NiCad and NiMH

The main difference between the two is that NiMH battery (the newer technology of the two) offers higher energy density than NiCads. In other words, the capacity of a NiMH is approximately twice the capacity of its NiCad counterpart. What this means is for you is increased run-time from the battery with no additional bulk or weight. NiMH also offers another major advantage: NiCad batteries tend to suffer from what is called the "memory effect". NiMH batteries are less prone to develop this problem and thus require less maintenance and conditioning. NiMH batteries are also environmentally friendlier than NiCad batteries since they do not contain heavy metals (which present serious landfill problems). Note: Not all devices can accept both NiCad or NiMH batteries.

Lithium Ion

Lithium-Ion (Li-Ion) has become the new standard for portable power in consumer devices. Li-Ion batteries produce the same energy as NiMH battery but weighs approximately 20%35% less. This is can make a noticeable difference in devices such as cellular phones, camcorders or notebook computers where the battery makes up a significant portion of the total weight. Another reason Li-Ion batteries have become so popular is that they do not suffer from the "memory effect" at all. They are also environmentally friendly because they don't contain toxic materials such as Cadmium or Mercury.

 

The Do’s and Don’ts of Battery Use

Battery Do's:

Fully charge/discharge battery up to 4 cycles before achieving full capacity of a new battery

Fully discharge and then fully charge the battery every two to three weeks for battery conditions.

Run the device under the battery's power until it shuts down or until you get a low battery warning. Then recharge the battery as instructed in the user's manual.

Remove from the device and stored in a cool, dry, clean place if the battery will not be in use for a month or longer,

Recharge the battery after a storage period

Ensure maximum performance of the battery by optimizing the device's power management features. Refer to the manual for further instructions.

Battery Don'ts:

Do not short-circuit. A short-circuit may cause severe damage to the battery.

Do not drop, hit or otherwise abuse the battery as this may result in the exposure of the cell contents, which are corrosive.

Do not expose the battery to moisture or rain.

Keep battery away from fire or other sources of extreme heat. Do not incinerate. Exposure of battery to extreme heat may result in an explosion.


My new battery isn't charging. Is it defective?

Usually NO. New batteries come in a discharged condition and must be fully charged before use. It is recommended that you fully charge and discharge the new battery two to four times to allow it to reach its maximum rated capacity

It is generally recommend an overnight charge (approximately twelve hours). It is normal for a battery to become warm to the touch during charging and discharging.

When charging the battery for the first time, the device may indicate that charging is complete after just 10 or 15 minutes. This is a normal with rechargeable batteries. New batteries are hard for the device to charge; they have never been fully charged and not “broken in.” Sometimes the device's charger will stop charging a new battery before it is fully charged. If this happens, remove the battery from the device and then reinsert it. The charge cycle should begin again. This may happen several times during the first battery charge. Don't worry; it's perfectly normal.

How can I maximize the performance of my battery?

There are several steps you can take to help you get maximum performance from your battery:

Prevent the Memory Effect - Keep the battery healthy by fully charging and then fully discharging it at least once every two to three weeks. Exceptions to the rule are Li-Ion batteries which do not suffer from the memory effect.

Keep the Batteries Clean - It's a good idea to clean dirty battery contacts with a cotton swab and alcohol. This helps maintain a good connection between the battery and the portable device.

Exercise the Battery - Do not leave the battery dormant for long periods of time. We recommend using the battery at least once every two to three weeks. If a battery has not been used for a long period of time, perform the new battery procedure described above.

Battery Storage - If you don't plan on using the battery for a month or more, store it in a clean, dry, cool place away from heat and metal objects. NiCad, NiMH and Li-Ion batteries will self-discharge during storage; remember to recharge the batteries before use.

Sealed Lead Acid - (SLA) batteries must be kept at full charge during storage. This is usually achieved by using special trickle chargers. If you do not have a trickle charger, do not attempt to store SLA batteries for more than three months.

Next:

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. Since the delta V bump is not always easy to see, slight overcharge is probable. For this reason PowerStream does not recommend using minus delta V as a termination criterion for nickel metal hydride batteries.

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.

Overnight Charging

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 http://www.powerstream.com/NiMHWM.htm. This charger uses a microprocessor to report the state of charge via an LED as well as performing the timing function.

Faster Charging

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.

Fastest Charging

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).

This board also has the ability to sense voltage and current for more sophisticated algorithms.

When terminating on temperature rise the dT/dt value should be set at 1 to 2 degrees C per minute.

Trickle Charging

In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and .05 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.

dT/dt versus -dV/dt

These two termination methods work well for NiCads, and are both applied to NiMH as well. dT/dt measures the temperature rise at the end of charge. After the battery is fully charged it starts new chemical reactions in order to absorb the unneeded current. In nickel hydroxide style batteries this consists in generating and recombining oxygen. This process heats the battery. The sudden increase in temperature rise can be used to terminate the charge.

Another effect of the oxygen generation/recombination cycle is to depress the voltage of the battery slightly. If you can detect this voltage depression you can use this signal to terminate the charge. Of course, -dV/dt is the easiest because it doesn't require a temperature sensor. The best method for NiMH is the dT/dt method. There are two main reasons. With the NiMH battery the voltage depression is smaller, and harder to detect than with the NiCad battery. This almost always ensures an overcharge, which will limit the total number of charge/discharge cycles before battery failure. Second, a new NiMH battery has false peaks early in the charge cycle, and so the charger will terminate too soon.

There are new algorithms that use microprocessor control to use the -dV signal to detect the end of charge. These can work very well and several of our chargers use this technique, which involves pulsing the charger on and off to do the voltage measurements. This technique seems to be sensitive to imbalance in the capacity of the cells. The dT/dt is still more reliable, especially for large packs, but in cases when only two wires are available solutions are now available.

The Ultimate Charger

Sometimes the most important issue is the lifetime of the batteries or the total lifetime cost of the system is in a good  charger one that is in the area of microprocessor controlled battery chargers and power supplies. Specs to look for the ultimate charger are:

1. Soft start. If the temperature is above 40 degrees C or below zero degrees C start with a C/10 charge. If the discharged battery voltage is less than 1.0 Volts/cell start with a C/10 charge. If the discharged battery voltage is above 1.29 V/cell start with a C/10 charge.

2. Option: if the discharged battery voltage is above 1.0 Volts/cell, discharge the battery to 1.0 V/cell then proceed to rapid charge.

3. Rapid charge at 1 C until the temperature reaches 45 degrees C, or the dT/dt indicates full charge.

4. After terminating the fast charge, slow charge at C/10 for 4 hours to ensure a full charge.

5. If the voltage climbs to 1.78 V/cell without otherwise terminating, terminate.

6. If the time on fast charge exceeds 1.5 hours without otherwise terminating, terminate the fast charge.

7. If the battery never reaches a condition where the fast charge starts time out the slow charge after 15 hours.

8. Fuel gauge, communication to the device being powered, LED indicators all possible.