Hello,

I am in the middle of assembling my ARF Kadet LT 40 and came across 2 questions. I am using an Airtronics RDS8000 radio with 4 standard Hitec servos.

Question 1: I am looking for a battery pack that will charge from the charger that came with my radio. The instruction booklet recommends a 4 cell 4.8V Nicad pack but does not specify how much capacity it should have. Since the Tx comes with a 700 Mah pack, I assume I don't need much more for the Rx? Would this Hitec pack keep the Rx up with my Tx? It is 12 bucks, and 600mah. http://www3.towerhobbies.com/cgi-bin...?&I=LXN731&P=7


Question 2: From the reading I have done, it seems like the most common cause of a crash causing a total loss is a failure of the switch or Rx battery. Would it be a good idea to use 2 switches and/or 2 battery packs? Is this even possible and how would I go about wiring it up?

Thanks for any info.

It doesn't matter what capacity pack you use, your charger'll charge them up... it'll just take longer for bigger packs. You can (and probably should) use a 6V pack instead, and use NiMH instead of NiCad... you can charge them with the same charger, but of course you'll get better results & more info with one of the new computerised chargers - you can pick them up cheap on ebay from Hong Kong, plus they let you discharge, cycle etc automatically. The 5 cell NiMH pack will weigh as much as the 4 cell NiCad one, it won't have the same "memory effect" as the NiCads, and you'll get better results from servos and receivers. I personally wouldn't fly with anything less than 2200mah pack but I know of plenty of people who do. If you can solder you can save a bunch of cash on batteries by buying individual cells (with solder tabs) and soldering up your packs yourself.

Yes, you can use 2 switches in parallel and 2 battery packs as well - it's called a redundant power system. Make sure you use the same voltage packs (eg: 4.8V or 6V) and try to use similar capacity packs (eg: don't use a 700mah + 2200mah) to avoid one pack trying to charge up the other pack. You can connect them to your receiver with a Y adapter after the switches, or if you have an unused channel just plug the second switch into that. Be aware, you'll need to balance the desire for redundancy with the desire to keep the weight down as much as possible. Don't forget to re-check your CG if you add a second pack to a plane that's already set up.

Thanks Sandman. When you say you do not fly with less than a 2200 mah pack, I assume this would also apply to your Tx? I also assume you are flying much more complicated aircraft than my kadet. Would I need that much juice for 4 standard servos and a 2.4 ghz Rx?

Also, could you drop me a link if possible to one of the cheap chargers from HK that you refered to.

If it were me.. and I do this all the time, I would use a 5 cell (6 volt nominal) pack. This stems back to the original Spektrum DX7 receiver days when a low voltage caused the receiver to "reboot" which could cause serious problems if in the air. That has pretty much been eliminated by changes in how the receivers function, however, I found that I really liked the 5 cell packs in my aircraft.

A rule of thumb for receiver packs, when increasing from 4 to 5 cells is to increase the capacity by at least 20%. This is to overcome teh differences in current draw with the higher voltage. (constant load, higher voltage = higher power factor which means more current draw from the battery pack for a given configuration).

A 5 cell pack will give you more torque AND faster servo action. Again, the negative side of that is that you will draw more current. I consistently use 1200 mah NiMH packs, either in single pack configuration or a parallel pack, as Sandman suggests. You can use any capacity but consider that 20% power factor and increase accordingly... if you go to 5 cell packs.

If you prefer to stick with the 4.8 volt packs, then, well, depending on the servo, and in this case, you said "standard" servos, so I would have to assume it is equal to the Airtronics 94102Z servo, which is their "standard" servo. Most transmitters come with 600 mah receiver packs. In the case of the RDS8000, though, if you bought one of their special packages, you probably got a transmitter, two receivers, and no servos and no RX battery pack, which is fine, it allows you to configure it any way you wish. In this case, it is prudent to err on the positive side and just go with a higher capacity pack, especially considering that you have not yet purchased a receiver pack.

If I may, I would recommend you visit

www.radicalrc.com

and take a look around. He has a FAQ section that really tells the tale about batteries, packs, soldering your own (really not recommended). And, you can purchase a pack suited to your needs, in any shape and size, with any connector you wish. This is one of the better sites for purchasing battery packs. AND, they guarantee them. I bought a LiPo pack from them, and when I got home, I put it on the charger. I think I accidentally, and momentary shorted the leads together which destroyed the packs. I wrote an Email to them and he (the owner) said to return it and he would replace it free of charge. How can you beat that!!!!

Best of luck with your new LT 40. Let us know what you decide.

CGr.

My Tx pack is still the original one that came with the DX7 which (from memory) is a 9.6V 1500mah NiMH pack. I've thought about upgrading it a few times but haven't yet. I guess the difference is that I can see the battery gauge on the radio, and it'll beep to warn me when it's low whereas I can't tell what the planes are doing when they're in the air. I've done about 8 flights in a day without the Tx battery getting dangerously low.

I fly three planes - a Great Planes Super Skybolt that runs five servos, a Seagull Models Dual Ace with seven servos, and a Seagull Models Harmon Rocket 3 (modified to have flaps) with eight servos. In all cases the servos are the DS821's that come with the DX7 (they're a reasonably priced digital servo that's a bit more capable than average) except for throttles and flaps where I use standard analogue Hitec servos. All my planes have "lost model alarms" and LED volt meters on board, and the Dual Ace also has an on-board glow driver to help with the twin engines (with its own independent battery). Don't forget though, it's not really how many servos you have on board, it's how many you're likely to use at once and what load you're going to put on them.

As CGR has pointed out some 2.4Ghz receivers (most notably Spektrum ones) have issues with low voltages "rebooting" the Rx. It's never happened to me even when I was flying 4 servo trainers on 4.8V packs, but I'd rather avoid it where possible. CGR's also pointed out that that higher voltages also equates to higher current draw, which is why I won't use less than 2200mah packs where I'd be happy to use a 1700mah pack in a 4.8V system (if I was willing to go with 4.8V of course. ) Having said that I was able to use the 4.8V 1100mah pack that came with my DX7 without an issue in three different (all now defunct) planes - I just tend to err on the side of caution. I should also say that none of my planes have ever finished the day with flat batteries, and I'd like to keep it that way.

[link=http://cgi.ebay.com/B6-LiPo-Li-Po-NiMH-2S-6S-Battery-Balance-Charger-Black_W0QQitemZ270427522693QQcmdZViewItemQQptZRadi o_Control_Parts_Accessories?hash=item3ef6bc8685&_t rksid=p3286.c0.m14&_trkparms=65%3A12|66%3A2|39%3A1 |72%3A1205|293%3A1|294%3A50]Here's a link[/link] to one of the cheap battery chargers I mentioned. If you search for "battery charger" in toys & hobbies/radio control you'll find plenty that'll work, probably cheaper than the one I've listed. You usually have to get a 12V power supply too, or run it off a car battery. I got it because finding a charger for a single NiMH cell for the on-board glow was proving more difficult than it should have been, and the digital charger was going to cost about the same as the plug pack.

there is nothing wrong with using 4.8 volt nicad batteries.....you can use 6 volt if you wish, the battery will be of bigger capacity....you just don't see 6 volt 600 mAh batteries......as stated you can run 2 switches in parallel. where voltage will stay the same but current will be additive....in other words 2 4.8 volt 600mAh batteries, in parallel will give you 4.8 volt 1200 mAh capacity.....and will take twice as long to charge fully as a single 600mAh battery. or you can run the second battery to an open connection in the receiver.....
I have never run a dual power system....I have never had a power failure.....If you charge the night before you fly for 15 to 17 hours you will be ok...remember a second battery is extra weight.....If your worried about a brown out with your 2.4 system...get the smallest capacity 6 volt battery and you will be just fine

The RDS8000 has a low voltage cutoff of something like 2.5 volts. You will see problems with your servos long before your receiver shuts down. What this means is that a 5 cell, 6 volt pack is definitely not needed, just an option if you want faster servos with more torque.

I have the same radio. I use a 1600mah pack for the radio itself and it lasts a good two days of flying without needing a recharge. I got the battery from RadicalRC.com.

You can get 2000mah battery packs and that should last you quite a while, again probably two days of flying without a recharge, for your receiver.

People do run two "redundant" switches and batteries, but usually on larger, more expensive aircraft. If you want, you can do something like two switches with two 1000mah battery packs. This would give you added protection and would be approximately the same flight time as using one 2000mah pack.

Gee, I seem to recall someone else saying the exact same thing somewhere above. Hmmm...

I can happily confirm that the Airtronics 92824 receiver that comes with the RDS8000 is quite happy with a 4.8v 600Mah receiver battery. I've been flying my RDS8000 for about 18 months now, and I've experience nothing but happy flying using standard receiver packs on four different trainers and sport planes while using my 2.4Ghz radio system. I do hava a 6.0v NiMH pack on another plane, but it was more for improving servo performance than anything else.

In fact, I recently purchased a pair of new Hitec 4.8V 600Mah NiCd receiver packs from http://www.servocity.com for $5.00 each. Feel free to spend more if you wish.

Good luck and good shopping.

I use 600 mah nicad receiver battery packs (that came with my radio outfit) on my planes and get at least 7 flights out of each of them before I call it a day. And that battery pack is operating 5 servos installed in some of them. If I feel like flying more I give it a quick charge at the field with my hobbico fast charger.

Great info here! Thanks, the links provided had great info, as did your posts. A 600 mah Rx pack for 5 bucks seems like a deal to me!! Once I move up to more expensive, higher drain applications, I'll just upgrade my Tx and Rx at the same time to NIMH. And sine ya'll have been flying for ever with no issues, I guess I'll save the redunduncy issue for when I get into more pricey planes.

Thanks again!

Just a word of caution, Bonza. It doesn't matter what anyone says, if you buy cheap, you get cheap. One place where you do not want to skimp on, for the sake of an additional 15 or so dollars is the receiver and transmitter batteries. There is a reason why they are cheap. In many cases, way they bond the batteries in a pack is a point of failure in cheap packs.

Be very careful. Consider that after you lose control either by a transmitter failure or a receiver failure, which is more llikely because of the vibration, you don't know where that plane will come down. These things are not harmless if they hit something or, worse, someone.

Vibration can and will de-bond a connection between battery packs if they are not done right. That will cause the pack to go dead in a heartbeat.

CGr.

Yeah I gotta agree with CGR (if only to emphasise his point) - buy quality packs. Don't go making your own unless you're sure of your soldering etc skills and if you do, buy quality cells. It only takes one dodgy cell to kill your plane.