NiCd Batteries

The normal charge rate for NiCds is C/10 or the capacity of the NiCd divided by 10. For example, a 600mAh pack should be charged at 600/10 or 60ma. This is known as the overnight rate. Although, ideally, a pack should be fully charged in 10 hours, due to inefficiency, it will probably take between 12 and 15 hours.

After being charged overnight, the battery should either be removed from the charger or the charge rate should be reduced to C/100 (the capacity divided by 100). This is known as the trickle rate. The 600mAh pack in our example would have a trickle rate of approximately 6ma. The battery may remain on the trickle rate indefinitely. Keeping your system’s batteries on trickle charge is a great idea as it will ensure that your batteries are fully charged when you go to the field. NiCds, just sitting around will probably loose 1% of their charge each day.

Most NiCds may also be charged at a higher rate such as a rapid charge of C (charge rate equal to capacity) or a quick charge of 4C or four times the capacity. This is normal practice with the packs used in powering electric models. Some cells are better at accepting a fast charge than others and these are usually denoted by being an “R” type cell or “SCR”. In fast charging NiCds, however, one has to be very careful to ensure that they do not get overcharged. Applying these high charge currents to a battery that is fully charged can at the least ruin the battery and at worst make the battery explode.

There are two basic types of NiCd fast chargers. The first is a timed charger where the charge rate is applied by turning on a timer. Timers are usually for 15 minutes although it will probably take between 20 and 25 minutes to fully charge the pack. Although timed chargers are not the best choice, they are often the most economical. Most timed chargers will come with a discharge circuit. The discharger is used to fully discharge the pack before charging so the current state of charge for the pack is known and to reduce the chance of overcharging it.

A second type of fast charger is the peak detection charger which can automatically charge your battery packs for you without the need for a timer. A circuit monitors the voltage of your pack during charge. As a NiCd charges, the voltage will increase at a slow rate. However, once the battery is fully charged, the voltage will “peak” and actually drop back slightly. The circuitry detects this drop and reduces the charge rate to trickle. You can safely charge your batteries with this type of charger and there is no need for initially discharging them.

Even though NiCds can be fast charged, it is important to slow charge your batteries at the overnight rate periodically, or about every 5 charges. This helps stabilize the cells to retain their full capacity and will lengthen their life.

Another important characteristic of NiCds that should be noted is their self discharge. Self discharging is the energy that the cell loses just sitting on the shelf with no load. Typically for NiCds, they will lose 10 to 15% of their capacity within 24 hours of coming off charge then reducing further by about 10 to 15% per month thereafter. Self-discharge rates increase with increased ambient temperature. It is a good idea to charge the batteries just before you go flying rather than charging them as soon as you get home and then not again before flying a few weeks later!

NiCd batteries are not environmentally friendly. They contain Cadmium which is dangerous to the environment. These batteries must be recycled and cannot be simply discarded in the waste.

NiMH Batteries

Nickel Metal Hydride (NiMH) batteries are very similar to NiCds, and have been accepted with open arms to
the radio control community. A NiMH cell has the same nominal voltage of 1.2V as a NiCd so they can form packs in the same way. Where the NiMH cell really shines and has the advantage over the NiCd is in its capacity. For the same size package, the NiMH cell carries double the energy capacity. For example a typical ‘AA’ cell NiCd has a capacity of 700mAh whereas the NiMH ‘AA’ has about 1400mAh of capacity. The energy to weight ratio for NiMH is much better—almost double.

There are tradeoffs, however. The NiMH cell has a higher internal resistance (almost double that of the NiCd) and as a result is limited to the speed at which energy can be charged or discharged with the cell—lower charge rates and lower discharge rates. This is particularly true of the smaller NiMH cells such as ‘AAA’ and ‘AA’, however, some of the larger capacity cells (currently in the 3000 to 3300 mAH range) are designed for higher current rates. In some applications, such as powering electric model aircraft, the higher internal resistance can be noticeable on the power output when compared to NiCds. To help offset the drop, an extra cell is often added to the pack. For instance, where a 6-cell 7.2V NiCd pack is used, select a 7-cell 8.4V NiMH pack. Internal resistance increases with drop in temperature and decreases with increased temperature.

Another disadvantage to the NiMH is the useable working life. “Lifespan” in terms of age for NiCds and NiMH cells is similar, around 4 or 5 years, however, NiCds are capable of a greater number of usage cycles. Typically NiCds are capable of 1000 or more discharge/charge cycles whereas NiMH cells are limited to around 300 to 400. This may or may not be limiting for the user, depending upon one’s application.

Self-discharge also applies to NiMH cells. In fact, one may experience a higher self-discharge rate with NiMHs than with NiCds.

Although NiMH cells have been touted as not having “memory” like NiCds, this is not completely true. The nickel component in these cells is partially responsible for the memory effect. In a NiCd, both the nickel and the cadmium contribute to the memory while in the NiMH cell, the nickel is the only component contributing to the effect. As a result, NiCds show the effects of memory considerably more than NiMH. Cyclers can still play an important role in the exercising of NiMH to keep them delivering optimum capacity and output.

Charging characteristics for NiMH is slightly different than NiCd, particularly around the full charge point. Peak chargers used should be designed for NiMH cells and not just for NiCds to assure they turn off after peaking.

One other advantage for NiMH cells that should not be overlooked is that they are more environmentally friendly than NiCds—NiCds containing cadmium which is a chemical not desirable to have loose in the environment. NiMH may be simply discarded in the waste when they have reached the end of their life.
Li-Po Batteries