How to take care of your NI-CD Batteries !

Solving Battery Problems

If a battery pack is giving you a problem, replacement of the whole pack is not always necessary.    Most of the time, only one or two cells are weak and if they are replaced the capacity of the whole pack will come up to par.  In order to determine which cell is defective, it is best to discharge the pack down to 1.1V per cell and with the load still on the pack, use a voltmeter to test each individual cell.  If one or more of the cells are lower in voltage than the rest, it is the weak one and needs to be replaced.   If a cell has completely opened up or shorted, it will show no voltage at all.  

Replacement batteries have to be of the same capacity and type as the rest of the pack and, if possible, made by the same manufacturer so the cells are matched as close as possible. DO NOT USE ACID CORE SOLDER !  If a pack is completely dead, make sure the charger is operating properly with a voltmeter as described earlier.

What is Cycling? "Cycling a rechargeable battery is to discharge the battery to a pre-determined voltage (at a fixed discharge current) and recording the time it takes to do so."  It is not desirable to discharge the battery below this voltage because of the possibility of causing a cell reversal within a pack (due to unequal discharge).  In the case of ni-cd batteries, this voltage is 1.1V per cell; i.e. 4.4V for a four cell receiver pack, 8.8V for an eight cell transmitter pack.

In order to give valid readings, the discharge current should be such that the batteries are cycled in about one to six hours.

Why Cycle?  The only sure way to test the overall goodness of a ni-cd battery is to cycle it.  No other test exists that will tell whether or not a ni-cd is deteriorating and losing capacity.  Since we know the amount of time it takes to discharge the pack, we then know the amount of safe flying time that exists.   since we also know the amount of current that was being removed from the pack, we can calculate the actual operational capacity and compare it to the rated capacity of the pack.  For example, if a pack that is rated 500mah is only delivering 300mah in a cycle, than this is an indication that the pack is not functioning properly and needs attention before real problems develop - and you wind up crashing an airplane. 

Also, it is healthy to periodically cycle the batteries to keep their capacity at a maximum.  This clears any internal shorts that may be developing and erases any "memory" effect; that is if a ni-cd is repeatedly asked to deliver a certain amount of capacity, it may not supply much more if asked to do so.

How Do You Cycle?

There are two ways to cycle ni-cds: "manually" and "automatically".  To manually cycle the batteries, you need to put a resistive load on them and monitor three parameters: Current, Voltage and Time.  It is suggested that they be discharge at 200 to 300mah: a 47 ohm 3 watt resistor will provide a suitable load for the 9.6V transmitter and a 22 ohm 2 watt is about right for the 4.8V receiver pack.  You will need to devise a set up which will allow you to apply this resistor across the pack and to measure when the voltage reaches 1.1V per cell.   Be sure to remove the resistor at this point! (You don't want to completely discharge the batteries.) If you record the time it take to discharge, you will then have all the elements of the cycle.

Although awkward, the abo ve procedure will do the job.  If you want more convenience and accuracy, commercially-made battery cyclers are available that will do the job for you and record the time of the cycle automatically.  See Ace Radio's Digipace 3, which is a lab quality instrument that accurately and dependably cycles both receiver and transmitter packs.

One other advantage to using a battery cycleer is that it can detect if a battery has a vibration problem.  It will prematurely switch out of the discharge mode if the defective pack is subjected to vibration while being cycled. 

Using an E.S.V.  In the field, it is impossible to monitor the condition of you batteries by cycling them.  The next best thing is to usa an "Extended Scale Voltmeter"(ESV) which puts a load on the batteries and read the voltage.   Because it has an "expanded scale", it only looks at a certain range of voatage (not the whole range from zero up) so the measurement can be much more accurate.   This type device can tell you whether a cell has failed or the batteries are reaching a discharged state.  It can't tell yo how much charge you have left in the pack but it can indicate that you have enough for at least one more flight.  Such a piece of equipment is valuable and should be kept in the flight box and used regularly. See Ace Radio's "Super SmarTest" - It is a highly accurate device with four voltage ranges and two load discharge rates to fill all your needs.

Care of you NI-CDS

Besides cycling, there are some other good things you can do to insure long and dependable life out of your batteries.  Several factors cause damage to ni-cds and should be avoided.   One of the big killers is temperature extremes, particularly heat.

* Brief periodic excursions to high temperatures will not cause problems, but repeated long exposure will decrease life.  Very low temperatures can cause similar problems.  Avoid exposure by keeping the plane and transmitter in the shade - avoid leaving the system in a hor (or cold) car trunk all day.  Charge and store the batteries in a cool place.

* Store your batteries when they're not being used in a full charged state.  If they are fully charged, they are less likely to acquire a permanent internal shor while sitting around.

* Avoid completely discharging your pack by accidentally leaving the system on and running them down.  You run the risk of cell reversal which will affect the life of the cell.

* By the same token, avoid, overcharging the batteries by leaving them on too long or charging at too high a rate.  This could cause them to vent and lose electrolyte which affects life.

* Subjecting ni-cds to high vibration can cause physical damage to the cell internally and should be avoided by isolating the batteries with foam rubber.

* Physically inspect the battery pack's condition including the wiring and joints.  Look for bad welds and/or dented cells.  If a white powder substance appears on the positive terminals, it indicates that venting has occurred.   Although a ni-cd can stand some venting repeated loss of electrolyte will decrease capacity and lead to battery failure.  Keep you eye on the pack and double check your battery treatment procedures to prevent more problems, including  corrosion or fatigued joints.

Remember, good care and monitoring of your batteries can save disaster.   If you consider all the time and money spent putting a plane together, a small investment in your batteries is cheap insurance.