Can I have two Life batteries powering my Futaba receiver?

[Tim Flyer]

I am currently in the process of getting set to assemble a Seagull 30cc Hurricane Warbird. I was wondering about adding receiver battery redundancy protection. To make my assembly simple I’m thinking of using the Futaba 14ch receiver which will leave me 5 spare slots. I use LIFE batteries and was wondering whether it might be sensible to use two 1500mah batteries rather than the single 2000 mah battery I was planning to use. I will be using 8 servos in the plane . The retracts will be electric and powered by a separate battery not in the receiver circuit. If I slot two batteries into the receiver plugs might they “fight each other “as they run down? As mentioned the third battery will be separated from recent power by removing the middle wire of the lead on the connection . 

Edited By Timothy Harris 1 on 07/06/2018 12:53:31

Edited By Timothy Harris 1 on 07/06/2018 12:55:37

[Roo Hawkins]

Have you thought about using two rxs one battery to each one one rx will control the ail and flaps one side and the other rx the other side. Plus two servos for the elevator and split the elevator one controled by one rx and the other by the other. The you will have two batteries and two rx with split control and if one battery or rx go wrong you still have some control.

Edited By Roo Hawkins on 07/06/2018 13:12:05

[Rich too]

Tim,

Yes, that is a system I use the most. The batteries will not fight each other. One receiver, two LiFe batteries and switches, plugged into separate channels. I recommend that you invest in decent switches, as this is a common failure point.

[Tim Flyer]

Thanks Roo that sounds a good idea. I hadn’t thought of that . This model uses two elevator servos so that sounds a good idea

[Tim Flyer]

Cheers Rich too I will weigh up costing etc . So far I haven’t had receiver failure but have suffered switch failure so well noted the 2 battery 1 receiver might be the way I go on this . This forum is a great knowledge pool ! Cheers 😊 Tim

[Rich too]

Yep, receiver failure is rare, and I would only consider two receivers on much larger models.

[robert chamberlain]

Hello all,--I am new to LiFe battery technology and am uncertain as to the charge rate. AS luck would have it, my cell phone battery has one, rated at 5 volt and 1000MA and my wall charger says the same thing on the label. All new wall chargers for flip phones say 5 volts 2000 ma. Does this mean to imply that these batts can take a 2C charge rate? Of course, I do not plan on using this cell phone batt in an aircraft, but I was wondering if this was indicative of all LiFe batts?-------thanks in advance, Bob in Kansas

[robert chamberlain]

Concerning Tim's comment above: My question is, if using 2 batts plugged into the receiver, what would happen if one batt should fail? Wouldn't' the other batt try to feed into it? As I only have 6 ports on the rec, why couldn't I just use a "Y" extension, plugging the two batts into one end and then only having to use one rec port?

If we are talking about a 2 cell LiFe batt, isn't that a little too high voltage? Every thing on my radio is 6 volts. Thanks for any input. Maybe I'll just go to NiMh--------Bob

[Biggles' Elder Brother - Moderator]

Hi Bob,

How's things in Kansas?

Regarding charging your LiFe batterries: yes you can charge at 2C - generally I wouldn't do so though. I doubt that it makes a lot of difference but I believe in charging only at 1C.

Note though, you will of course need to use a LIpo sytle charger with a LiFe setting.

BEB

[Rich too]

Hi Bob,

Pretty much all of my models use two LiFe batteries. There is nothing wrong with using a Y lead, and I tend to share with a servo. Most systems can take the slightly higher voltage, certa8nly Futaba that I use,

I agree with BEB and charge at 1C....

[Former Member]

[This posting has been removed]

[Frank Skilbeck]

Posted by robert chamberlain on 22/09/2018 03:43:52:

Hello all,--I am new to LiFe battery technology and am uncertain as to the charge rate. AS luck would have it, my cell phone battery has one, rated at 5 volt and 1000MA and my wall charger says the same thing on the label. All new wall chargers for flip phones say 5 volts 2000 ma. Does this mean to imply that these batts can take a 2C charge rate? Of course, I do not plan on using this cell phone batt in an aircraft, but I was wondering if this was indicative of all LiFe batts?-------thanks in advance, Bob in Kansas

Bob, be aware that the phone wall charger is just a power supply, the phone actually controls the charge process. My Tx uses a single 3500 mah LiFe cell for power, I charge it using a 2000mA phone charger but the Tx limits the charge rate to 1500 mA .

Note a fully charge LiFe cell is 3.6v per cell and a 2s LiFe pack is a good replacement for a 5 cell Nimh pack.

[Allan Bennett]

A simple way to prevent two separate power sources interfering with each other is to simply install a Schottky diode (with a suitable amps rating) into each +ve battery lead before they go into the Y-plug or into two separate receiver inputs. Then if one fails in some way, the other will not be able to feed power into the duff one, but will continue to power the receiver. I've got such a system in several of my aircraft -- fixed wing and helis -- using BECs rather than batteries as the power sources. I've also added an LED to each 'channel' to give an indication if one of the power sources has failed.

[Martin Harris - Moderator]

If you have any concerns over the slightly higher voltage of 2S LiFe over 5S NiXX then using silicon diodes will drop 0.6v while performing the same function.

[Rich too]

If it’s good enough for DA....

**LINK**

[Paul Marsh]

I seem to use twin rx packs on models larger than 20cc nowerdays or if a new build which might have powerful servos

I use these as well, the Powerbox ones are ok, but at £80 each very expensive and at that price it makes you think about not using dual batteries. So Rapid have them for £25 and have a few.

.

[Former Member]

[This posting has been removed]

[Paul Marsh]

Posted by Percy Verance on 23/09/2018 10:21:09:

A Powerbox switch at £80 actually isn't that expensive if it means you avoid an expensive crash Paul.

Smashed model, damaged engine, broken servos, possible internal damage to receiver. What might all that cost to put right?

True, but as I sell things to people at shows, it's surprising what people will do to save a quid! A chap came in saying his model crashed due to a faulty switch , which he had with him, and was looking at a new one. I showed him our best switches, but they were £8 each, the heavy duty Futaba/JR ones. He said it's too expensive and have I got anything cheaper.
We also had some normal switches, good quality, but £5 each. he said it's too expensive and will try to repair the switch he had with him to save some money... I give up...

The model is only worth as much as it's weakest link... the switch is a weak link and if it's worth £2.5 then the model is worth that.

[Engine Doctor]

Posted by Allan Bennett on 22/09/2018 21:10:21:

A simple way to prevent two separate power sources interfering with each other is to simply install a Schottky diode (with a suitable amps rating) into each +ve battery lead before they go into the Y-plug or into two separate receiver inputs. Then if one fails in some way, the other will not be able to feed power into the duff one, but will continue to power the receiver. I've got such a system in several of my aircraft -- fixed wing and helis -- using BECs rather than batteries as the power sources. I've also added an LED to each 'channel' to give an indication if one of the power sources has failed.

Thats my preferred method too. I also use a separate switch for each battery fitting the diodes into a "y" lead between the switch and Rx .

[Piers Bowlan]

Posted by Engine Doctor on 23/09/2018 11:31:09:

Posted by Allan Bennett on 22/09/2018 21:10:21:

A simple way to prevent two separate power sources interfering with each other is to simply install a Schottky diode (with a suitable amps rating) into each +ve battery lead before they go into the Y-plug or into two separate receiver inputs. Then if one fails in some way, the other will not be able to feed power into the duff one, but will continue to power the receiver. I've got such a system in several of my aircraft -- fixed wing and helis -- using BECs rather than batteries as the power sources. I've also added an LED to each 'channel' to give an indication if one of the power sources has failed.

Thats my preferred method too. I also use a separate switch for each battery fitting the diodes into a "y" lead between the switch and Rx .

OK, so to ask a silly question, what is a 'suitable amp rating"?

[Dave Hess]

Surely, if you want two batteries into one receiver, you put the two batteries in parallel with a simple Y-lead to the balance leads and keep them like that. There's no need to have any diodes. The only important thing is that they're at the same voltage when you connect them. After that, they will be self-balancing with each other, i.e. they will always have the same cell voltages as each other. No harm can come to either battery.

Note that some LiFePO4 batteries don't have balance leads. If yours don't, then you can still put them in parallel with a Y-lead using the power leads.

I wouldn't advise connecting two batteries to one receiver with or without diodes unless you can see inside the receiver that all the battery positives and negatives are connected directly by common rails, in which case it's the same as a Y-lead on the power connectors.

[Piers Bowlan]

Posted by Dave Hess on 23/09/2018 12:03:16:

The only important thing is that they're at the same voltage when you connect them. After that, they will be self-balancing with each other, i.e. they will always have the same cell voltages as each other. No harm can come to either battery.

What happens if one of the batteries has a duff cell (or one shorts out)? That one cell will drag down both batteries, which is why I assume you use the diodes?

[Paul Marsh]

Linking batteries is a no-no, even if the same voltage. Resistance is what does it, slight variation on battery parameters, leads, and they both will try and discharge each other, or worse...

Lipo's for example where there are two cells in parallel are matched in the factory and placed next to each other, so the above is minimized, but notice how little choice there are for Li-pos in 3p configuration?

[Tim Kearsley]

I think it might be slightly better to run two batteries into two separate ports on the Rx, rather than into a "Y" lead and then into the Rx, as it gives some redundancy against the connection becoming faulty. It's all about trying to avoid single points of failure after all. I've run two LiFe batteries on a Hangar 9 Spitfire (the 30cc one) for a year or so with no issues at all. I don't use diodes at the moment, but I might change that, as, on reflection, it does offer some protection against a battery failing and pulling down the other one. But then every component you put in circuit is another point of failure. I also avoid switches for the same reason, simply plugging the Rx batteries into a short extension from the Rx. If I do use a switch I use an electronic one - I simply don't trust mechanical switches!

Tim.

[Peter Beeney]

To try and answer the question regarding the duff cell in a pack that’s connected in parallel with another, both packs will discharge equally timewise but the total capacity will always be less than two good packs. If they are (say) 2Ah packs but one has a faulty cell reducing the capacity to 1.5Ah then the total will be 3.5Ah. This will happen seamlessly with or without any diodes.

If a pack shorts out certainly a diode will stop any back feeding but I’m wondering how this might occur anyway. If there was a short within the pack I think the resistance might be low enough for enough current to flow briefly to perhaps cause a fire - which might then quickly become a bigger cause for concern. I’ve never read of any instances of a pack shorting out internally; if it were to happen I’m sure it would soon be reported. (Other than problems relating to counterfeit batteries that is; but then this situation could also equally apply to modellers as well, I suppose). Then there is the lipo hazard which is very familiar and well recognised but rather than go open circuit they simply self destruct and go on fire anyway. But as yet do many folks generally use lipos as a receiver power supply?

In my view all this simply reiterates the importance of testing the individual pack’s capacity from time to time; at least annually perhaps; and just as importantly new items before they go into service. Cells fail gradually over time, and there is no outward indication of this. However, It’s possible to avoid this by about 99.9%, a simple onboard led receiver battery voltage indicator gives a constant readout of the battery state and I’m sure it would be very difficult to ignore a brightly glowing red lamp and take off regardless! Again, I’d put this very close to the top of any priority list.

Please consider this as just my opinion…

PB

Edited By Peter Beeney on 23/09/2018 15:58:43