AA Rx Batteries

[Steve Hargreaves - Moderator]

Come on now Peter...surely you aren't suggesting that manufacturers overstate the capacity of their packs are you...that would be plain silly !!!

 

But you raise an interesting point about voltage monitors......I fitted monitors to all my models after a dodgy Rx battery nearly caused the demise of a favourite. I had bought a brand new Sanyo pack & fitted it to a model...charged it up the night before...everything seemed fine..took off, flew around fine for a few minutes then on an inverted & low, slow pass the motor dropped to idle.....help....bicycle clips time....!!! Thinking it had gone lean I pulled the throttle back to idle & forward again & the motor picked up.....phew!! Later on it did it again...back to idle but picked up OK when I closed & re-opened the throttle....then it dawned....I had a PCM Rx in this one set to cut the motor to idle on low battery.....Aaarrgghhh!! land, Land, LAND!!!! Luckily I got down with no problems....later testing confirmed that a brand new battery was junk!!!

 

Since then I've fitted voltage monitors & this has added to my paranoia when they do just as you describe & flick down towards the red when "stirring the sticks"...it is largely this which has caused me some anxiety & driven my search for an Rx battery which will hold up the voltage when the servos are working hard.

 

This throws up an interesting conundrum regarding voltage monitors when A123 cells or even a LiPo plus regulator is used as both these methods will show exemplary voltage stability & maintain the top green light on the monitor right up until the very last gasp of the battery which kind of renders them a bit useless wouldn't you say? It was for this reasoon that I tried to use a Lipo regulator with a low voltage alarm that sounded when the LiPo voltage dropped......I think you joined me in that thread too with all its attendant problems regarding interference from the inverter

 

Personally I am convinced that the A123 cells offer the best way forward, simple, rugged etc etc but I do still have some concerns about the higher voltage & how it might affect servos etc... I think I will have to bite the bullet & try some out!!!

 

Tried out my new Instant Rx packs this weekend....the green LEDs stayed lit even after a fair bit of stick stirring which hasn't been the case lately & do my eyes decieve me or are the sevos a tiny bit faster too......

[Peter Beeney]

   Steve,
            I’m not suggesting that some manufacture’s overstate the capacity, I’m actually saying they do! And yes, you’re right, I am plane silly, I should know better at my age!

   I’m afraid I don’t have the luxury of a PCM radio with a low battery volt throttle shutting mechanism so that I can fly low inverted to test my new battery pack; so I simply have to do it the other boring old way, that’s testing the pack before I install it.

   The voltage monitors. My take on these might go something like this. They are actually an expanded-scale voltmeter, over perhaps one volt. A 5 cell pack Ni-MH might be considered flat at say 5.3V, if the load voltage stays above 6.3V then there is no danger of it being very discharged so we can ignore anything above this. So the scale might be between 5.3 and 6.3 volts; if you have 8 leds say, then each led represents a 125 milli-volts change. I don’t think the flickering leds are a problem, this is simply a measure of how sensitive they are, they are just responding to the volts drop over the wiring harness etc. Some of this might be due to cell resistance, too. Maybe a spot of damping would be in order, a small can of capacitance close to the input of the monitor might do the trick.
   Regarding the A123’s, interestingly enough I’ve just recently done some accurate discharge tests at different rates for a heli friend who’s keen to use them as a rx pack in a new heli. The discharge curve is flat but that’s not a problem. The snag is the difference in voltage. The range for 2 123’s might be from a low of 6 volts up to 7 volts; so straight away we can see that when the pack is flat the monitor still thinks it’s operating more or less in the fully charged Ni-MH area. Hence there is no flickering either, the high voltage is keeping the monitor firmly on.
   Possibly there will be monitors for A123’s later, one interim way might be to make a voltage divider to adjust the voltage difference.

   However, one other way would be to ignore the monitor all together. I reckon you have a ‘safe’ 2Ah capacity in a large 123. If you consider that the average sports model consumes 150 mA’s per hour, that’s a complete guess, then that’s 13 hours constant flying. Even at 500mA, that 4 hours; and I’d say 500mA is a very big bird indeed.
If you check the capacity of the pack carefully, make good solid solder joints using solid copper wire and a good wiring connection that will not fail you. I reckon you could have 2/3 hours solid flying without even bending the battery. Modellers have done that for years. Very few people seem to consider the battery capacity, and providing the pack is in good nick, get very little problems. At least in terms of capacity. Simply because they never ever get anywhere near to using all the watts in there.
   I started out on Sunday to try and do some sort of power consumption test but it was a good sunny day, I got very busy, became side tracked and lost it! I’ll have another try, though.

   I’m also about to try and cause some confusion amongst a pack of Eneloop’s, hopefully they will confuse me!                       PB

                                                                                                                                        

[Steve Hargreaves - Moderator]

Our good friend Mr MacKey has done some interesting workmin the field of servo current consumption...see here....

[Lee Smalley]

jUST A SIMPLE NOTE 

 

if i charge my pack at 2 amps the wires to the pack get warm, this indicates they are approaching the limit of there current handling capacity  i would be very worried if any of my packs are being pushed to 3 amp draw, that would indicate  a problems with the servos you have to be worried if 5 servos are pulling 3 amps 

Edited By Lee Smalley on 19/10/2010 12:59:36

[Steve Hargreaves - Moderator]

Totally agree Lee & as you say if 5 servos pull 3A continuously then we have a problem as the system is not designed for this sort of current (as you have found with your hot wires) but the interesting thing is the transient loads & how the voltage of the pack holds up under them......

[Lee Smalley]

yeah definately and i think i will do soem tests when the weather is too rubish for flying i will loan some test equipment from work to run some proper tests of servos under loads and bat voltage during i use the vapex instants 6v packs and i have never had a problem !! but it would be nice to know !! one for when the weather is rubbish i think !!

[Steve Hargreaves - Moderator]

I've just started using 4 cell "Instants" & I'm pretty impressed so far (one flying session!!!) they seem to hold up the voltage pretty well with very little flickering of the monitor when I stir the sticks...this compares to my other batteries that used to drop right down into the red LEDs when all the servos were in action bfore recovering into the green.....

 

How do you treat yours? I was wondering about instigating a new regime whereby I discharge the cells to about 1.1v/cell after a flying session before charging them back up again...my reasoning is that I understand these cells like to be excercised (ie discharged & recharged) rather than just topped up & then having recharged them fully I can take them straight out & fly 4 or 6 weeks later with no problems.....

 

Any thoughts??

[Lee Smalley]

well i have been using them for about 2 years now and i never do that until the end of the flying season i cycle them once a year other than that i just top them up, i did hear that they do not like slow charging so i now charge at 1 amp until full, my charger detects this, and its fine for me never had a problem mine last for about 3 months without serious loss of capacity but i allways check before flight anyway 

[Peter Beeney]

   Lee,
        With the greatest respect, and I’m not intending to contend your view in any way, but I am a little intrigued by your statement: ‘if i charge my pack at 2 amps the wires to the pack get warm,’. Can I then ask, please, is this just the charger wires or the wires from the plug to the battery? Also do you have any indication of the actual temperature rise?
   The reason I’m asking this is because in the past I’ve applied some pretty hefty currents to some Futaba battery leads, this was to see at what sort of current levels the insulation melted and caught fire. Needless to say, these were not connected to the battery at the time! I have to say, in my opinion anyway, with all things being equal, you would almost need a contact thermometer to register the temperature rise when charging at two amps; so do you have something that’s slightly different?
   Regarding the 5 servos, if all five operated simultaneously then the inrush current might total 3 amps, after all, that’s only 600 milliamps each, but that would quickly fall away as the motors ran up to speed. I would personally have no problem with rating the Futaba switch harness at 3 amps continuous, but I think, that in the main, the load in yer ordinary model is an average of milliamps rather than amps.
   As a fiery footnote, it is definitely possible, in the right circumstances, for a standard rx battery to heat up the connecting wires to the point where the insulation melts and catches alight. I have seen the charred remains of one such happening, including the little scorch marks on the carpet where the burning insulation dripped off the red hot wires.

   There is another newish kid on the block, the Maplin Hybrid. I’m reliable informed that these are currently King of the Hill! One of it’s advertised claim’s to fame is a a good performance at low temperature. So, a discharge in a Cold Climate might be the first order of the day.

   Steve,

         A little contrivance that might go some way to fixing the volts discrepancy problem with the voltage monitor and the A123’s would be to pop a couple of forward biased diodes, in series, in one leg, that would lower the voltage at the monitor by around 1 - 1.2V. Together with a capacitor this might just make something a little bit more accurate. I shall see my mate with the Travel Air tomorrow, I’ll give him all this guff about the monitor and see what he says. He likes playing with a soldering iron!                           PB

                                                                                                        

[Steve Hargreaves - Moderator]

Hi Peter...yes I've wondered about incorporating a couple of 5A diodes in the the battery harness to drop the voltage slightly....(plus a reverse biased one for charging too).....

 

It might assuage my paranoia about the higher voltage of the A123s....

 

I'd thought about mounting them on a small pice of veroboard covered with heatshrink to make a nice job.....but then I'd get to worring about how much current the copper track on the veroboard could take........

 

You see I really am a hopeless case!!!

Edited By Steve Hargreaves on 19/10/2010 22:32:00

[Bill R]

Hi Guys,

did a little test on a NiMH pack of mine [2100maH] On my tester, which is little LED's, 1 red 2 yellow and two green the pack showed 2nd green dropping to first green on aappplication of a 1/2A load. the voltmeter showed 5.15v open circuit and 5.01 volts with the load on. I applied a 11 ohm resistor of 25watts and the current drawn  was 487mA at 5.01 volts. After 1.5 hours the volts had dropped to 4.95 volts and 467 mA current draw.

This is continuous load. The battery was still cold, leads etc cold. resistance quite hot to the touch. At this stage the led showed yellow [medium] with no change on 1/2A load.

The next step is to fully charge the battery and exercise the Servo's to check the load ciurrent.

Sseems like you can fly for at least 1.5 hours on a 2100maH  NIMH battery without appreciable loss of power.

 

regards,

Bill R.

[Chris Bott - Moderator]

Steve I'd be careful about putting in a reverse diode for charging, because then the battery would not reach the chargers set voltage.

 Maybe a second connector connected straight to the battery just for charging? 

 

Personally I'm a big fan of the Sanyo Eneloops. (and of course A123's)

[Lee Smalley]

i did not use any more than the human finger to register that the leads were a tad warm at 2amps charge it took about half an hour for this to happen and did not concern me these were the wire to the pack the newer packs come with slightly bigger silicone leads and should be better all i know is that in my 20cc yak with older 5kg servos (hitec) i can fly all day about 10  flights of 10 mins and i hove only lost 20% tops from the pack (2200mah) so its fine for me, i doubt that even on switch on the servos demand 3 amp but when the weather is rubbish and i have nothing better to do i will loan some serious equipment from work and do some tests when i do i will let you know until then im off flying !

[Steve Hargreaves - Moderator]

Good point Chris...I didn't think of that...

 

Lee, so you use a standard 4 cell AA pack in your Yak yes....Wow...that makes me think I've been worrying about nothing after all...(collective groan from the Forum!!)

[Peter Beeney]

 

   Steve,

          Yes, I think if I were putting any components in line I would always put them in the switch harness; thus you would not require the charging diode and even if you used Veroboard you would tend to solder the wires directly on to the diodes so there would be no problem with the tracks. If you think the track needs to carry some current, run a thickish coat of solder over it, to increase the current carrying capacity.
   I’ve spoken to my Travel Air colleague about the monitor, he was already ahead of me and thinking along similar lines, so I’m going to calibrate a couple of monitors for him, that’s making a note of the exact voltage at which each led extinguishes. Then hopefully we will know where and how we want to adjust the supply to the monitor on the A123’s.
   He’s also heavily into servos, he has a shed full and he uses the Spektrum DS821 as a standard servo for i/c models; in his comparison tests he reckons these are as good as any, and better than most. Although he doesn’t use Spektrum radio. Also you can sometimes buy them relatively cheaply too, I understand, because the helicopter flyers don’t use them, and they get taken out of new sets. He’s more than happy that they run ok at the higher voltage, maybe they might wear a little quicker, but, as I said, he’s got plenty!

   I managed to do a sort of ‘flying capacity check’ today, I’ll look at the info. later and see if it is worth posting a companion thread. It might give us a clue as to how much airborne power we do actually consume.                   PB

                                                                                                                                                

[Lee Smalley]

Steve i use 6v packs in all my stuff as when i started flying again it was recommended by spektrum users so i just bought loads of 6 volt packs, its a simple as this for me big model lots of servos (higher torque ones as well) = 2 packs and a regulator /backer unit everything else gets a standard 6v 2200mah vapex instant pack and i have never had a problem, but it would be nice to known what the current drain is whilst flying, i bet even 7 servos doing a blender will not pull anywhere near 2 amps

[Steve Hargreaves - Moderator]

Cheers Lee....thats food for thought...

 

I did wonder about using two 4.8V packs feeding the Rx via a "Y" lead...mind you I might as well just go for a set of Sub C cells instead....the weight would be the same!!