I just purchased a dynmite prophet plus AC/DC 4-7 cell prediction charger for ni-cd & ni-mh battery packs. I have a few questions:

1) how low is to low to fly, voltage (futaba 7c tx & rx) 4v battery in rx

2) how long should charging take (at the field, 12v car battery) the rx to charge @ 1, 2 or 4 amp rate? understanding it depends on how low the battery is before charging.

3) what amp rate is better for the life of my battery?

4) with peak prediction circuitry, should I charge battery pack with the prophet charger at home or use the futaba home charger?

5) the charger didn't come with an adapter for the tx, I have ordered it, but I heard from LHS owner that tx charging didn't do as well with the charger, is this true?, should I not charge the tx with anything but the futaba wall charger?

6) what volt reading should full charge on the rx and tx be?

7) what is the difference in running a 4v battery vs a 6v battery? can you use either with the futaba rx flight pack?


I know there are a lot of questions for one post, please answer the ones you feel comfortable answering.



thanks in advance!!!

1) You can fly a 4-cell receiver pack until it is down to 4.4 V under 300 mA Load
2) Charging will only take about 20 minutes at 1 amp charge rate if the pack needs 300 mAh. Will take one hour if it needs 1000 mAh, etc.
3) The absolute best rate to charge is one tenth of the capacity for 14 hours. But charging at the "1c" rate with a peak detector is OK too. Charging at 2C is marginal. I generally don't charge above 2C because it may shorten the life of the pack.
4) Use the home charger overnight for routine charging when you remember. Save the filed charger for the field or for when you forget to charge overnight.
5) The field charger should work OK with the transmitter pack, as well as the receiver
6) A full charge 4-cell pack (nominal 4.8 V) may read as much as 5.5 or higher for a few minutes in use. A 8-cell transmitter pack may read over 10.5 Volts for a while.
7) You can use either a 4-cell or 5-cell pack in your receiver. The 5-cell will give more speed and power to your servos, but will not last as long. Because of the increased voltage, the system is actually using more amps per minute, so you can fly 20% fewer minutes on a 5-cell pack.

thank you very much for that information.

i have one other question.

i flew yesterday, and my rx battery reads 5.07v now, 29 hrs after it was last turn on. I won't fly again until maybe thursday or sunday. should this battery be charged (the night before flying) a full 12 to 14 hrs?

Yes, I generally charge again. But if when you flew yesterday you only flew one flight (say 15 minutes), then you can safely fly in a few days without charging. Some packs self-discharge as much as 1-3% per day. So after 10 days they might be discharged by 10 - 30%. (I've never experienced this great an amount, but I think I read this somewhere). My mind is never reliable enough to remember whether a pack was partially discharged, so when I know I might go flying, I charge overnight the night before. If I cannot fly and the next weekend rolls around, I charge again. repeated overnight charging (using the C/10 charger that comes with the system), does not seem to hurt the packs. I believe you could charge them once every weekend overnight and they would still work for many years. I have accidentally left packs on for a couple weeks and no apparent harm. All of this is with NiCd or NiMh packs charged at a low rate (50 mA).

I ALWAYS recharge when my receiver pack gets to 4.8 volts. Letting it fall to 4.4 is really pushing your luck.

That charger may not perform well on a transmitter pack because most transmitter packs are 8 cells (9.6 volts) and your charger is rated for 7 cells.

Dennis-

Using a wall timer set for one hour a day is a good way to keep your batteries maintained at full charge. These are pretty cheap and you can charge a bunch of airplanes and field equipment the same way, just hook your timer up to an outlet strip, plug in all your chargers, set it for an hour a day and the next time you want to go flying it is ready to go.

When you get back you can turn the timer on for 15 hours(leave it set for 1 hour-just turn it on and set the dial so it turns off in 15 hours or so) ,this does a full recharge,and then the next day it will start charging for 1 hour again.

Remember to fully charge before starting the 1 hour a day charge.

I will put up a small argument on the 4.4V thing. Reciever dropout (loss of signal from the transmitter) becomes a real factor at 4.6V. At 4.8V depending on how the plane is flown is getting close to the time I charge. Mild standard non-digital servos do not draw much current when moved one at a time. Now start moving them all at once and see how much voltage drop you get. Even a momentary drop to 4.4V for just a few milliseconds is enough for the receiver to drop out. Five JR537 Standard servos moving all at once can drop the voltage by .3 - .5 volts for several milliseconds and that's assuming there is absolutely no binding in the control rod or surface. Any bind what so ever and the amount of voltage required goes up not to mention the air loads on the surface creating a need for more voltage if the model is flying.

Once the receiver drops out (loses signal) from too low a voltage there may not be enough voltage left to get control back since now you are in panic mode and moving everything to try to get control back. Or you go into the dropout/get-it-back/dropout round robin and once again you have very little to no control.

I will not even get into what digital servos will do.

To me, it's not worth the risk going below 4.8V and with a decent field charge, you can be charged in an hour or less.