Charging NiMH Batteries is not as straight forward as with other battery types. Here are a few tips to help you.
Fundamentals of NiMh and NiCd Battery Charging: NiMH and NiCad batteries are usually some of the most difficult batteries to charge accurately. Unlike lithium ion and lead acid batteries where you can control any overcharge by simply specifying a maximum charge voltage, Nickel chemistry based batteries don't relate to a float charge voltage. Therefore, the battery cell charging is founded upon pushing a current through the battery. The forced voltage required to do this is not as set as it is for the other battery technologies.
Parallel NiMH Battery Charging: The consequence of the lack of a set charge voltage is that it makes charging NiMH & NiCd Batteries difficult to charge in parallel. This is because you can't be certain that each cell or battery pack is of the same resistance (or impedance), and therefore some cells will consume more current than others even when they are full. Consequently, you need to use a separate charging circuit for each string in a parallel pack, or provide current balancing in an alternative manner, for example by employing resistors that provide a resistance that it will impose the current control.
The charging efficiency of nickel metal hydride batteries is normally 66%, which means for every 100 amp hours of energy out put you must put 150 amp hours into the battery in the form of charging. This situation deteriorates when you attempt to charge the battery faster.
However, modern charging algorithms have been developed to enable accurate charging without using a thermistor. These chargers have special measurement techniques to detect a full charge, usually involving some kind of pulse cycle where the voltage is measured during the pulse and between pulses. Luckily, NiMH does not mind being overcharged, which allows the charger to balance the cells during the trickle charge. As the NiMH battery reaches it's end of charge phase, oxygen starts to form at the electrodes, and be recombined at the catalyst. This new chemical reaction creates heat, which can be easily measured and is the safest way to detect end-of-charge during a fast charge.
The cheapest way to charge a nickel metal hydride battery is to charge at C/10 (10% of the rated capacity per hour) or below. Therefore, a 100 mAH battery
would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. The latest cells have an oxygen
recycling catalyst that prevents any damage to the NiMH battery on overcharge. This only applies if the charge rate is over C/10. The minimum voltage
you need to get a full charge varies with temperature.
For the fastest charging rates of NiMH Batteries, If a temperature monitor is can be used and NiMH batteries can be charged at rates up to 1C (in other
words 100% of the battery capacity).
Trickle Charging NiMH Batteries can be achieved in a standby mode and means that you can keep the battery topped up without damaging it. For safety reasons
this should be done at a current of between 0.03 C and .05 C. The voltage required for this is dependent on temperature and therefore the current in
the charger needs to be regulated.
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