RX Batteries - how long do they last

[Eifion Herbert]

With most of my fleet being of the electric powered variety, I know very little about receiver batteries, how long one charge will let you fly for, how to test them and charge them, as all my receivers and servos just run off the BEC.

Was flying my Alula today, it has a 300 mAh NIMH pack, my old Futaba wall charger's RX channel puts out 100 mA, so it'll be fine for charging I guess? it's the equivalent of 0.3 C in LipoLand, and should take 3 hrs ish for a full charge.

So I chareged it up, and it seems to work. However how long will it let me fly for? I know it's a small pack, but it's only powering an AR400 and a pair of sub-micro servos. Also how do you tell? My Lipo Checker has a port that I can plug a RX battery into, and it'll give me a voltage and a %age but it doesn't make sense to me. After half an hour or so's flying it was reading down to about 25% on the tester, but still reading about 5.2 volts. Now the NIMH cells are 1.2v nominally, so the pack should be 4.8v right? So how come the tester reckons 5.2v is three quarters empty?

Also how easy is it to run the receivers and servos in a pure glider (or even, - gulp - IC plane) off Lipos? I suspect there'd be a weight saving to be made - or extended flight times for the same weight.

Sorry for the basic questions but it's really something I've not had to think about before.

[Malcolm Fisher]

Hi Eifion,

I don't know if this will help, but a number of yeas ago I flew a two channel glider for about three hours in one flight. The receiver battery was a 500 mAH NiCd and there seemed to be plenty of life in it at the end. At the time I could only guess at the battery state by the rate at which the servos moved. To me there was no discernible slowing down with them at the end of the flight. They had been charged overnight by the standard 50 mA charger which came with the radio.

Malcolm

[FlyinBrian]

Typically I have four 10 minute flights a session with my Aeromaster biplane, this has five digital servos and a recharge normally puts back 350 - 400 MAh so I would say with two small servos in an Alula you would be good for well over an hour of continuous operation.

Do you have any way of measuring the current draw? If so simply divide the pack size by the current draw and take off a margin, say 15% - 20% for safety

Edited By FlyinBrian on 30/06/2013 09:42:08

[Piper Cub]

Nicads \ Nimh cells 'fall off a cliff' when they get close to being discharged. By that, I mean your cells may show a nominal 1.2v and a bit and seem as if they have bags of life in then. So you have another flight and bang, the pack is effectively discharged with very little or no warning.

If you've ever seen an electric RC car run to discharge, yes you'll see the speed drop off but as the pack gets fully discharge, it'll just stop.

There's two ways to check how long the battry pack will last in your model. One is to find out the current draw of all servos, radio and motor and use the resultant figure against battery capacity to calculate time then, allow a 20% margin of error.

The second way is to fix the plane to a work bench then power the motor and servos as if in flight and just measure the time to you notice a drop in motor power (this may take a while and do it carefully!).

No system is foolproof though because even tethered to a bench, its not a true representation of flight neither can a pure mathematical calculation take into account a host of variables such as any servo binding using more current.

If you're really concerned about how long you pack lasts and you want to do more than 4 long flights a day, the simplest answer is to buy a spare pack and swap over every 4th or 5th flight.

We spend lots of money on planes, motors and flight packs but sometimes overlook separate receiver packs which may cost, what, 5 or 6 quid?

[Jim Newberry - Nuviation Ltd.]

When I was flying slope soarers I used to work on a general rule of thumb of 100 ma per servo per hour. So a rudder/elevator model with two servos and a 500 mah RX battery should be good for around two hours' flying with a safe amount of spare capacity. Put the same battery pack into a model with a servo on each aileron and the flight time drops to around an hour. Like all thumb rules it is of course very approximate. If you get a charger that can tell you how much charge has been put into your battery pack you can start keeping an eye on the numbers and you'll quickly get a feel for how long is safe.

[Eifion Herbert]

Yes extra RX packs probably work for IC planes, or electrics where you have dedicated RX/Servo power. Thing is the Alula's RX pack lives under the belly skid, which is taped to the body, so would mean undoing and redoing the tape every time, which isn't great, as not only would you be getting though a lot of tape, but eventually it would start to degrade the foam I would think.

And then you charge the RX packs overnight, but the forecast changes so you don't go sloping, then you don't get a chance to go for six weeks or so later, now are your RX packs still charged? Do they need a top-up? How long for? Will another overnight charge ruin them?

Searching on youtube I have found details of a lipo / UBEC powered Alula, even had a little LED battery checker built in. Seems the way to go to me, with such changable weather in this country we have to be ready to go flying as and when the weather and other stuff allows. The number of times I've planned to go flying, found it blowing a gale, why don't I take the Zagi out asks Mrs Eifion, because the RX pack isn't charged, and it takes overnight to charge it.

I can bring a Lipo from storage to flying charge in about 20 mins. I guess you *could* even do that off the cigarette lighter socket in the car on the way to the slope at a pinch, lipo in a suitable fireproof bag of course, though I'm not sure I'd even risk that, driving and having a battery go pop and fill the car with toxic smoke, better do that onsite and just admire the view while the pack's chargine.

Just need to get some numbers on a source of 2-cell lipos, ubec and see what weight and size options are available, and work out how to plug it all together.

[Piper Cub]

Eifion, to answer a couple of your points, I see it this way.

I appreciate removing tape and re applying may affect the foam but, what's more important, safety or looks? In all honesty, I'd look to change the attachment method to something like velco straps. They are good strong and easily removable causing no damage to foam.

My advice is always charge batteries before going flying. I certainly wouldnt go flying on batteries last charged a week ago never mind 6 weeks without a re charge. A 50mah or 100mah Nicad charger wont damage a recently charged pack. Fast charging them is a different matter!

If you're that worried, you should discharge the pack (its easy to do with a torch bulb or two!) and re charge overnight.

I personally wouldnt charge a lipo pack in the car while driving! I think a fire is more likely rather than smoke of any kind.

[Eifion Herbert]

Don't think velcro would work well for the Alula, the belly pan is a very snug fit and designed to be taped on. I can find many lipos of the right capacity and weight, but not one of the right physical size and shape to fit the existing opening. Whatever I get I certainly won't be charging it while driving!

[Allan Bennett]

Seems to me one problem you've got is, it doesn't matter how well you guestimate the consumption of all your servos, you still don't know how the battery's capacity is holding up -- after a few years (maybe less) the 300mAh pack may only be able to deliver 200mAh of its rated capacity.

When I used nickel Rx batteries I used to charge them on a cycler which would show me how many mAh it had taken out. I got a "feel" for what the right figure should be after an afternoon's flying and a week's sitting, and if the figure was unexpectedly large without any obvious reason, I would ditch the pack. That rarely happened -- my "normal" consumption was about 200mAh out of a 600mAh pack.

LiPos should never be charged in a model because of the risk of puffing and fire, so they're not ideal if you're concerned about regularly unsealng the battery bay. Also, in my opinion, having to add a BEC is a retrograde step when it comes to risk of failure.

An ideal solution for you might be a 2-cell 1100mAh LiFePO4 pack, which is nominally 6.6v and probably acceptable to your radio and servos without a BEC. The cells are in cylindrical metal containers about the size of an AA cell, are lighter than nickel cells, and are safe to charge in your model at high C rates.

Here's an example of a 2200mAh one which, of course, will be physically bigger than the 1100 size I mentioned **LINK**

[SkippyUK]

The link is to a 220mA pack - not 2200mA?
If it's any help, I've used a timer on standard ripmax wall char gers to pump an hour a day into my my zagi's and ic planes just so they are always ready for flight.
I give them 4-8 hours when I come back from flying then resume the hour (during summer).
I've never had a problem over many years and occasionally do a cycle to test capacity of each pack.
Personally i've found the 800mA AA batteries last better than the higher capacity ones.

[John Lee]

Posted by Eifion Herbert on 30/06/2013 11:56:17:

And then you charge the RX packs overnight, but the forecast changes so you don't go sloping, then you don't get a chance to go for six weeks or so later, now are your RX packs still charged? Do they need a top-up? How long for? Will another overnight charge ruin them?

I use a 800Mah Enerloop AAA square pack in my Weasel Evo to get over this problem. Enerloops NiMh batteries do not self discharge to any great extent and will have 85% capacity after 6 weeks from a full charge.

From your first post I agree that the NiMh port on the Smart Guard checker is incorrectly calibrated & the % left should be ignored on that function. You can discharge to 4.8v when you will have about 30% of capacity left but as Piper Cub says the voltage falls off a cliff just below this. You can find the discharge curves on the Panasonic/Enerloop website.

[Eifion Herbert]

Lots of options there. The A123 / LiFePO4 option looks good, might have a look at that for the Zagi where i have a bit more space and weight to play with. The Alula is very tight though - I've got 32g of battery pack and 20g of lead at the moment, might see if there's any suitable eneloop cells that could make up a pack around the 50g mark, a bit more capacity would always be nice. Otherwise I think I'll just ignore the %age output on the smartguard, keep an eye on the voltage and stop when it reads 4.8 volts. Even if the thing does cut out as long as I'm only discus launching on a flat field it ain't going to go that far! Might not want to push it on a slope though. Using a timer is a great idea for keeping the packs topped up and ready to go, hadn't thought of that.

[Bob Cotsford]

52G? How about one of these then? 700mAHr LiFe and in stock in the UK depot. I've been using the 1100mAHr version for a couple of years without issues, that's on .60 powered models so I'd expect the 700 to be fine for two or three hours of flying time with just a couple of servos.

[Eifion Herbert]

That look like it might be just the ticket Bob! How do you charge it? The AR400 RX I use can cope with up to 9.6v input, so will be fine for that, the servos are Blue Bird BMS-306BB units, which the manufacturer list under their 4.8 - 6v category, so should be fine too.

Thanks!

[Bob Cotsford]

You will need a LiFe capable charger Eifion, you can't use a LiPo one or a Futaba/JR/whatever trickle charger but most multi-chargers have a LiFe setting these days.

[John Privett]

I have a 300mAh battery (described by the seller as an "Alula battery" in my Speedo. That powers 2 (small) digital servos in the wing and a Hitec HS55 on the elevator plus a Frsky FASST-compatible rx. I usually fly it for a little over an hour, 90 mins max.

When I've done a discharge before recharging (just to check capacity left in the battery) I've always got at least 90mAh out of it.

[Eifion Herbert]

Just checked through the manual of my charger and it can indeed be switched to do LiFe batteries. Ordered the 700 mAH from HK - just over £2, unbelievable! Twice the capacity of the stock Alula battery for the same weight as the stock battery + balast, and can be charged in under an hour instead of overnight. Fantastic. Even better I can probably hook the charger up to the leisure battery in the caravan for some alula flying while on trips away. Now just need to sort out a charge lead and a balance port extension so it can be charged in situ.

If this works out well I'll probably go LiFe for all my RX batteries in the future. Thanks all for the suggestions.