Twin motor setup with receiver questions

[70sflyer]

Building Cessna 421 Twin,will be using 2 hacker 40-10,2 castle 75 ESC, 2, 3300 mah battery one for each motor.Question is I want to us Fife 2100mah battery for the receiver.it is 6.6v,will that be to much or do I need a BEC to bring the voltage down.Also i have a 7 ch receiver,do i plug it into ch 7.the rest are full.Thanks

[kyleservicetech]

Originally Posted by 70sflyer

Building Cessna 421 Twin,will be using 2 hacker 40-10,2 castle 75 ESC, 2, 3300 mah battery one for each motor.Question is I want to us Fife 2100mah battery for the receiver.it is 6.6v,will that be to much or do I need a BEC to bring the voltage down.Also i have a 7 ch receiver,do i plug it into ch 7.the rest are full.Thanks

Question, are you using the CC ICE series with the switching power type of BEC?

If so, I've used those with complete success. With a bit of wiring, you can connect both BEC's of the two ESC's to the receiver. But, they have to be separated with a pair of diodes. The diodes called for are special low voltage drop Shottky types, with a current rating on the order of 9 amperes each. Shottky diodes are used all over the place in power supplies and similar items. WWW.digikey.com sells them for about $2.00 each or so. Digikey sells to anyone with a credit card.

The two attached photos shows how it is done.

Let me know if you've got questions.

[fhhuber]

Matched Castle ESCs are safe to parallel their BECs
No diodes needed.

2X Thunderbird 35 for example.

This has been true for at least 5 years.

Some brands are not... older Dynam ESCs (I haven't tried newer ones... last tried about 6 yrs ago) will "fight each other overheating both even with diodes...

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But I think that's not the original question...

I am not sure what the max RX buss voltage is when using Castle ESCs that do not have built-in BEC.
Please contact Castle on this one.

**********

You can plug in RX power to any channel. I commonly Y in with throttle (when all channel ports are used already) since that is a very low load channel.

You can plug dual RX power in just the same as dual ESCs... any channel. (its fun to mix twin engines on 1 & 7 to rudder. Very scale for taxiing a DC3.)

[Larry3215]

Most receivers can handle that voltage easily - its the servos that may give you problems.

Digital servos in particular are very sensitive to rx voltage and you should never exceed the listed max voltage.

Analoge servos tend to have a little more play, but its still not a good idea to run the servos above their max rated voltage.

Check your servos and go by the lowest listed max voltage.

[rcers]

Well I am one with a bit of a different opinion. While I understand Castle ESC's can be paralleled I never do that. I think your LiFe pack is 100% the way to go on a large twin like this.

The ICE BEC cost me a $1200 model - all with just 4 digital servos. So I don't trust those either. It is well documented - if you do a search on the ICE BEC's with digital servos the internal BEC's are very easily overwhelmed.

I never use BEC's on models of this size - period. That is just one more point for failure. Again just another guy's opinion.

But the LiFe packs have served me well on the larger models that I fly. The voltage matches a 5 cell NiCad or NiMh pack - so if the servos are rated for "6v" you are just fine. I have never had an issue with digital servos and LiFe packs. In fact the voltage is very little over that of a 5 cell pack. Within .1v as a matter of fact.

I like the increased speed and torque that voltage gives you too.

So if it were me - I would disable all the BEC's and use a single LiFe pack. The 2100 mAh pack would give you gobs of flight time.

Now the LiFe packs are NOT LIKE NiCad's or NiMh packs however. You should not use voltage as an indicator of how much power is left in the pack. The discharge curve is much too flat for that to be reliable. And they dump voltage quickly as they are near the end.

Mike

[kyleservicetech]

Originally Posted by rcers

Well I am one with a bit of a different opinion. While I understand Castle ESC's can be paralleled I never do that. I think your LiFe pack is 100% the way to go on a large twin like this.

The ICE BEC cost me a $1200 model - all with just 4 digital servos. So I don't trust those either. It is well documented - if you do a search on the ICE BEC's with digital servos the internal BEC's are very easily overwhelmed.

I never use BEC's on models of this size - period. That is just one more point for failure. Again just another guy's opinion.

But the LiFe packs have served me well on the larger models that I fly. The voltage matches a 5 cell NiCad or NiMh pack - so if the servos are rated for "6v" you are just fine. I have never had an issue with digital servos and LiFe packs. In fact the voltage is very little over that of a 5 cell pack. Within .1v as a matter of fact.

I like the increased speed and torque that voltage gives you too.

So if it were me - I would disable all the BEC's and use a single LiFe pack. The 2100 mAh pack would give you gobs of flight time.

Now the LiFe packs are NOT LIKE NiCad's or NiMh packs however. You should not use voltage as an indicator of how much power is left in the pack. The discharge curve is much too flat for that to be reliable. And they dump voltage quickly as they are near the end.

Mike

Parallel connecting two switching power supply types of BEC's is also something I'd NEVER do. There is no guarantee that they will share the load equally.

IMHO, any $$$$ model, or giant scale model should have dual battery supplies of one type or another. In fact, most of my club members have changed over to either LiFe or A123 batteries for their receiver/servo power on their giant scale models. Several are even using them on their smaller glow powered models.

On my two giant scale 2500 Watt models, my setup consists of a CC 10 Amp uBEC connected to a "Tap" of 6 cells on my 12S2P A123 cells, PLUS a backup two cell 2300 Mah A123 battery.

The CC 10 Amp uBEC is set to 6.5 Volts DC.

I've got undervoltage alarms on the receiver that are set to go off at 6.0 Volts DC. The two diodes on the backup battery drops its voltage to below where the uBEC operates, so after a days flying, very little is pulled out of the A123 pack.

Two of these setups have over two hundred flights on them with two different models, with zero problems.

Take a look at the attached to show how it was done.

[70sflyer]

Thanks for all your replys.I think I have a handle on it know.

[fhhuber]

I generally disable the BEC (if the ESC has it) for anything using 4S 3000 or larger and go to a separate RX pack. When the plane uses that large of a LiPo the weight of a 4 cell NiCd or a LiFe pack wouldn't be significant to performance.

I never provide the RX two different types of power... I either use a BEC or battery(s). The BEC might be a Smart-Fly regulator that has dual battery inputs.

I don't mix direct connecting a battery to the RX with use of any form of BEC. If you mix direct connecting with a BEC you are either charging the battery (overcharging probably) or draining the battery with the BEC idling on sensing high voltage, so its not doing you any good to use both anyway. (and it could overload the BEC if its trying to charge the added RX battery + run servos)

[rcers]

Originally Posted by kyleservicetech

Take a look at the attached to show how it was done.

Yep Denny I think I have seen that about 20 times now. LOL!



I will stick with the LiFe unregulated pack. In fact I am sticking a 200 mAh LiFe 2s pack (it is tiny!)in my Alula as we speak. I am very fond of LiFe power - even in my 6oz glider.

Mike

[kyleservicetech]

Originally Posted by fhhuber

I don't mix direct connecting a battery to the RX with use of any form of BEC. If you mix direct connecting with a BEC you are either charging the battery (overcharging probably) or draining the battery with the BEC idling on sensing high voltage, so its not doing you any good to use both anyway. (and it could overload the BEC if its trying to charge the added RX battery + run servos)

Yeah, direct connecting a battery in parallel with the BEC of an ESC is a big NO NO. These two supplies must be isolated from each other with a quality diode, such as those 10 Amp Shottky types.

Same with direct parallel connecting of two different Nih batteries. They may not share load properly. I've actually ran across many cases at work where two parallel connected 24 volt Nicad packs went into thermal run away during the charging process, destroying both $250 Nicad packs.

Batteries such as the LiPos, LiFe's and A123's can be safely parallel connected. In fact, my models typically are 6S2P, 10S2P, 12S2P A123's and so on for motor power.

[kyleservicetech]

Originally Posted by rcers

Yep Denny I think I have seen that about 20 times now. LOL!



I will stick with the LiFe unregulated pack. In fact I am sticking a 200 mAh LiFe 2s pack (it is tiny!)in my Alula as we speak. I am very fond of LiFe power - even in my 6oz glider.

Mike

20 times? You are not reading the same article twenty times???

Yup,
Those LiFe batteries are a very nice direct drop in for replacing a 5 cell Nih battery pack. Their discharge curves are very similar, per the attached graph.

For the larger models though, the 2000 Mah LiFe batteries will far surpass the performance of the typical 5 cell "AA" type Nih receiver battery where the model uses 6 or more high current servos. Note the performance of the A123 pack at 32 degrees F. For what its worth, I'd NEVER use Nih cells in a receiver pack in freezing temperatures. Some day I'll run tests on this, but at work, standard Nicads didn't perform well at below freezing. We had to go to a special Nicad chemistry that would work to 40 F below. And, those batteries were expensive.

Take a look at the second battery test, of a new LiFe battery vs a brand new 2000 Mah five cell "AA" Nih battery at 12 Amps. That "AA" battery could result in a 2.4 Ghz receiver going under its minimum voltage requirement, and re-booting itself. The graph represents a time frame of a few seconds. (Note the graph indicates 1.0 Amps, the correct value of the Nih pack was 2000 Mah.)

I have measured a peak current of 13 Amps of servo current on my $$$$Fluke 87V digital multimeter on its 1 millisecond peak hold function. That 13 Amps was reached by just spinning the transmitter sticks round and round. The model is my Giant Big Stick with its seven Hitec 645MG servos.