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Can I have two Life batteries powering my Futaba receiver?

Receiver battery redundancy protection

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Dave Hess23/09/2018 12:03:16
258 forum posts
18 photos

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 Bowlan23/09/2018 12:44:41
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1513 forum posts
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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 Marsh23/09/2018 12:49:14
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3397 forum posts
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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 Kearsley23/09/2018 12:49:54
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578 forum posts

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 Beeney23/09/2018 15:54:55
1531 forum posts
59 photos

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

Percy Verance23/09/2018 16:21:26
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7099 forum posts
142 photos

Good point Tim. My radio manufacturer offers electronic switches for airborne installations for those whom prefer to go with them, They also use an electronic switch on every transmitter bar one that they presently market.......and I think that one is soon to be dropped from their line-up.

 

Edited By Percy Verance on 23/09/2018 16:22:59

Rich too23/09/2018 18:59:46
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2839 forum posts
1031 photos

Cheap switches asking for trouble ( I’ve had them fail), I use these in larger models **LINK**

Martin McIntosh23/09/2018 19:53:48
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2592 forum posts
963 photos

Definitely go with Percy V`s recommendation of an intelligent switch regulator. I use quite a few of the HK Failover switches which are identical and only about 12 quid. Just remember to disconnect the packs after flying or they will discharge into the switch and be destroyed (after a month or two that is).

Allan Bennett23/09/2018 20:51:29
1403 forum posts
37 photos
Posted by Piers Bowlan on 23/09/2018 11:47:30:
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.

yesThats 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"?

Suitable amp rating depends on how many amps you think your system is going to draw. I use a twin MBR1545CT Schottky rated at 15A per section. In my models it's usually fed by 10A BECs rather than by batteries. But whatever your power source, the rating depends on the power consumption of all your servos and any other accessories which might be running off the receiver supply.

Dave Hess24/09/2018 12:35:36
258 forum posts
18 photos

Yes, lots of urban myths about connecting batteries. Whilst there are some potential problems, there should be no risk in OP's case provided that he checks that the two batteries are the same voltage when they're put in parallel.

I've been working with electric vehicles (mainly bicycles) for the last 8 years, where we use batteries anywhere between 10S and 20S and typically with anywhere between 5 Ah and 20Ah, Working wit lipos, we'd make something like a 16S 8Ah battery from 8 individual 5000mAh 4S hard-packs - 4 in series and two in parallel or a 12S 20Ah might be a 2S4P pack of 6S soft-pack lipos to make a 12S4P pack.

Most ebike batteries these days use 18650 cells, so a typical 48v one would be 65 cells in 13S5P arrangement. If it were a problem to put cells in parallel, we'd know about it!

here are some examples, so that you can see what I mean:

20ahbattery_zpsa67a9f42.jpg

batt2cells_zps4c8a9ad4.jpg

Edited By Dave Hess on 24/09/2018 12:44:44

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