Mranga
Are you using 4 or 5 cell packs?

you will love gas! long flight times and cheap fuel! well, cheaper than batteries or nitro

kmeyers,

Iam using a 4 cell pack (4.8 volts). Why is this relevant?

Thanks

This is another thread (not related to the original question)but what are the smallest available gas engines equivalent to (compared to an equivalent nitro) ? I understand they are much more reliable as well.

Ranga

JBA makes a 15 cc. One would use this on a .60 sized plane. That is the smallest I know of. Although I do know some folks that have installed it on a hangar nine twist .40. Gas is very reliable and once it's tuned you rarely have to ever mess with it again. Most guys file with two batteries, one for the engine ignition and one for the Rx. You will also need an opto kill switch. This allows you to kill the gas engine is something goes wrong. There are two basic types of kill switches. One that works directly with the engine ignition and battery and another types that will work with just one battery including the igntion. Which eleminates the need fro the ignition battery, thus savign some weight. The switch is activated by a switch of your liking on the Tx. Some guys will also install a micro servo on the choke of the engine as engaging the choke will also kill the engine. Ignition batteries for most gas engines cannot exceed six volts. Another thing to remember is that you have to use a gas compatable plug for your fuel tank. Nitro fuel tank plugs will basically melt when gas contacts the surface. Special fuel tubing is need as well. And the clunk line in the tank should be checked regularly (once per year is what I do) , gas causes the clunk to harden when not used for awhile. I change out the clunk line every year just to be safe. Flight times are the best thing about gas!! I have a 24oz tank on my PT 19 that has a 26cc SPE gaser on it. I can fly for 45 minutes easy and have plenty of fuel in the tank for landing!! I have never flown for 45 minutes at a shot. but after 25 minutes, I still have over a 1/4 tank left.

If you are using 2.5 rx, a 4.8 will not provide enough power and could cause your rx to shut down in flight, (known as brown out). the 2.4 rx will reset itself but by the time it does, plane is usually on the ground. If you are flying 2.4 you need at least six volts for the Rx pack. Irun at least 5 cell, six volt packs.

The very best battery info, period! This guy knows what he talks about and his batteries are the best. http://hangtimes.com/rcbattery_faq.html

Constantly recharging batteries before they get down to a cut off voltage is not good for them either. The best way to know the condition of your batteries is to use a loaded volt meter of a least a 250 mAH load for sport planes and 500 mAH load for giant scale or 3-d planes. Also you need to load test your batteries periodically on a charger to know what they are capable of. As a general rule once they deliver less than 70% of their rated capacity they get replaced. Again the best battery FAQ out there: http://hangtimes.com/rcbattery_faq.html

I hope you are assuming 250mA for EACH servo average under flight loads when NOT at stick center.

If not you are underestimating.

A standard servo can pull down 2000mA at full stall. Ideally that should NEVER happen, but 250mAh and above is fairly normal at higher deflections.

At rest the load drops down to a small fraction of that.

Put a meter on your pack and move ONE control surface, say the elevator... while holding it like this, put a 4-8oz weight on the trailing edge of the surface and see how much draw you are getting from the servo... you will be surprised!

IMHO you should calculate a minimum pack size for a standard servo by taking 250mA x the number of STD servos used, to give you a good safety margin and proper duration for 4-6 flights without worry.

I always thought the best way to see the condition of my packs was to cycle them. When they go below 80% of there rate it's time for a new pack. I do cycle my packs about 4 times a year, more often if I have nothing better to do. Having a good charger on hand that will cycle is something I think of as a must have. My loaded volt meters only show me the packs voltage under load, if there is a bad cell in the pack it may not show up. I have had packs drop there voltage after one flight, when I get home that pack gets a couple of charges and cycles so I know what is going on with it. Field charging is hard on NIM packs, they don't like the heat. I have noticed over time that if they get quick charged they don't last as long. Maybe two years instead of three. I still use the fast charger at the field, replacing a pack a bit early is a lot better then not flying that plane the rest of the day.

Opjose,

Correct. I am assuming 250 ma per std servo and 100 ma per micro servo, giving me a total draw (for this plane)of 550 ma. Thus, an estimated flying time of about 1.75 hours.

Thank you for responding,

Best regards,

Ranga

Very good!

What size and kind of plane is it?

Opjose,

I made a mistake : 2 standard servos x 250 ma +3 micro servos x 100 == 800 ma. So I'll get about 1.25 hours of flying time with 1000 MaH. Safe to go 4 flights at 10 minutes each flight.

The plane is a biplane ( 40 size with micro servos for the aleorons and for the throttle ).

Thanks,

Ranga

I agree that the static load will not equal the dynamic loads; however this should give a rough estimate. Bad batteries would quickly be recognized with this method, and assuming you took voltages every 10 min you could compare your inflight numbers to the “recorded data� and find out how close (or far) the method really is. The point I was trying to get across is batteries last longer than you think, but always compare battery voltage to known data
Just a warning, small batteries can’t deliver high current. Most 350mah nimh batteries are good up to 2-3 amps, go over that and your voltage drops like a rock.

I agree. Smaller packs seem to have a critical voltage at a higher percent than larger packs. It is why I work down to the size of pack I want in the model. The price of small packs is less than 10 bucks, and with careful cycling, monitoring, et cetera, I can use a very light flight pack. I'm to the point such that switching out a rx pack every flight is worth it compared to hauling around dead weight.
As I said, I am flying some pretty light designs.

Having said this, I have some 1500 and 1800 mAh LiFe packs I haul around in trainers and put in 45 minute flights with students and don't worry a bit about battery capacity.

Disregards servo when no load I'm mean when it's on the ground. Timer start from taxiing to engine shutdown after landing. Actually your servo load is based on the size of your control surface and speed. also weight of an aircraft affect energy requirements. Considering your data, take the ff:

milliampere hour requirements, mAh

mAh = average current consumption of all servo * time * 1000 * 1.25

time = flight time in hours


Power requirements = servo voltage * (full load current of all servo + 25% of highest pair servo current) / 80%

Let me know if you have any question.

Sorry your looking for time, just reverse it, thanks...

All of the aboveis good advice. Know your batteries capacity and the usage of your plane. I have an easy method that has worked well for me.
Cycle the battery a couple of times and fully charge it. Make a resistor lead for your volt meter(I use 20 Ohm 10 Watt). Test the voltage of the fully charged battery after a few seconds of load when the voltage stabilizes. Leave the load on it and record voltage every 5 minutes until the voltage falls rapidly. If you graph the results you will see the voltage drops very slowly over its capacity, and falls off at the end. Back up 10 or 15 minutes from where it begins to fall off and mark this voltage on the pack. Test the voltage every flight, and Do Not take off when the voltage reaches this point.
You will find out very quickly how many flights you can get out of a fully charged pack. Remember, Nicd batteries have a memory, and they do not like to be partially discharged and recharged. You can charge at the field if you need to get a few more flights, bud it is better for the battery if you don't. From the sound of it, you are not even using half of the capacity of your batteries.

I've seen some post here, taking the 80% capacity of a fully charged pack and devided by the sample data taken from one flight (mAh used per flight) and you'll get flying time.

Are you talking a 4 or 5 cell pack? If 4 cell why are you charging when the voltage gets close to 4.8? Seems to me you are wasting most of the battery charge. Also do you ever slow charge or do you always use the field charger?

I would base the battery size more on weight than electrical capacity... I would never fly a 600mahr pack when the weight of a 2300 pack is almost the same.

I never fly 4.8 batteries ... I have switched entirely to 6 volt 5 cell batteries. More head room before a brownout. One crash due to batteries and you will start thinking the same.

I never run a NiMH down to zero. When the charger says I have put 2600 mahr into a 2300 mahr battery (example), I know that it cannot be because of losses and heat produced, therefore when I put 1800 mahr into a 2300 mahr battery I know that did not happen either.

I know weight is important, but flying safely with more battery power than necessary is more important.

The best advice I could give you is to PM Red Scholefield.  Red knows more about batteries than probably the total of the rest of us have forgotten.  The key is to "do what he suggests".

hi
i only use NICADS for 20 plus years in two cycle glo planesonly batteries there were 20 years agonever a battery issuewith proper maintenance
if the battery cycles twice every 2-3 months i will fly it if it has not lost 20 % of its capacity
always cycle a new battery 2-3 times to bring up its capacity ( this is the number you will deduct 20% from in the future )
do not care how old the pack is it has passed the required cycle testI FLY IT
slow charging is always best
keep NICADS fully charged when storedprevents shorts
find that the peak chargers i have used have shortened my battery pack life a lot over slow charging at 10c or trickle charging which is best
fast charging is damaging your best cell in a packas that cell is almost at a capacity charge when you start the charging

you will feel the servo's get slower -when the battery gets weakeras you are flying the plane learned this from my trainer flying for 30 years

get my best results charging NICADS with a wall chargerand cycling with an ACE DIGIPACE- went back to doing what i used to do after using a peak chargerthat truly shortens battery pack life far to much for my liking
BEST REGARDS TONY

Tony here is some battery advice I got off the net. Dan.
http://www.camlight.com/techinfo/techtips.html

*Q: What’s ‘cycling’ and why do I need to do it?




A: Battery‘cycling’ in it’s simple form is the two step process of charging and then discharging the battery under controlled conditions. Normally the purpose of the procedure is to ‘condition’ or ‘format’ a new pack or to verify the capacity or check the condition of a pack that’s been in service. ‘Routine’ cycling, or discharging before recharging a pack after every use is not necessary for either NiMH or Nicad packs, and discharging to ‘zero’ voltage under load is in fact harmful. A ‘cycler’ or controlled discharger will limit the discharge voltage level to prevent damage to the pack, whereas ‘leaving the switch on’ in your radio system to discharge a pack is very poor practice and may result in damage your radio system components or the battery.

-


Most ‘hobby’ cyclers or dischargers operate at a predetermined load and cutoff level that is different than the cell mfg’s rating system. This leads to discrepancies that can be a high as 10-15% of rated capacity between the mfg’s rating and the rating presented by the cycler or discharger you are using. If your cycler or discharger system’s numbers are lower than the comfort zone described above then you need to verify the calibration of the cycler and check the charge rate and charge time to insure the pack did indeed get a full charge before the discharge function began. If the numbers are still uncomfortably low after re-running the test with your new pack, we suggest you contact the assembler or retailer for evaluation assistance in determining what the cause is before using the pack in a critical application. Any pack cycling more than 25% below the mfg's rating or the original numbers established for the pack with your equipment when it was new should be removed from critical application service.

*


Q: How do I know if my pack is fully charged?

*


Temperature is the key.. always, always, always! IF THE PACK AIN'T WARM AT THE END OF THE CHARGE ROUTINE, IT AIN'T CHARGED! Note I said 'warm'. Not HOT! Hot is NEVER GOOD at the end of the charge routine.. but warm is OK. If using a temp probe, set it for 10 degrees above AMBIENT. If it's a peak controlled charger and the charger shuts down before the pack is slightly warm to the touch.. IT AIN'T FULLY CHARGED. If it's a timer controlled charger and at the end of the charge period the pack is not warm.. it's NOT fully charged yet! By far and away the biggest reason for low capacity numbers is undercharging.. if you have a 5 gallon pail with only 3 gallons in it, your only gonna get 3 gallons out of it. Engage your brain.. check temperature at the end of the charge routine!

*


Q: Temperature?? I thought voltage was how you could tell the pack was charged??

*


A: More packs have been fried by guys looking for a 'number' instead of temperature on a charger than any other cause of premature battery failure. The 'finish' voltage of a pack will vary under an astounding number of variables.. but temperature rise ALWAYS signals that the cells are charged in a slow charge routine. Why? Because when the cells can no longer absorb the energy being shoved at them by the charger, they begin to give the unstored energy off as heat.

-

Taking the Temperature story to the next level.. fast charging. Here we can generate even more heat. Some high-impedance cells like AA NiMH's can get warm when being charged at 'normal' 1C (fast) or even 10% (slow) rates. They can get warm BEFORE they get to full charge. Why? Because high-impedance cells don't absorb energy as efficiently at high current levels.. just like they don't give up energy efficiently at higher discharge rates. So.. before you crank up the current on your charger on your new NiMH Txpack.. check the pack label on our packs.. and set the charge rate recommended; which is often LESS than the 'industry standard' for NiMH cells.


Final word on temperature: Aside from being a key indicator for correct charge time, rate and final charge condition bear in mind that heat is the bitter enemy of NiMH Cells. Drive the pack into thermal overload (hot) on the charger and it's likely you'll ruin it. Pay attention to the temperature!



Taken from http://hangtimes.com/rcbattery_faq.html