How do you trickle charge a NiMH battery?

What is a NiMH Battery?

NiMH stands for "nickel-metal hydride." These batteries are integral in various applications, ranging from consumer electronics to hybrid vehicles. Their development has evolved from earlier nickel-hydrogen batteries, which were notable for their long cycle life and robust performance, particularly in challenging conditions like those faced by satellites. Nickel-hydrogen batteries utilize pressure vessels to store hydrogen gas, whereas NiMH batteries use metal-hydride compounds to accommodate hydrogen atoms, creating a more stable and safer environment for energy storage.

NiMH Battery Charging Techniques

Charging Basics

Correctly charging NiMH batteries can pose challenges due to their unique chemistry. Unlike lithium-ion and lead-acid batteries, which allow simple overcharge prevention through a maximum voltage, NiMH batteries do not have a defined float-charge voltage. Charging depends on passing current through the batteries, and the necessary voltage fluctuates based on various conditions.

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Charging in Parallel

Charging multiple NiMH batteries in parallel requires caution due to varying resistance levels among battery cells. This inconsistency can lead to some cells charging faster than others, necessitating separate charging circuits for each battery string, or implementing current balancing methods.

Typically, the charging efficiency of NiMH batteries hovers around 66%, meaning that 150 amp hours may need to be input to achieve 100 amp hours of output. This relative inefficiency increases under rapid charging conditions.

End-of-Charge Detection

Traditional methods of detecting the end-of-charge cycle for NiMH batteries can be unreliable since they lack a pronounced voltage drop compared to NiCad batteries. This makes it crucial to use modern charging algorithms that employ advanced measurement techniques, allowing for more accurate charge detection through methods such as voltage pulsing. Fortunately, these batteries tolerate capacities being slightly overcharged, which can facilitate balancing during the trickle charge phase.

Overnight and Fast Charging Methods

For economical charging, it is recommended to charge NiMH batteries at C/10 or lower—charging a 100 mAH battery at 10 mA over 15 hours. This avoids the need for an end-of-charge sensor while adequately filling the battery. With modern technology, these cells come equipped with an oxygen recycling catalyst, which mitigates overcharge damage as long as the rate stays below C/10.

For faster charging, employing a timer can allow charging at C/3.33 rates for a duration of 5 hours, although this should only be attempted when batteries are fully discharged to prevent venting. More advanced methods exist for managing charge cycles but may not be commonly available at present.

Trickle Charging Techniques

To maintain charge without damaging your NiMH battery, trickle charging can be implemented safely at current levels between 0.03 C and 0.05 C. The exact voltage required for this varies with temperature, necessitating careful regulation of charger current levels.

Ideal Charging Practices

To ensure optimal battery health, NiMH charging practices should focus on preventing overcharging and managing temperature throughout the charging process. Effective monitoring could extend battery life significantly.

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