NIMH or SBEC and lipo for ic?

[Gordon Brown]

Not sure if this is the right forum but hopefully I'll get some advice nontheless. I'm currently putting together a CMP Mitsubishi Zero powered by an ASP FS120, which is the first ic model that I've built and I'm looking for some advice on the electronics side of things. I've built biggish all-electric models before and if it's using a big motor I'd use an opto esc plus a separate SBEC or UBEC and lipo battery to power the receiver and servos. However, most of the ic models I've seen at the club use a NIMH pack for rx and servo power, something I have no experience of. My inclination is to use something like a Turnigy 15A UBEC and a 3 cell lipo as the plane will have 6 Futaba 3003 servos installed running on 6V to give as much torque as possible. However, I'd like to power the UBEC using a NIMH pack as I want to put the battery as far forward as possible to minimise the additional balancing weight that will be necessary in this model, and I want to be able to charge it in the model. Can anyone advise if this is a reasonable approach? Is there a better alternative that is of a comparable cost?

[Martin Harris - Moderator]

2S LiFe with an optional 3 or 5A silicon diode in series if your servos are of a sensitive disposition...

P.S. Hope your 3003s are the real thing - there are a lot of fakes around (usually with white splined output shafts).

Edited By Martin Harris on 21/03/2017 21:41:43

[Frank Skilbeck]

Yep 2s LiFe or 5 cell (sub C) Nimh, I've used both with Futaba 3003s with no problems, just make sure you use a good quality switch, no need for a UBEC.

[Denis Watkins]

NiMH pack is a good choice Gordon

But just check your receiver operating voltage as some are not rated for 6 v

[ChrisB]

You should be fine with just a Nimh pack Gordon. 3003's in the type of model you have won't put a huge load on the system.

You're a brave man for choosing that model for your first I.C. Keep the speed up and remember that the engine won't react as quickly as a motor. If you've never had a deadstick then make sure you keep the nose down when it happens, which is rare if set-up correctly but does happen from time to time.

I would go for a single 3300mah, SubC Nimh and with a HD switch and leave it at that. The more bits and bobs, the more to go wrong.

If you were building a 1/4 or certainly 1/3 scale Zero, then i'd say use separate supplies for the RX and servos, but not a 70" model with a 120.

Cheers

CB

Edited By ChrisB on 22/03/2017 08:23:14

[Frank Skilbeck]

Posted by ChrisB on 22/03/2017 07:54:48:

If you were building a 1/4 or certainly 1/3 scale Zero, then i'd say use separate supplies for the RX and servos, but not a 70" model with a 120.

Cheers

CB

Edited By ChrisB on 22/03/2017 08:23:14

Or a receiver designed for large models with a high capacity power supply, connectors and internal distribution system.

[Engine Doctor]

If you need the weight in the nose why not keep it simple and use a 5 cell" Sub C" pack . That will make sure that you get enough power for servos and Rx . Alternatively go for LifePo4 packs, two 2 cell packs in parallel will give you all the current you need and run at 6.4 v so you wont need a UBEC , and they don't self discharge like some NiMh.

[iqon]

Sub C nimh

[Nigel R]

3003s are a basic standard servo so don't have massive power requirements. IIRC I have read 0.6A as a reasonable guess at their stall current at 6V, so all six fully stalled (i.e. worst case) would draw 3.6A. Sub C nimhs are easily capable of 3.6A.

Weight around 350g I think for 5 cells.

[Gordon Brown]

I've got one of these, looks suitable, any views? As ChrisB says, this may be a brave or foolhardy choice but as is the nature of these things the model came up at a swapmeet for an extremely good price (£70 complete with servos and retracts, never completely built) and I couldn't resist it. I've installed split flaps to try to slow the landings down and beefed up the retract mounts as well. Looks pretty good so far, but my heart stills gives a little flutter when I think of flying it.

I'm a reasonably competent and experienced pilot, although I don't get dead sticks with the electric models I do practice dead stick landings with my 1.8m Monolog, simply because it's good pracice to do so. I've only seen one dead stick on an electric model, a balsa AcroWot where a magnet came loose in the motor can producing an interesting smoke trail as the motor wires burned out. The lad flying it did one of the best dead stick landings I've seen, bringing it down from an upwind position at relatively low altitude by dropping the nose and doing a fast low level turn spot on to the landing strip. Hope I don't have to emulate him!

[John Lee]

I think that this is a case where there are multiple solutions that will work well & it comes down to personal preference.

I prefer to use a 2s3p Lipo with either a Digiswitch or Jeti BEC. Both of these eliminate the use of a mechanical switch, if the switching mechanism fails they fail 'on', and for me have been 100% reliable. I have used a couple of Turnigy BECs but having experienced a failure with one decided the extra expense of quality items was worthwhile.

[Jon H]

Another vote for sub C 3000+Mah 6v packs. I use them in my 80 inch warbirds running 3-12kg servos with a handful of digitals in there. I do use a beefier switch but thats about it.

The battery in your photo is using smaller AA cells and while man enough in theory i would probably go to the larger sub C cells. Never trust a swapmeet battery until you have cycled it a few times and are 100% sure it is behaving

If you arent used to warbird flying make sure you balance the model accurately and dont use excessive rates, especially on the elevator

Edited By Jon Harper - Laser Engines on 22/03/2017 11:48:11

[ChrisB]

As you're using 3003's you'll probably get away with using the AA pack that you have linked Gordon. Any bigger servos and i'd go for a Sub C **LINK** as they will stand a higher current draw.

[chris larkins]

I have used a 2s LiFe in the last 5 I.C models that I have put together, unless I really have an issue with nose weight I will always use these in the future. For their size they offer a really good compromise between weight/capacity and of course offer the benefit of a higher voltage for increased servo speed/torque and are actually cheaper than a "good quality" 4.8v NiMh of similar capacity, they also hold their charge for months. Hobbyking sell a 2s LiFe which they market as a transmitter battery however it is perfect for use as a receiver pack as it is fitted with a standard JR plug **LINK** .........just make sure your charger is LiFe compatible !!

I bought a second hand model that is also fitted with 3003's and I have used the LiFe pack with no voltage issues whatsoever, however as Martin mentioned in an earlier post fitting a diode similar to this one **LINK** inline in the positive wire of the switched side of the RX switch will drop the voltage to approx. 5.9v

Unlike a LiPo there is no risk of the battery catching fire during charging so they can be safely charged in the model, you may just need to fit a balance extension lead so that you can balance charge if the battery is not easily removable. There is no need to use a separate SBEC or UBEC.

 

Edited By chris larkins on 22/03/2017 22:49:16

[Grant Webb]

Have to say I am firmly in the LiFePo camp. Having suffered model loss through "Black rot" in the wiring while using NiCad packs, I made the switch to LiFe cells a few years ago after research placed the blame on the Ni-Cad cell chemistry. From my research it was suggested that NiMh are of similar chemistry to Ni-cads and subject to the same potential problem. You can read a good description of it here.

So, to be on the safe side I decided to switch to LiFePo4 technology for all my IC powered models.

I still have two 27MHz transmitters waiting for re-wire due to the black wire rot.

My 2 cents worth....

Grant W.

NZ

[Jon H]

Black wire can effect any cable but NiCd are quite vulnerable due to the gas emitted when overcharged. High charge rates and overcharging were primary causes of black wire in the NiCd days and while NiMh is not totally immune they are much more resistant.

[Denis Watkins]

The Japanese some decades ago, and I expect NASA did and do try to eradicate "black wire", very cleverly with "pulsed" electrical flow and more easily with AC, alternating current.

We suffer from DC direct current action, braking down the boundaries within metal wire, and blighting this wire, with little we can do about it.

The boffins findings were that with current swishing one way, then the other way, continually, that this somewhat undid the damage done from say the forward current, as the current reversed.

See how tricky this is?

But this is the reason many electrical/electronic devices survive years, eg cheap watches, as their action is pulsed and not direct current.

So I have read

[Gordon Whitehead 1]

What do the folks who recommend NiMH use for charging their batteries? Peak detect chargers or 150mA wall chargers.

My recent experience of NiMH is that the low self-discharge ones, including Sanyo Eneloops, is that they don't live up to their hype regarding self-discharge rate. Ordinary NiMH self-discharge fast enough that I'd want to charge the same day I was going to use it. Additionally, I've suffered false-peaking when using a peak detect NiMH charger, which is a pain when you expect to set off to the field having just had your battery on charge for an hour, and it stopped charging within 10 minutes.

Funnily enough, 12 years ago when I was flying 20-cell 1kW 90mm EDFs with Gold Peak GP3200 NiMH cells (that was high power back in the day though well eclipsed now of course), charging at 5A or more to get 'em hot ready for the plane, false-peaking never occurred.

Recently I decided to try a 3300 NiMH and heavy-duty switch for my current build which is a bipe with a Laser 155 25cc 4-stroke. But have chickened out for the above-mentioned reasons as I've had complete satisfaction with 1500 lipos and Powerbox Sensor Switches in Wren 44 jets and Saito 36 petrol-powered bipes. I looked inside the J Perkins HD switch body I'd bought, and discovered a 2-pole changeover switch which looked a bit anaemic compared with the Noble 4-pole changeover switches widely used in early propo outfits for many years.

If the battery does have to be in the model for charging, although I've never used them, the LiFE batteries look suitable as they have no false peak problem, retain their charge, and their charging and operation will be familiar to anyone used to operating lipo-powered electric models.

[Denis Watkins]

Ideal charge would be 1/10th C

But if you arrived at the field with a flat pack

Then get flying after 1C NiMH charge