positive electrode and react with the electrolyte.
Simultaneously during charge, the charger pumps electrons into the negative electrode. This causes a reduction reaction to occur, and causes this electrode to absorb hydrogen from the electrolyte.
Based upon the type of battery charger, the battery charger will automatically stop pumping electrons out of the positive electrode into the negative electrode after most of the hydrogen has been removed from the nickel. The battery charger can tell when this happens in one of three ways: through a change in temperature, a change in voltage, or by a built-in timer.
Powering The Electronic Device
During discharge, all of the electrochemical reactions that happened during charge happen again – in reverse.