Digital camera is a high drain device consumes large amount of power in each click. Digital camera with LCD and flash light can draw over 1000 mA quickly and depleting the alkaline batteries after a few shots. Alkaline batteries have approximately 3000 mAh capacity with a 1000 mA load. So we have to use a high efficient battery such as NiMH ( Nickel Metal Hydride) battery. NiMH battery outperforms other rechargeable batteries and has higher capacity and less voltage depression. The toxic Cadmium is absent in NiMH battery so it is the most Eco friendly one.
Typical NiMH battery is rated 1.2 volts/cell. This is the nominal voltage of the cell that is discharging at the rate of C/10 at 25 degree centigrade to an end voltage of 1 volt. The battery pack has three 1.2 volt cells making a total 3.6 volts. When a fully charged battery is used, the discharge voltage starts from 1.5 volts followed by a sharp drop to 1.3 volts. The voltage in the cell remains 1.3 volts to 1.2 volts until another voltage drop occurs during a deep discharge.
The NiMH battery is similar to NiCd cells in structure but it uses a hydrogen absorbing alloy as negative electrode instead of cadmium. The positive electrode is a nickel oxy hydroxide (NiOOH).NiMH batteries can easily handle the high current levels and maintain their full capacity. AA type NiMH cells have a nominal capacity (C) ranging from 1000 mAh to 2700 mAh at 1.2 volts with a discharge rate of 0.2C / hour.
Important facts to be considered while using NiMH battery are
1. NiMH cells are good for 500 to 1000 cycles if properly charged.
2. A new battery needs full charging (Reforming) before use. It should be `charged and discharged minimum 5 times to attain full charge capacity.
3. When a new NiMH battery is charged and connected to equipment, it will show full charge only for 10-15 minutes. If this occurs, remove the battery and allow it to cool for 15 minutes and then charge.
4. Fast charging with 100%C should be terminated after 1.5 hours.
5. Charger should be switched off if the battery temperature rises above 55 degree centigrade.
6. Do not discharge the entire cells in the battery pack below 0.8 volt per cell to avoid voltage depression.
7. NiMH cells do not accept full charge and discharge if the temperature is above 50 degree and below 0 degree.
8. NiMH cells like 0-40 degree temperature.
9. NiMH cells can hold full charge for a month if stored in normal temperature. If it is kept near hot objects or exposed in sunlight for hours, the charge will drop in one day.
10. ‘Condition’ the battery once in a month. This is done by completely discharging the battery by running the equipment in battery power and then allow to charge completely.
11. If the battery is not used for long periods, first charge the battery fully and then remove it from the equipment and store in a cool dry place. Recharge again once in a month.
12. Do not charge NiMH battery using raw DC (12or 24 V) available from the outlets of car or aircrafts since the voltage and current are not regulated.
13. If emergency charging is required from such power sources, use a voltage and current regulator circuit in the charger.
14. Do not ‘flatten’ the battery below 0.8V/cell and do not over charge beyond 100%C
What is Inside?
The NiMH battery has a special chemistry.There is an ‘Inter metallic compound in the NiMH battery’. Most NiMH cells use AB5 or AB2 type metallic compounds. The common one is AB5 where ‘A’ represents a rare earth mixture of lanthanum, cerium, neodymium, praseodymium or titanium. The ‘B’ represents nickel, cobalt, manganese etc. But some NiMH cells use higher capacity negative materials based on AB2 compounds. Here ‘A’ represents titanium or vanadium and ‘B’ is nickel modified with chromium, cobalt iron etc.
The metallic compound reversibly forms a mixture of ‘metal hydride compound’. When the battery is undergoing charging process, hydrogen ions are forced out of potassium hydroxide electrolyte solution by the charging voltage. This avoids the formation of gas inside. During the discharge process, the same ions are released to participate in the reverse reaction.
When over charged at low current, oxygen produced at the positive electrode passes through the separator and recombines at the surface of the negative electrode. Hydrogen production is suppressed and the charging energy converts to heat. This makes the NiMH battery maintenance free.