I was wondering just how often you should change your batteries in both your airplane, and in the transmitter? I have a square 4.8V in my plane (a trainer), and a 7.2V in my transmitter (Conquest Futaba 4 channel). Also, when it says that "batteries should be checked under load"- what does this mean?
Thanks for all the help. Airzinho

You should change your batteries when the no longer hold about 70% of there rated capacity
It depends on how well you take care of them.

Checking under load is accomplished with a tool called an ESV (expanded scale volmeter) it test the capacity while at the same time drawing current (load) this is what is meant by checking under load. my tool test 4.8v 600mah nicads with a simulated 100mah load

Also go over to the battery forum and do some reading

You should be able to get 4 or 5 flights out of a freshly charged receiver pack, many more from the transmitter.

Crashem was right on the money on the ESV meter. You can buy one at Tower Hobbies for about $15 to $20, and it's worth every dime. Check you receiver pack between each flight to be safe.

You should eventually buy a charger that is capable of "cycling" the packs. That means discharging, then recharging, while keeping track of the bpacks capacity in milli-amps. A 600 mAh pack should be tossed when the capacity drops below about 580 (in my opinion), because the danger of a cell failing grows higher.
Dennis-

The modern NiCd can be good for up to 12 years of use. (that is an exceptionally good pack with excelent maintenance...) Average will be closer to 4 years service.

To maximize battery life, you need to ensure the pack is discharged and recharged at least monthly. This means that the people who don't fly for 6 months each year need to cycle the batteries durring the non-flying season. A NiCd which is not cycled for 6 months will often go bad.

If you have a cycler and can test the capacity of a battery when its new.. write the tested value and date on the pack with permanent marker. If a cycle test shows it has lost 15% of capacity don't trust the battery. There's about 2 charges difference between 15% loss and 30% loss... then another charge or two and its dead.
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How many flights on a charge? :::: What plane? What engine? What type of flying are you doing?

You can get over 2 1/2 hours of flying of a 2 meter glider using a 500 maH pack if not doing much aerobatics. You can get about 45 min of average .40 size trainer flying from a 600 maH pack. Under adverse conditions, you can get as little as 7 min flying out of a 1200 maH pack flying a 1.20 powered Pattern Competiton aircraft. Depending on how I fly it, my .90 powered Fokker Dr1 can get from 10 min to 90 min per charge from the 700 mah pack in it.

You REALLY need to have a meter to test the pack at the field and see if its safe to take off for the second flight, no matter what you fly.

Btw, when testing your batteries, you want to use a meter bought from a hobby source. If you get a voltmeter from Radio Shack, you'll get a falsely high reading. The meters meant for our use include a small resister to draw some current from the battery, and test the battery while this current is flowing. That's what is meant by testing "under load". It means testing while the battery is actually providing some current. You can also (and I have) get a resister from Radio Shack, and wire it up to a female servo lead and plug that in to a normal voltmeter to do the same thing. But it's cheaper just to buy the ones from the hobby store.

This is the meter i use. Its a noload/load volt
meter. It will test either way.

Inspect your batteries also. I had some from around 96-97 or so? and one pack had some sort of crystalization on the end of it. I bought two new packs. Costs around 30 total from tower. Not to bad when comparing to a new tx and flight pack.

If you are good at soldering.. you can make your own replacement packs.

You can salvage plugs from old computers. The LED, speaker and other similar plugs will fit the pins of a Futaba, HiTech or JR RX. The wire is usually heavier guage than the original battery pack's so it is an upgrade. For JR you have to slightly file the plug corner to fit. For Futaba.. its just like using a universal plug... watch the color code.

You can buy Solder tab NiCds for about $1.25 a cell (AA size 700 maH) or you can get 1800 maH AA NiMH cells at $9 for a 4-pack! The NiMH cells need the contact points sanded/filed a bit to get a god solder joint. (plan on destroying a cell or two if you haven't soldered batteries before)

There is no good reason to pay $32 to $45 for a Futaba replacement TX pack... $18 and a little work gets you a pack with 3 times the capacity.

Oh my god, I'm way new, building my first ARF. Ive been buying servo extensions and such at the LHS for big bucks wanting to make my own. I've been a computer geek for 15 years, and kept looking at the connnections of planes knowing I've seen that before.

I'm all set now. Just go root around in some of my old computer boxes in storage and I'll have a bunch of good plugs in no time... - Joe

Receiver Battery packs seem to last me about 2 years before they are no longer needed or wanted (a lot of fast charging). TX packs last the life of the device.....8U (sold) 9C (Now have)