BU-407: Charging Nickel-cadmium

Nickel-based batteries are more complex to charge than Li-ion and lead acid. Lithium- and lead-based systems are charged with a regulated current to bring the voltage to a set limit after which the battery saturates until fully charged. This method is called constant current constant voltage (CCCV). Nickel-based batteries also charge with constant current but the voltage is allowed to rise freely. Full charge detection occurs by observing a slight voltage drop after a steady rise. This may be connected with plateau timing and temperature rise over time (more below).

If you want to learn more, please visit our website recharge ni cd batteries.

Battery manufacturers recommend that new batteries be slow-charged for 16&#;24 hours before use. A slow charge brings all cells in a battery pack to an equal charge level. This is important because each cell within the nickel-cadmium battery may have self-discharged at its own rate. Furthermore, during long storage the electrolyte tends to gravitate to the bottom of the cell and the initial slow charge helps in the redistribution to eliminate dry spots on the separator. (See also BU-803a: Loss of Electrolyte)

Battery manufacturers do not fully format nickel- and lead-based batteries before shipment. The cells reach optimal performance after priming that involves several charge/discharge cycles. This is part of normal use; it can also be done with a battery analyzer. Quality cells are known to perform to full specifications after only 5&#;7 cycles; others may take 50&#;100 cycles. Peak capacity occurs between 100&#;300 cycles, after which the performance starts to drop gradually.

Most rechargeable cells include a safety vent that releases excess pressure if incorrectly charged. The vent on a NiCd cell opens at 1,000&#;1,400kPa (150&#;200psi). Pressure released through a re-sealable vent causes no damage; however, with each venting event some electrolyte escapes and the seal may begin to leak. The formation of a white powder at the vent opening makes this visible. Multiple venting eventually results in a dry-out condition. A battery should never be stressed to the point of venting.

Full-charge Detection by Temperature

Full-charge detection of sealed nickel-based batteries is more complex than that of lead acid and lithium-ion. Low-cost chargers often use temperature sensing to end the fast charge, but this can be inaccurate. The core of a cell is several degrees warmer than the skin where the temperature is measured, and the delay that occurs causes over-charge. Charger manufacturers use 50°C (122°F) as temperature cut-off. Although any prolonged temperature above 45°C (113°F) is harmful to the battery, a brief overshoot is acceptable as long as the battery temperature drops quickly when the &#;ready&#; light appears.

Advanced chargers no longer rely on a fixed temperature threshold but sense the rate of temperature increase over time, also known as delta temperature over delta time, or dT/dt. Rather than waiting for an absolute temperature to occur, dT/dt uses the rapid temperature increase towards the end of charge to trigger the &#;ready&#; light. The delta temperature method keeps the battery cooler than a fixed temperature cut-off, but the cells need to charge reasonably fast to trigger the temperature rise. Charge termination occurs when the temperature rises 1°C (1.8°F) per minute. If the battery cannot achieve the needed temperature rise, an absolute temperature cut-off set to 60°C (140°F) terminates the charge.

Chargers relying on temperature inflict harmful overcharges when a fully charged battery is repeatedly removed and reinserted. This is the case with chargers in vehicles and desktop stations where a two-way radio is being detached with each use. Reconnection initiates a new charge cycle that requires reheating of the battery.

Li ion systems have an advantage in that voltage governs state-of-charge. Reinserting a fully charged Li-ion battery immediately pushes the voltage to the full-charge threshold, the current drops and the charger turns off shortly without needing to create a temperature signature.

Full-charge Detection by Voltage Signature

Advanced chargers terminate charge when a defined voltage signature occurs. This provides a more precise full-charge detection of nickel-based batteries than temperature-based methods. The charger looks for a voltage drop that occurs when the battery has reached full charge. This method is called negative delta V (NDV).

NDV is the recommended full-charge detection method for chargers applying a charge rate of 0.3C and higher. It offers a quick response time and works well with a partially or fully charged battery. When inserting a fully charged battery, the terminal voltage rises quickly and then drops sharply to trigger the ready state. The charge lasts only a few minutes and the cells remain cool. NiCd chargers with NDV detection typically respond to a voltage drop of 5mV per cell.

To achieve a reliable voltage signature, the charge rate must be 0.5C and higher. Slower charging produces a less defined voltage drop, especially if the cells are mismatched in which case each cell reaches full charge at a different time point. To assure reliable full-charge detection, most NDV chargers also use a voltage plateau detector that terminates the charge when the voltage remains in a steady state for a given time. These chargers also include delta temperature, absolute temperature and a time-out timer.

Are you interested in learning more about exit sign requirements osha? Contact us today to secure an expert consultation!

Fast charging improves the charge efficiency. At 1C charge rate, the efficiency of a standard NiCd is 91 percent and the charge time is about an hour (66 minutes at 91 percent). On a slow charger, the efficiency drops to 71 percent, prolonging the charge time to about 14 hours at 0.1C.

During the first 70 percent of charge, the efficiency of a NiCd is close to 100 percent. The battery absorbs almost all energy and the pack remains cool. NiCd batteries designed for fast charging can be charged with currents that are several times the C-rating without extensive heat buildup. In fact, NiCd is the only battery that can be ultra-fast charged with minimal stress. Cells made for ultra-fast charging can be charged to 70 percent in minutes.

Figure 1 shows the relationship of cell voltage, pressure and temperature of a charging NiCd. Everything goes well up to about 70 percent charge, when charge efficiency drops. The cells begin to generate gases, the pressure rises and the temperature increases rapidly. To reduce battery stress, some chargers lower the charge rate past the 70 percent mark.

Figure 1: Charge characteristics of a NiCd cell [1]

Charge efficiency is high up to 70% SoC* and then charge acceptances drops. NiMH is similar to NiCd. Charge efficiency measures the battery&#;s ability to accept charge and has similarities with coulombic efficiency.

* SoC refers to relative state-of-charge (RSoC) reflecting the actual energy a battery can store. Full charge will show 100% even if the capacity has faded. (See BU-105: Battery Definition and what they mean)

Ultra-high-capacity NiCd batteries tend to heat up more than standard NiCds when charging at 1C and higher and this is partly due to increased internal resistance. Applying a high current at the initial charge and then tapering off to a lower rate as the charge acceptance decreases is a recommended fast charge method for these more fragile batteries. (See BU-208: Cycle Performance)

Interspersing discharge pulses between charge pulses is known to improve charge acceptance of nickel-based batteries. Commonly referred to as a &#;burp&#; or &#;reverse load&#; charge, this method assists in the recombination of gases generated during charge. The result is a cooler and more effective charge than with conventional DC chargers. The method is also said to reduce the &#;memory&#; effect as the battery is being exercised with pulses. (See BU-807: How to Restore Nickel-based Batteries) While pulse charging may be valuable for NiCd and NiMH batteries, this method does not apply to lead- and lithium-based systems. These batteries work best with a pure DC voltage.

After full charge, the NiCd battery receives a trickle charge of 0.05&#;0.1C to compensate for self-discharge. To reduce possible overcharge, charger designers aim for the lowest possible trickle charge current. In spite of this, it is best not to leave nickel-based batteries in a charger for more than a few days. Remove them and recharge before use.

Charging Flooded Nickel-cadmium Batteries

Flooded NiCd is charged with a constant current to about 1.55V/cell. The current is then reduced to 0.1C and the charge continues until 1.55V/cell is reached again. At this point, a trickle charge is applied and the voltage is allowed to float freely. Higher charge voltages are possible but this generates excess gas and causes rapid water depletion. NDV is not applicable as the flooded NiCd does not absorb gases because it is not under pressure.

References

[1] Source: Cadex

Nicds and Nimh cells are the same voltage ?~ 1.20 ?and charge at the same voltage so ??essentially? ? you can charge them both in the same charger. I charge Nimh?s in my old Nicd charger all the time, but they take about 2x as long to charge because Nimh?s have a much higher rated capacity.

7hrs ? Nicd ..600mah ?(milli-amp-hours)
14hrs ? Nimh mah

A lot of the newer Nimh chargers charge at a faster rate because of the higher capacity of Nimh?s (about 2x) - versus Nicd?s which are normally 600-700 - but there are high-capacity Nicd?s rated mah. My old Nicd charger charges AA at ?150ma ? 2.8volts (2 cells) and has no autoshutoff so it?s basically a trickle/slow charger. .

So Nicd?s will charge in a Nimh charger and their charging time will be about half that of Nimh?s (depending on their capacity).

Almost all chargers that charge higher than 0.1C (10% Capacity) usually have an auto sensor that either steps down charging to trickle mode when finished or shuts off. As Jamarno pointed out, if the charger has autoshutoff sensor -it might not sense the voltage correctly so it might shut off too soon or go too long??? Quick chargers also have a heat sensor in case the batteries get too hot. Rayovac has a quick charger that will charge Nimh?s in about an hour ? charge rate = 1.0C.

Both Nicd and Nimh have built in overcharge protection in their chemistry - but it only goes so far. The battery info sites state you can essentially overcharge Nimh?s at ? 0.1C ? (slow charging/trickle)? for a year without doing damage to them - although you will shorten the life somewhat. That?s why they?re rated 500- charges. Best to take them out when charged.

If your Nimh charger charges mah batteries in just over 3 hrs it would probably charge those Nicd?s in 1.5hrs or less. This is a very fast charge rate (much more than 0.1C). I wouldn?t want to leave the Nicd?s in much longer than that.

My brother has an auto charger that charges both Nicd and Nimh. Although there is a switch to set the type of battery I believe it?s for the ?auto-sensor? (and not the charging rate). The voltage peaks for Nicd/Nimh?s when they reach full charge react slightly different so the sensor has to change it?s settings.Nicds and Nimh cells are the same voltage ?~ 1.20 ?and charge at the same voltage so ??essentially? ? you can charge them both in the same charger. I charge Nimh?s in my old Nicd charger all the time, but they take about 2x as long to charge because Nimh?s have a much higher rated capacity.7hrs ? Nicd ..600mah ?(milli-amp-hours)14hrs ? Nimh mahA lot of the newer Nimh chargers charge at a faster rate because of the higher capacity of Nimh?s (about 2x) - versus Nicd?s which are normally 600-700 - but there are high-capacity Nicd?s rated mah. My old Nicd charger charges AA at ?150ma ? 2.8volts (2 cells) and has no autoshutoff so it?s basically a trickle/slow charger. .So Nicd?s will charge in a Nimh charger and their charging time will be about half that of Nimh?s (depending on their capacity).Almost all chargers that charge higher than 0.1C (10% Capacity) usually have an auto sensor that either steps down charging to trickle mode when finished or shuts off. As Jamarno pointed out, if the charger has autoshutoff sensor -it might not sense the voltage correctly so it might shut off too soon or go too long??? Quick chargers also have a heat sensor in case the batteries get too hot. Rayovac has a quick charger that will charge Nimh?s in about an hour ? charge rate = 1.0C.Both Nicd and Nimh have built in overcharge protection in their chemistry - but it only goes so far. The battery info sites state you can essentially overcharge Nimh?s at ? 0.1C ? (slow charging/trickle)? for a year without doing damage to them - although you will shorten the life somewhat. That?s why they?re rated 500- charges. Best to take them out when charged.If your Nimh charger charges mah batteries in just over 3 hrs it would probably charge those Nicd?s in 1.5hrs or less. This is a very fast charge rate (much more than 0.1C). I wouldn?t want to leave the Nicd?s in much longer than that. energizer? ? has some real cool tech information about Nimhs and Nicds.

Contact us to discuss your requirements of 3.2 volt rechargeable batteries. Our experienced sales team can help you identify the options that best suit your needs.