rctt

My Feedback: (1)

ON a 1650 nimh 5 cell pack whats the typical no fly volts /and at what load to read @ 200 or 300 ma ?

THANKS RON

thomasb

Five-Cell No Fly Volts: 5.99 VDC if you you like to be cautious, 5.94 if you live on the edge. 300mA load.

Please be aware that a NiMH has a very steep v-drop when you hit the end of the charge. If you tend to fly to the last drop then check the voltage before and after every flight.

rctt

My Feedback: (1)

THANKS AGAIN !

thomasb

I should mention that I allow NiCD packs to drop a tiny bit further before quitting time. They offer a little more advanced warning.

mglavin

My Feedback: (31)

It seems common that there is this magically SAFE voltage at which we decide to stop flying, logic suggest's to me there is a better method. A voltage reading is just that. It tells you nothing about remaining capacity, which is what you are interested in.

All batteries have a plateau voltage which they hold for a finite amount of time. NiMH have a slightly flatter one than the say NiCds. Watch your volt meter on your TX sometime. Voltage drops to 9.8 quickly, but then stays there for a very long time and then slowly drops to 9.6, stays there and so on. It is not until the very end of the useful capacity that the voltage drops off quickly.

All the discharge curves I have seen and my own cycling suggest to me that you can burn down to 1.1 volts safely before falling off the end of the discharge capacity/voltage curve.

So while you can measure your RX battery voltage and certainly be safe when you first get to 5.99 volts, you are clearly stopping flying long before you need to. You can demonstrate this by cycling your batteries and realize the actual remaining capacity.

A secondary problem with using a voltage cutoff is that you can be at this safe voltage and then drop quickly to an unsafe voltage. Knowing the voltage again has not really given you any information beyond that the pack is delivering a specific voltage. Knowing the actual available mah capacity available is far more valuable!

Go fly and time all your flights. Cycle the pack at a 1 amp discharge load. Use a 1.1 volt per cell cut off. Record the remaining capacity. Subtract that figure from the last recorded full charge cycle capacity. You now have actual data for capacity used while flying or cumulated minutes of flight.

If you were to consume 20ma per minute based on your previously cycled battery pack's after flight, you could calculate an approximate flight time down to say 60% of the cells rate capacity.

Hypothetically 60% is 1980mah capacity of a 2]1650mah packs. 1980mah divided by 20ma flight load would yeild 99 minutes flying time.

Test the voltage loaded before and after each flight to make certain that a cell has not dumped, connector failure, excessive current or similar.

Actual voltage reading's tell me little about when the useful capacity has been expired. Timed flight, known capacity and mah consumption provide a much better method of evaluating when to stop flying...

rctt

My Feedback: (1)

This sheds some light on why just this evening I was cycling that pack/ And was noticeing the left over charge after 6 volts

at a dicharge of 1.5 amps It went for nearly 30 minutes before
the 4.5 volt cut off ! There would have been well over 45 min
left over of flying time on this pack ///

It seems to have a lot left over at 6 volts //I have been noticing on this 5 cell pack //
Thanks Ron


I keep a close EYE on It to what It actualy cycles

thomasb

Agreed! I worked on a project a couple years ago that was designed to reduce the measurement burden to a simple gas gauge style device. It was NOT voltage based. It reported true remaining capacity of an R/C battery pack (in mAH).

Here is some info on the prototype (beta test version): http://www.digitalproductsco.com/batmon/

R/C hobbyist interest seems to be near zero. Lack of understanding of the pitfalls of the existing methods and the love of the voltmeter test seem to dominate our hobby. Despite its excellent accuracy, no one really cared.

In the meantime, the voltmeter can keep you out of trouble, but as you say it is not the Holy Grail. The extra steps required to determine the remaining capacity are not a big deal, so I recommend them too. A battery maintenance system with mAH measuring abilities will help reduce the effort.

mglavin

My Feedback: (31)

rctt

4.5v cutoff is to low for NiMH battery packs. Or at least it appears to be when viewing discharge curves or graphs for NiMH cells.

.9v x 5 cells = 4.5v this is typical for NiCds. I prefer to see 1.0v on NiCd's for our uses.

1.1v x 5 cells = 5.5v which I believe is a safe stop flying measured voltage under load.... Again this dependent upon your cells and the cycling capacity... Some will very. If you have a volt meter or equipment that displays the voltage while discharging you can view the elapsed time and voltage as they near the plateau or voltage drop off.

Something you should consider:

Your radio equipment will function below 5.5v or at 4.5v all the way down to 3.8v. However there is minimal capacity or current left in a NiMH below 1.0v per cell. They will continue to discharge but will fall off very quickly.

f2racer

I'm kinda confused about your numbers and calculations. Consumption at 20mA per minute would result in 1200mAh... Since packs are are rated in mAh, if you used your 1980/1200 you get some 1.65 hours to completely drain the capacity that you mentioned. Two observations, first is that I don't know of any under 120 sized plane that consumes 1200mAh (you'll see between 250-400mAh for most 40-90 sized planes, and possibly 400-1000mAh for larger 120 to 1/4 scale planes with many high torque servos). Second, you never want to drain a battery down to no capacity. I usually want to stop flying when there's at least 20% capacity left. So let's take your example for 1980mAh of capacity from your battery on a larger model draining 600mAh. The calculation that I'd make is 1980*0.8/600 which gives 2.65 hours of flight time. I'd probably stop flying after 2 and 1/2 hours of RX on time. I can fly my 40 sized mechanical retract equipped Mustang (6 servos) for hours before needing a recharge...

mglavin

My Feedback: (31)

Do the math.

1980mah @ 20ma is 99 minutes * 1 hour-39minutes * 1.65 hrs.

Looks like it's all the same to me......


Yes, I agree you do not want to fly/drain a battery down to rated capacity! That's why I suggested the stop flying time of/at 60% of the batteries rated capacity. I have allowed for a safety margin of 40% of rated capacity. Most batteries actually deliver more than rated capacity for our use. Cycling will validate this assertion.

You missed the statement that the 1980mah is 60% of 2]1650mah NiMH battery packs.

2 x 1650 = 3300mah
3300 x 60% = 1980mah

Larger models with hi-power servos will draw more than 10ma per minute......... 20ma is more realistic. Obviously the load is dependent on the servo count and type. 20ma represents an average draw for [8] servos on a 33% model...

You certainly could fly your model down to 20% of it's rated capacity. Problem I find with this is there is little reserve and if you viewed a discharge curve you'd see that after 80% of the rated capacity has expired the voltage begins to fall off sharply.

There is little room for error or safety margin at 80% battery depletion... If you were diligent and monitored your battery pack, were aware of it's cycled capacity, kept track of flight time and new the average flight load [ma's consumed] you successfully can run them down to within 20% of rated capacity. Again I would not recommend it...