Most of the batteries we use in our hobby today are the rechargeable type.There are several kinds of rechargeable batteries and these include NiCads(Nickel Cadmium), lead - acid, sealed lead-acid, and gel-cell, among others.NiCads are used to run our radio systems as well as power our model cars,boats, and planes. Generally they are wired together in packs of four ormore cells, depending on the application. The other types of batteriesare usually 6 or 12 volt and are used to power flight boxes and large scaleboats.
NiCad Batteries
NiCad batteries are used in just about every radio system that comes withrechargeable batteries, and they power just about every electric car, boat,and plane in the hobby. A NiCad cell, regardless of capacity has a nominalvoltage of 1.2V. When fully charged it will have slightly higher voltageand it is considered to be fully discharged when it is down to 1.1V.
The capacity of NiCads is measured in milli-amp hours (M.A.H.), the averagecurrent drawn times the time in hours. A NiCad cell of 1000 m.a. capacitycould supply 1000 m.a. of current for one hour. It could also supply 2000m.a. of current for a half hour or 500 m.a. of current for 2 hours. NiCadscan be found with capacities ranging from 50 m.a. to 4400 m.a. in differentsize and shape packages.
Most radio systems have battery packs made up of AA size cells, havinga capacity of 600 m.a. The airborne system battery pack will generallyhave 4 cells wired in series producing 4.8 Volts and the transmitter willhave 8 cells producing 9.6 Volts nominal. Only the voltages add when thecells are wired in series; the capacity remains 600 m.a.
Battery packs to power large models are usually made up of the Sub-C sizecells. At one time These were rated at 1200 m.a. in capacity, but withprogression in technology, Sub-C's are commonly found with 1400, 1500 andeven 1700 m.a. capacities. These packs are usually in either 6-cell (7.2V)or 7-cell (8.4V) configurations but some models, especially large airplanesmay have 28 or more cells wired in series to form a pack.
If you are wiring your own pack, DO NOT wire NiCad cells in parallel toget higher capacity. NiCads have an extremely low internal resistance comparedto ordinary, non- rechargeable cells. When they are wired in parallel,if one cell is at a different charge level than the other, thus producinga slightly different voltage, a great deal of current can flow from thehigher voltage cell to the lower voltage one. This could cause a fire byheating the wire joining the cells or from the heating of the cells themselves.If you need higher capacity, go to a larger cell.
When operating a radio control system it is very important to know thecondition of the batteries powering it. The life of your model, and thesafety of those around it, depends on this. Always be certain your transmitterand receiver battery packs are fully charged before you operate your model.Your transmitter will usually have a meter indicating the current stateof your transmitter battery it can be easily monitored during operation.The only way you can determine the state of your receiver battery, is toplug an expanded scale voltmeter into your pack and measure the voltageunder load.( which is generally done through the charging plug ) doingthis after each flight during a flying session is a good habit to get .
NiCad batteries discharge differently than alkaline or other non-rechargeabletypes. When the cell first comes off charge it will usually show a fairlyhigh voltage (1.4V possibly). This will drop off quickly as the cell isdischarged until close to the nominal voltage of 1.2V. The voltage willthen drop off slowly throughout the bulk of the discharge. However, oncethe cell is near fully discharged, it will drop off very quickly again.
NiCad batteries have one bad characteristic in that they can develop amemory. If a NiCad battery is repeatedly fully charged and then used anamount that is less than full capacity (let's say you charge and regularlyhave three or four flights in a session), after a period of time, it maynot be able to deliver any more than the capacity frequently used. Thisis called NiCad memory.
To avoid NiCad memory, it is IMPORTANT to cycle the batteries. Cyclingis where the battery is fully discharged under controlled conditions, asdescribed below, and then recharged. By fully discharging the pack in thisway every so often , ( about once a month ) the cells in the pack willnot develop memory and will remain at maximum possible capacity.
It is also a good idea to measure the capacity of your battery packs everyso often. This will tell you how long you can safely operate your modelin one session. You will also discover when your batteries need to be replacedwithout destroying a good model in the process. ( it is a good idea toset a schedule for this that fits your flying habits. )
It is possible to check the capacity of the battery while cycling. If youdischarge your pack at a constant, known rate, and measure the pack voltageat various time intervals during discharge, you can determine the capacityby multiplying the discharge current rate by the time it takes to fullydischarge. Your pack is considered discharged when it reaches a value of1.1 volts per cell. For example, if you are discharging a 4-cell receiverpack, it would be fully discharged at 4.4 volts ( 4 cells x 1.1 volts/cell). Do not discharge a pack below this level or cell reversal could result.
There are pieces of electronic equipment available which can do this automaticallyfor you and you will find these in your favorite hobby catalog. It is definitelyworth investing in one of these units as one crash due to poor batterymaintenance will usually cost you more than a cycler, not to mention thehours you have put into building your model.
The normal charge rate for NiCads is C/10 or the capacity of the NiCaddivided by 10. For example, a 600 m.a. pack should be charged at 600/10or 60ma. This is known as the overnight rate. Although, ideally, a packshould be fully charged in 10 hours, due to inefficiency, it will probablytake between 12 and 15 hours.
After being charged overnight, the battery should either be removed fromthe charger or the charge rate should be reduced to C/100 ( the capacitydivided by 100 ). This is known as the trickle rate. The pack in our examplewould have a trickle rate of approximately 6 ma. The battery may remainon the trickle rate indefinitely. Keeping your system's batteries on tricklecharge is a great idea as it will ensure that your batteries are fullycharged when you go to the field. NiCads, just sitting around will probablyloose 1% of their charge each day.
Most NiCads may also be charged at a higher rate such as a rapid chargeof C ( charge rate equal to capacity ) or a quick charge of 10C or tentimes the capacity. This is normal practice with the packs used in poweringelectric models. Some cells are better at accepting a fast charge thanothers and these are usually denoted by being an "R" type cell or "SCR".In fast charging NiCads, however, you have to be very careful to make surethat they do not get overcharged. Applying these high charge currents toa battery that is fully charged can at the least ruin the battery and atworst make the battery explode. For this reason, fast chargers are equippedwith a discharge circuit and a timer.
The discharger is used to fully discharge the pack before charging so thecurrent state of charge for the pack is known. The charge rate is appliedby turning on the timer. Timers are usually for 15 minutes although itwill probably take between 20 and 25 minutes to fully charge the pack.
A second type of fast charger is the peak detection charger which can automaticallycharge your battery packs for you without the need for a timer. A circuitmonitors the voltage of your pack during charge. As a NiCad charges thevoltage will increase at a slow rate. However, once the battery is fullycharged, the voltage will actually drop back slightly. The circuitry detectsthis drop and reduces the charge rate to trickle. You can safely chargeyour batteries with this type of charger and there is no need for initiallydischarging them.
Even though NiCads can be fast charged, it is IMPORTANT to slow chargeyour batteries at the overnight rate periodically, or about every 5 charges.This helps stabilize the cells to retain their full capacity and will lengthentheir life.( on an average you batteries should be changed every two years.)
Other types of batteries such as lead-acid, sealed and non- sealed, andgel cells should also be charged with care. Do not charge your field boxbattery with an automotive car charger or with any other "fast charger".These batteries can boil dry and be damaged by high rates of charge andovercharge. ( and possibly blow up ) Use a charger meant for the job atthe overnight rate of C/10.
