rayg1022

Hey guys I'm new to the electric thing and have I guess a dumb question.. How can I test my battery to fiqure out how long I can fly for on a full charge?
Is there at chart or some math you use to fiqure this out?

BarracudaHockey

My Feedback: (11)

If you have a good charger that tells you how much you put back in you fly a couple of short flights and charge and that will give you an average consumption per minute, then you multiply the capacity of your pack by .8 and thats how many Mah (or a little less is better) that you want to put back in.

So if you have a 1000mah pack, you don't want to be putting back more than 800mah per charge or you're risking damaging most lipo batteries

guver

My Feedback: (325)

2 other ways: (if lipo pack)

Check the remaining capacity after the flight by resting voltage. Kinda the same as barracuda said , but with out the re-charge. You would've still had to time the flight and check the voltage.

Another way is to measure the current average and divide that into the capacity that you wish to use up. It may be somewhat difficult to measure the average current on the ground though.

rayg1022

Still kinda foreign to me here with theses batteries. I have Hobby People TAZR multi-chemistry charger/cycler. People say its not good to run your batteries all the way down is this true? Also when i charge my batteries I have it set on balance when charging. What does it mean to cycle your batteries and how often should that be done?

guver

My Feedback: (325)

It is true , it is not good to run them all the way down. Lipos should be kept above 3.6 volts/cell (resting)
Cycling is simply a back to back discharge and charge (or charge and discharge) and never needs to be done.

rayg1022

ok, so when charging is it good to have the charger set on balance? I guess what I am dreading is getting my plane in the air and run out of battery and have to try to land? gas I had pretty good idea but IM not sure of.

jdetray

Formula for Run Time

run time in minutes = (Battery Capacity in Ah/Motor Current in A) x 60

This is just a rough guide, and it requires that you know approximately how much current your power system draws. A wattmeter will tell you. The formula also needs your battery capacity in Amp Hours (Ah). Divide mAh by 1000 to get Ah.

The simplest method is to measure your current at full throttle and use that number in the formula. In reality, you probably won't fly at full throttle all the time, so your average current usage probably will be less than the full-throttle value. Also, a power system will often draw less current in flight than it does during ground testing.

So if you use the full-throttle current in the formula, it will generally give you a worst case flight time. Actual flight time will usually be greater.

I've included some examples below.

- Jeff


Here are some examples.

Example 1:
1500 mAh battery
power system draws 12A

1500 mAh = 1.5 Ah

1.5/12 x 60 = 7.5 minutes


Example 2:
1500 mAh battery
power system draws 18A

1500 mAh = 1.5 Ah

1.5/18 x 60 = 5 minutes


Example 3:
4000 mAh battery
power system draws 25A

4000 mAh = 4.0 Ah

4.0/25 x 60 = 9.6 minutes


Example 4:
5000 mAh battery
power system draws 25A

5000 mAh = 5.0 Ah

5.0/25 x 60 = 12 minutes

rayg1022

So jeff I have 3 batteries 1 @ 6cell 3200MAH and 2 @ 6Cell 4000MAH what does that tell me? And should theses all be charged on balanced?

jdetray

Without knowing how much current (amps) your power system requires, the cell count and capacity of the batteries does not tell you anything about flight time. To use the formula in my post, you need to know how much current your motor-and-prop requires. You might find this information in documentation for your plane or motor, but the best way to find out is to measure it with a wattmeter.

It is never going to hurt your batteries to balance charge them. Balance charging will help to keep the batteries in good shape, but it does not have to be done every time.

- Jeff

rayg1022

ok I will check that out.. Is it good or ok to discharge your batteries with the charger or not good?

jdetray

With lipo batteries, there is no need or benefit to cycle them (discharge them with the charger then charge them).

Other useful information:

A fully charged lipo should measure 4.2V per cell. For a 6-cell pack, that's 25.2V.

However, and this is where balancing comes in, it is important that each cell be at 4.2V when the pack is fully charged. If some cells are at more than 4.2V, and other cells are at less than 4.2V, the pack is said to be "out of balance," even though the overall voltage of the pack might still be correct.

Charging and operating a battery that is out of balance can damage the cells and result in failure of the battery. Balancing is designed to assure that all cells are charged to the same voltage - 4.2V per cell.

Lipos should never be fully discharged. You should not allow them to fall below 3.6V per cell when the battery is at rest. At a resting voltage of 3.6V per cell, a lipo is essentially "empty" and must be charged before being used.

- Jeff

049flyer

My Feedback: (18)

If you are going to fly electrics you WILL need a wattmeter.

They cost about $40.00 and will answer all questions of this sort. Such as which prop to use, which battery, which motor, flight time on a given battery, what C rating do I need and more. The first time you burn something up because of a wrong guess would have paid for the meter.

guver

My Feedback: (325)

These charts to follow may also help. Also if you post your specific plane and set-up then perhaps someone with a similar set-up will be able to give you an estimate on flight times. The way I do an unknown plane is to take a wild conservative guess and research the manual or posts and then set my timer to that (usually 5 minutes)

I get used to the plane and trim it then measure the remaining capacity when I land. Then I can accurately calculate a new timer setting kinda figuring if I'm going to get more aggressive or how I may fly it in the future. It is important to do this again when making major changes too such as battery,motor,prop,ect.

guver

My Feedback: (325)

100.00% 4.2
90 4.13
80 4.06
70 3.99
60 3.92
50 3.85
40 3.78
30 3.71
20 3.64
10 3.57
0.00% 3.5

guver

My Feedback: (325)

Resting Voltage
4.00V84%
3.96-77%
3.93-70%
3.90-63%
3.86-56%
3.83-48%
3.80-43%
3.76-35%
3.73-27%
3.70-21%
3.67-14%

guver

My Feedback: (325)

another chart

3.7=0%
3.8=20
3.9=40
4.0=60
4.1=80
4.2=100%

jdetray

guver's tables are very useful. Don't let the slight differences between the three charts concern you. In reality, all three are telling you the same thing, which is that you can tell the state of charge of a lipo by simply measuring its voltage.

4.2V per cell = fully charged
3.5V-3.7V per cell = fully DIScharged

And if it isn't obvious by now, a wattmeter is an essential tool for electric pilots.

- Jeff

rc-maniac

Is there anything wrong with just doing a timed run on the ground? Meaning, secure the plane on the ground with a rope to a pole, you just want to hold it in place. Run the plane at half throttle while you time it. The ESC should shut the motor down when the voltage gets to 3.5 (if it is a good ESC, check your instructions). Then do another run at full throttle. This should give you an average time.

I would do it in the cool part of the day if possible since I am not sure of heat will be an issue. I'm a noob to electrics also.

jdetray

Prolonged static running (i.e., on the ground with the plane tied down) at full throttle is quite hard on the power system. A lot of nice motors have been damaged when doing this due to overheating. Even when there are no obvious signs of damage, such as smoke, even one overheating episode can permanently weaken the magnets in the motor so that it never again delivers full performance.

I try to keep full throttle static running to 30 seconds or less. That is plenty of time to measure the motor current with a wattmeter, which tells you what you need to know.

- Jeff