NiCad and NiMH batteries are amongst the hardest batteries to charge accurately. Whereas with lithium ion and lead acid batteries you can control overcharge by just setting a maximum charge voltage, the nickel based batteries don't have a "float charge" voltage. So the charging is based on forcing current through the battery. The voltage to do this is not fixed in stone like it is for the other batteries.
Parallel Charging: This makes these cells and batteries difficult to charge in parallel. This is because you can't be sure that each cell or pack is the same impedance (or resistance), and so some will take more current than others even when they are full. This means that you need to use a separate charging circuit for each string in a parallel pack, or balance the current in some other way, for example by using resistors of such a resistance that it will dominate the current control.
The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.
The minus delta V bump that is indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it is very temperature dependent. To make matters worse, new NiMH batteries can exhibit bumps in the curve early in the cycle, particularly when cold. Also, NiMH are sensitive to damage on overcharge when the charge rate is over C/10 (C is the capacity of the battery, so a charge rate of C/10 would give a full charge in 10 hours). Since the delta V bump is not always easy to see, slight overcharge is probable. For this reason PowerStream does not recommend using simple minus delta V as a termination criterion for nickel metal hydride batteries.
However, modern algorithms have been developed to enable accurate charging without using a thermistor. These chargers are similar to the -delta V chargers, but 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. For multicell packs, if the cells are not all at the same state of charge, and if they are not balanced in capacity, the cells may fill up one at a time, blurring out the end-of-charge signal. In order to balance the cells it may take several charge-discharge cycles. Luckily, NiMH does not mind being overcharged at C/10 or less, which allows the charger to balance the cells during the trickle charge.As the battery reaches end-of-charge oxygen starts to form at the electrodes, and be recombined at the catalyst. This new chemical reaction creates heat, which can be easily measured with a thermistor. This 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 or below (10% of the rated capacity per hour). So 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. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours. Examples of this kind of charger are shown at /Ni-6-200.htm . This charger uses a microprocessor to report the state of charge via an LED as well as performing the timing function.
Using a timer it is possible to charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully discharged before charging. If the battery still has 90% of its capacity when the timer starts you would have a good chance of venting the battery. One way to ensure this doesn't happen is to have the charger automatically discharge the battery to 1 volt per cell, then turn the charger on for 5 hours. The advantage of this method is to eliminate any chance of battery memory. PowerStream does not currently have such a charger, but the microprocessor board used in the C/10 charger /NiMH-NiCad-solar-charge-controller.htm could easily be modified to do the discharge. A power dissipating package would be needed in order to dissipate the energy from a partially charged battery in a reasonable amount of time. More examples of 3 hour chargers are here /NiCad-NiMH-Catalog.htm .
If a temperature monitor is used NiMH batteries can be charged at rates up to 1C (in other words 100% of the battery capacity in amp-hours for 1.5 hours). The PowerStream battery charge controller shown in /product3.htm does this, as does the battery management board shown in /product5.htm.
This board also has the ability to sense voltage and current for more sophisticated algorithms.With high quality products and considerate service, we will work together with you to enhance your business and improve the efficiency. Please don't hesitate to contact us to get more details of Ni-MH Battery Pack AA2400mAh 7.2V, Ni-MH Rechargeable Battery Pack.