Battery level, voltage regulator and DX8

[Alan Bahia]

Hi Guys,

I've bought a new battery A123-6.6VB 2300mAh 2s1P Receiver Pack (Constant discharge: 6-10C) and added a DualSky VR-8L Linear Voltage Regulator. Now, my plane (Sbach 342 28cc) has 6 Savox servos SC0254mg which will be powered by this battery and I have some of the following questions:

 

1. The DualSky voltage regulator has to "outcome" cable which one is connected to my receiver and the other one I am thinking to connect my Sullivan Sky Writer pump. So far I had a separate battery to the pump, I wonder if my new system would cope and for how long? (why I am thinking to have everything running on this battery? To save weight) by the way I have another battery for my petrol engine which I will keep it.

 

2. I have a DX8 with telemetry on this plane which gives me the battery level and I can program an alarm for battery low voltage. What would be a secure low voltage on the above battery?

 

To see my whole project and some videos of my plane please visit my website where you can also find my project with links, costs, etc.

 

 

Thanks!

 

Alan Bahia

 

 

Edited By Alan Bahia on 28/04/2011 15:40:14

Edited By Alan Bahia on 28/04/2011 15:41

[Frank Skilbeck]

Alan

 

Voltage regulators are normally used with 2s lipo packs which are 8.4v fully charged, a 2s A123 will be 7.2v, about the same as a 5 cell Nimh rx pack so providing your servos are 4/5 cell rated (i.e. 4.8v or 6.0v) then the regulator isn't required.

 

Note the A123 cell voltage drops quite quick when they are fully discharged so I'd go for at least 6v as the alarm setting.

 

As regards the pump, the pack you've got will give 2.3 amps for 1 hour, how many amps does your pump use?

[Alan Bahia]

Hi Frank,

 

The A123 is a 6.6v battery and not 7.2v (at least is what is printed on the battery label.). The idea to use a regulator is to give constant 6.0v to all servos.

I was using two RX Packs of 6v Nimh in parallel but this meant I had a lot more weigh to carry in flight. Also, having a on-board battery checker, when the batteries drop just a bit (I could see on the on-board checker before flying) then flying pushing the plane on 3D with quick roles for example I use to get quick alarms from the transmitter meaning that the servos was draining the battery to a point that I though I was risking to loose connection with my receiver.

 

Regards how much the pump requires in terms of amps I have no idea. I have visited Sullivan's website and there are no technical data on this but the pump operates from 4.8v to 12v nominal.

 

 

Cheers,

 

Alan

 

 

[Chris Bott - Moderator]

Usually a voltage regulator needs an input voltage somewhat higher than the output voltage. You might run into trouble with this.

 

Also the A123's really do keep a constant voltage until they are suddenly flat. 3.6V off charge but settling to around 3.3V pretty quickly. I recon 6V might be a good setting for your alarm but I'd do some tests to see how much, if any, running time you have after they hit 6V.

Note: Their voltage will depend more on how much current you are pulling than on their state of charge.

[Frank Skilbeck]

Alan, as Chris says the nominal voltage of a 2s A123 pack is 6.6v but its 7.2v fresh off the charger, same with a 5 cell Nimh pack, the nominal voltage is 6v (1.2v per cell) but fresh off a charger they could be as high as 7.5v (1.5v per cell)

 

You say the Nimh cells were dropping voltage during 3D flying, if you were using high capacity AA cells I'm not surprised as they drop voltage quite a bit when on load. The A123 packs are often used at quite high amps for power applications, my 4 cell electric flight pack drops to just under 12v at 26 amps, I can't imagine that your servos will ever pull more than 10% of that. The A123 packs will quite happily deliver much more than your regulator or servo wiring will.

[Alan Bahia]

Right Guys,

 

I have changed the battery for a lipo 2200mah 2c 7.4v and wonder what would be my alarm on DX8 now?

 

What's the safe minimum voltage to get the plane to land?

 

 

cheers,

 

Alan

[Frank Skilbeck]

You should not let a lipo get below 3v per cell, so I'd err on the side of safety and set it at say 6.4v (3.2v per cell), but if you look at this post http://www.modelflying.co.uk/forums/postings.asp?th=11285 then you'll see the remaining capacity vs no load voltage so you might want to go a bit higher.

 

What made you go away from the A123 cells, these are getting quite accepted as a power source for Rx and servos due

 

- Similar voltage to a 5 cell Nimh pack (no regulator needed)

- Safety, you could charge them in the model, which you shouldn't do with a lipo.

Edited By Frank Skilbeck on 29/04/2011 17:36:05

[Alan Bahia]

Hi Frank,

 

 

Thanks for your reply.

 

Due to feedbacks on this thread I am moving the A123 to my other plane with no regulator.

 

Now, as I've mention I've got a lipo which I can easily remove for charging and I wanted something capable to give me constant current and enough capacity to operate my servos and my Sullivan pump.

 

I've noticed that now when using servos the current voltage drops from 6.0v while moving it and my DX8 alarm goes off every time that happens. So, I can't configure my DX8 at 6.0v as everytime my servos drain power it goes down on current and it bips.

 

As my pump when on I can see on my on-board battery checker that nothing happens and I believe that the pump is not draining much.

 

That's where I am at this moment.

 

 

Cheers,

 

Alan

[Chris Bott - Moderator]

Alan where is your DX8 alarm connected to? Is it to the battery side or the output side of the regulator?

 

If its the battery side and the battery is dipping below 6V then something is wrong.