The Battery Page

Ni-Cads have been the standby in battery technology since the inception of radio control 30 years ago. In today's market they are very cheap and are still the batteries of choice for all RC radio manufacturers. The ni-cad is still the best all around choice for durability, ease of use and longevity. They charge fast and discharge readily, making them ideal for electric's or anywhere high current draw is desired. Definitely the best bang for your buck.

If taken care of properly, the average ni-cad cell should last at least 3 years. You can determine the condition of your pack by cycling it. Cycling a battery pack is the only sure way to tell if you have a pack that is about to fail.

A good battery cycler will drain the pack at a specific discharge rate and until it reaches 1.1 volts per cell and then switch it back to charge for the proper time.

Note: Charging time is usually 10 to 16 hours. The typical charge rate for ni-cads is referred to as C10 which is 1/10 of the rated capacity of the battery. Example: A 1200 mah pack needs a 120 mah charger to charge it to full capacity.

The tell-tale signs of a pack that is about to fail is a loss of capacity or a high self discharge. If you have a 600mah battery and it consistently cycles to only 450mah there is a problem. Either a cell has failed, shorted or the entire pack is just old and losing capacity. The classic symptoms of a shorted cell is if the battery pack cycles fine after first being charged but if left for a week and then cycled, loses capacity.

A shorted cell will draw current from the rest of the pack and can have disastrous consequences if not diagnosed properly. Another word of caution. It's generally not a good idea to remove the bad cell from a pack and replace only that one, especially if the pack is over a year old. I'm of the opinion that, for the cost of a battery pack, I would rather not jeopardize my pride and joy because I was trying to save $30.00 on a battery!!

Battery memory is another issue I am questioned on all the time. When I say memory this refers to the battery memorizing a certain discharge point and developing the inability to produce it's rated capacity. What causes this phenomenon is constantly discharging the pack to a certain point, usually far less than it's rating. Example: making three flights every time you go out, draining 25% of the capacity of the pack and then recharging it again. If you perform this cycle many times the battery will eventually memorize this and start providing dramatically less capacity than it is rated at.

How you cure the memory problem is quite simple. Battery cycling 2 or 3 times a season virtually eliminates the problem. As long as the cells are deep cycled enough during this process, you shouldn't have anything to worry about even if you continue to only use 25% of the packs rated capacity every time you go flying.

One of the new kids on the block so to speak is NiMH batteries. When they were first released they looked like a shoe-in to depose the workhorse nicad. It hasn't quite worked out that way for a number of reasons.

On the surface, this new battery technology seemed to have all the positives of a nicad but with added benefits. The upside was the battery's capacity relative to its size. You can currently purchase NiMH batteries that have a 1400mah rating but are the same size as a 500mah or 600mah nicad pack. The weight savings are obvious.

The downside is that these batteries are more expensive relative to nicads and are sensitive to charge and discharge rates. Most can't be aggressively fast-charged like nicads and they do not like to be fast discharged either (more than 30 amps), making them less than ideal for larger electric aircraft or other high current applications. The other negative is their discharge rate when stored. Nicads can be expected to lose about 1 to 2 percent of their charge per day when left dormant. NiMH batteries are closer to 5 or 6 percent, meaning you will have a fully discharged pack in a week of no charging!

Update: 10/7/2003

As with all battery technology NIMH cells have evolved rapidly. The current generation of 2600 and 3300 sub-C size cells have become the battery of choice in the larger aircraft. High current draw (30 amps and more) and 8 amp charge rates seem to have very little adverse effect on these newer cells. They seem to be underrated also. These cells routinely cycle out to 2800-2900 MAH. An all-around excellent battery. For better performance in electrics fly them warm off the charger!

This battery is the latest on the market and seems to be the future of battery technology at the moment. The cells are very expensive compared to nicads and require special chargers, but they offer huge capacity and are 1/3 the weight of an equivalent nicad or NIMH cell of the same size!

Each cell is 3.7 volts, meaning it only takes two cells to make up an equivalent receiver pack. However, since the voltage is high, you must use a regulator to keep the voltage at 5.2v. They have no problems with memory and can provide very high current supply on demand if it is not sustained for long periods of time.

Unlike the NiMH batteries, the lithium's do not self-discharge and will retain almost 90% of their original charge even after one year. I should note that very high discharge rates can be achieved by connecting the packs in parallel with one another. This makes it possible to pull 40 or 50 amps from some of the larger cells. Wire the cells in series to get the voltage required and place them in parallel to get the MAH capacity and current load requirement needed.

Only one warning is required with these cells. If charged incorrectly they may catch fire. I personally feel the danger aspect of these cells is getting blown out of proportion. The real disasters occur when the wrong cell count is programmed into the charger and they are forced-charged at a higher cell count than they should be.

Most chargers will not allow you to do this without some sort of warning. The only other area to be sensitive to is if the cells have an impact (crash). If the membrane becomes ruptured it may ignite. In that case, place the cells aside for 15 minutes after a direct hit and if the cells do not swell, everything is OK.