Two batteries into an RX8R rx?

[Jolly Roger]

Is it ok to plug two LiFe rx batteries into an FrSky RX8R receiver, to get a kind of crude battery redundancy?

I appreciate there will be no diodes to prevent a failed battery draining the other one, but in my experience batteries fail open rather than short circuit.

Rog

[Cliff Evans]

I am no electronics expert but, for the sake of a few pennies, I would use the diode option! Depending upon how you value the model perhaps even £25 on a redundancy system like the Frsky. RB10 https://www.t9hobbysport.com/frsky-redundancy-bus-10

I am sure that someone will give an opinion or indeed the benefit of their knowledge to help you decide.

[Barry_Cole]

Diodes for me.....



BC

[Peter Balcombe]

I agree.
Diodes are a simple & cheap method (although you lose a little voltage, which can be minimised by using Schottky diode types).
In my experience, a single cell will often go, causing low output - which then drags the other cells or any parallel connected battery down.
Diodes will simply cause the faulty battery to be isolated from the good (higher voltage) one.

[terry white]

Here, here. another one for diodes.

[Jolly Roger]

You're right folks, I'm being lazy. For the sake of a few minutes soldering, I'll wire an extension between each battery and rx with a diode on the +ve wire.
Do these look OK?

https://uk.farnell.com/multicomp/p600d/ ... KU-9564985

Thanks,
Rog

[RobbieB]

Morning Rog.

As long as the servo installation isn't on the 'chunky' side, 5/6 ampers will be fine. I've used them for years without any problem. You could always go bigger for peace of mind for what they cost but that will only mean they will run a little cooler if they are being stretched to their limit - not that I've ever had a problem with that.

You will only get a max voltage drop across them of about 0.9V, give or take and that will only be when they are running at full load -well within two cell Life voltage capabilities.

What would be interesting would be to find out what the start-up load is on one of our little DC servo motors - significantly higher than their load currents for sure. It's very brief in this particular application but significant none the less. Ten/twelve servos all cranking up together........interesting.

I've got a schematic wiring diagram somewhere for charging options for batteries in/out of the model, I'll see if I can find it.

[Peter Balcombe]

RobbieB wrote:
>
> What would be interesting would be to find out what the start-up load is on
> one of our little DC servo motors - significantly higher than their load

I’ve recently built a low power wattmeter from a design by Chris Bott which should be appearing in RCM&E sometime.
This measures peak current, so I could check out servo start up draw.
Will have a look & get back on this.
Peter

[RobbieB]

Nice one Peter.
It would b e nice to discover that due to a light construction and hence low inertia in the rotors the start-up load isn't as high as I imagine.

[Peter Balcombe]

Robbie,
Just had a look with HS82MG, CS239MG & HS225MG servos for starters, but results are inconclusive I’m afraid.
The indication is that start-up surge with all of them is similar (approx. 200mA), but I cannot get it to register normal servo movement current unless I really load the servo up to almost stall current, when I get 200-300mA.

I think the measurement problem with the device I am using is down to both the 50mA measurement resolution, but more importantly, the relatively slow measurement sampling rate compared with the servo movement.
The wattmeter is intended for small electric power trains, so can get away with the relatively low sampling rate as the motor is either off or turning continuously, thus virtually DC. In this case, the peak measurement would often reflect the highest throttle setting during the test run.
However, I think it is missing the low current surges when the servo moves with no load in this test scenario.