radio testing

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radio testing

Postby no1dieselman » Thu May 05, 2016 9:28 am

Since the loss of my Blunderbird following loss of control I am getting a bit paranoid about my other models. I have checked thoroughly the installations and checked all contacts are clean. Every model/tx combination was range tested OK and finally I am checking the batteries. I have been giving them a full charge then leave them for at least 3 days. I then discharge at 100 milli amps. To save time I have taken 75% out of each one but they show good voltage stability. Is this a good enough test or do I need to discharge at a higher rate?. A lot of my batteries are only 200mah so didn't want to punish them too much.
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Re: radio testing

Postby Phil_G » Thu May 05, 2016 12:20 pm

Are these the batteries you're using for flight packs Doug?
200mAh is very small, even 'indoor' sized, so I take it they're lipos? whats the setup, a 2S and regulator? or just a single cell?
Are we talking about your diesel powered models so the load is a receiver and two 'analogue' servos?
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Re: radio testing

Postby no1dieselman » Thu May 05, 2016 1:32 pm

4 cell nimh just driving one servo in the small diesel/glow powered models. They are plenty big enough Phil I use 50mah on the small models like the 010 powered model you saw
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Re: radio testing

Postby no1dieselman » Fri May 06, 2016 12:33 pm

Can anyone confirm what sort of current 2 servos and a rx take and if my 100ma discharge test on a rx battery is realistic?
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Re: radio testing

Postby pchristy » Fri May 06, 2016 6:27 pm

Doug,

The current draw of a receiver is trivial and can generally be neglected. The servos stationary should similarly be very low - a few mA at most. The current draw of the servos whilst running and under load should be easily measurable with a milliammeter. For a small servo, I would be surprised if it exceeded 100mA, but some of the newer little digital servos could draw a bit more. What is difficult to measure is the startup current as the motor starts to move. This could be quite high. The resistance of an (analogue) servo motor when stationary is usually less than 10 ohms. That means that at the instant of start-up, it could be drawing 500mA - half an amp! Of course, as soon as the motor starts to spin, the back emf will build up and reduce the current draw - but you will still have that brief spike when it starts.

Now consider the batteries. These will all have an internal resistance which will limit the "short-circuit" current, but will manifest itself as a voltage drop when a high current is demanded. With a new cell in good condition, this voltage drop would normally be minimal. BUT (there's always one of those!) battery manufacturers have got involved in this capacity race - who can cram the most capacity into the smallest cell! Now for NiXX cells, the rule seems to be that the greater the capacity for a given size, the higher the internal resistance - and consequently, the bigger the voltage drop when a servo starts up.

I haven't measured it recently, but I do recall that around the time I was designing my first FM receiver (on 27 MHz!) I could measure a 0.5 volt drop on the battery voltage when a servo started! OK, we were still using DEACs back then, but the principle still holds true. It was a big problem when designing an FM receiver. AM sets using an active detector typically spat out 4 volt pulses to the decoder. If these wobbled by half a volt, it really didn't matter much. But in an FM receiver, the recovered pulses were typically only about 0.4 volts - and bouncing around by 0.5 volts! Building a decoder that could separate a 0.4 volt pulse from a 0.5 volt bounce was the biggest headache I had designing that receiver!

More recently, I have an old Hirobo Bell 47 helicopter with a 20cc spark ignition engine. The original Hirobo ignition system would run quite happily on 2000 mAH Eneloop cells. But when I changed the ignition to a modern Rxcel capacitor discharge system, the power loss was such that at full chat I might get half an inch of daylight under the skids if I was lucky! It took me a long time to figure out the problem, and in the end it was Steve Roberts at MacGregors who pointed me in the right direction. I replaced the Eneloops with 3200mAH sub-C NiMhs, and all the power came back! It wasn't the *capacity* of the cells - it was the higher internal resistance of the pen-cells - compared to the Sub-Cs - that couldn't provide enough *peak* current to operate the ignition properly.

So how do you test for this condition? Well, it isn't easy! If you have access to a decent 'scope, clap it across the power supply (batteries) and see how much the voltage bounces by when the servos are operated. If you see bumps of more than half-a-volt, then use physically bigger cells!

In short, there is probably nothing wrong with your batteries for the kind of constant current application for which they were designed. However, it *may* be that they can't cope with the short transient currents being demanded by your system. Your only solution then is to use a physically larger battery.

Hope this helps.....

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Re: radio testing

Postby jackdaw » Fri May 06, 2016 6:43 pm

Had a search through my links as I remembered some scope traces of servo tests http://www.bhabbott.net.nz/Servo.html
For batteries I would choose a couple of small LiFe cells in series to give a nominal 6.6v (head room for any potential voltage drop) and probably no heavier than your current batteries. Ebay may be a place to go as I got some from there in the past. If you want I could weigh a selection and post results.
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Re: radio testing

Postby no1dieselman » Fri May 06, 2016 7:08 pm

Thank you for your replies.
I am confident that in good condition the batteries I use are up to the job, I was looking to try and detect any possible loss of performance. What I have done in the past is twiddle the sticks for a few hours whilst watching tv but was looking for something a bit more scientific-the noise from the servos get irritating after a while.
I have discharge tested all packs and they are within 10% of claimed capacity. I then loaded them at 250ma and the voltage drop was 0.1 to 0.2 volt so I think I can safely say they will be fine for use.
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