tasesq

Hi All,

It's after midnight, and my mind is just not working that well. I am sure I am right, but would anyone be able to help me out?

I have a Kadet Senior I am building, with LED Nav lights, LED Becaon lights, 12V Landing light etc... I have set it all up using a single 2200 MaH LiPo running though a UBEC for the servo / reciever power, and through a bus / switching / flasher setup for all the lights (the nav's and beacons come on with the illuminated 12V rocket switch (conventional globe; not LED), and the landing light through a Pico Switch!)

The question:

I was wondering how many amps all this was pulling, so set up a little shunt thingy to connect to my MultiMeter (see pic).

With the Multimeter set to 10 the readings were
0.25 with no servo use, and up to 0.55 with both Aileron Servos (Hitec HS85BB) moving back and forth.

0.25 would be 0.25 of a single amp right? Making 0.25 = 250 Ma?
Likewise, 0.55 would be 550 Ma?

I read somewhere that a servo pulls about 250Ma, so by my calculations:

Lights = 250 Ma
2 x HS85BB = 300 Ma (550 Ma - 250 Ma as above)
3 x JR ES539 = 750 Ma

TOTAL = 1300 Ma

So if my LiPo is a 2200 MaH battery, I should be able to withstand this WORST CASE scenerio for 1 hour and 42 mins?

I.e - If I make sure that I never do more than 1 hours flying on the battery between charges, I will not run the risk of getting the cells to low and killing the battery and perhaps the whole aircraft and 2 years of slow labour! (fingers crossed)


I HOPE I HAVE THIS ALL CORRECT - COMMENTS / SUGGESTIONS etc WELCOME!

Thanks in advance.

FallDownGoBoom

My Feedback: (1)

Your math is dead on... .25 amp is 250ma, assuming the meter range is 0 to 10 amps. Which would have your HS85's pulling about 150ma each, so your estimate of 250ma for what I think is a larger JR servo makes sense. And your calculation of 2200/1300 = 1.4hours battery life is mathematically valid.

It also makes sense in real life. I regularly fly a 5 servo airframe using a 600mah battery setup for three 15 minute flights without recharging and have never seen a problem. I doubt that you're going to try to 3D a Kadet Sr, so your average amperage load shouldn't even approach the 1300mah level. The only thing I'd make sure of is that your LiPo is cabable of delivering 90+% of its rated capacity, and that all the cells are reasonably close to the same voltage.

Another way you could check would be to borrow a watt meter (like a Watts Up) and plug it between your battery and the electrical set up in the plane, then turn on everything and cycle the sticks on your transmitter for 5 minutes. This will let you watch the load under realtime conditions, as well as guage the actual mah consumed. It would be a way of double checking the reading on your multimeter.

tasesq

Hi,

Thanks for that - my brain was just not in gear last night!

How to I check that the LiPo can deliver 90% of it's capacity, and also how to i take a Voltage reading across each cell?

Thanks

Quigleywins

Just rember the load is on the servo only when its in the air flying,there will be more current draw when the servo is actually moving air to alter the direction of the aircraft. the more speed and angle of say the elevator the more electrical energy required. yours Paul T

FallDownGoBoom

My Feedback: (1)

Since you're doing some reasonably sophisticated things with electronics, I'm going to assume you've invested in good quality LiPo charger. It should have some sort of "LiPo Discharge" function, where it will put an amperage load on the battery (most folks use battery C/5 for the load, so your 2200/5=440mah, or between .4 and .5 amps). Charge the LiPo to full capacity, then discharge it to 3.0V/cell. Your charger should record the mah pulled out of the battery. Assuming it's more than 2000 mah, you're good to go.

Getting a voltage reading across each cell may be tougher (or at least more expensive). Some of the newer chargers can directly read each cell's voltage; my Triton doesn't do that, but they do sell a balancer called an Equinox that balances the cells during the charging process. If the LiPo cells get too far out of balance, then it won't charge the battery. I was up at the LHS the other day and they had this cute little PC board that showed the voltage on each cell. They said it was about US$15, if you want some more detail I'll try to find out manufacturer info.

For your application I wouldn't be too concerned about balancing LiPo cells, assuming you're using a fairly new battery. Your dischage rates are so low that it won't strain the battery, which is when cells tend to get out of balance. The only thing I'd really watch would be to totally disconnect the LiPo from your voltage regulator when you're done flying for the day. That way you won't inadvertantly discharge the LiPo to below voltage minimums, which leads to buying new LiPo's when you least want to... don't ask how I know!

tasesq

Hi All,

Thanks for the replies - yes the battery is new - was bought for the purpose.

As for the charger, at this stage i haven't invested in a 'good' charger yet. I am just using the GWS lipo charger / balancer as it was cheap (like a budgie!) and seems to do the job.

Perhaps i'll put in a christmas present request to the minister for war and finance for a better one!

Thanks!

FallDownGoBoom

My Feedback: (1)

Ah, the joys of going to the Appropriations Committee...

Even with a GWS charger, you should be OK. One very poor method of approximating how many mah you use would be to fully charge the pack, fly around 60 total minutes, then time how long it takes the GWS to re-charge the pack. If you know the charge rate of the GWS charger, that'll give you a rough idea of how many mah get put back in the pack. That'll give you some idea of how much power you use per flight, and you can plan appropriately.

Let us know how things come out... I've been thinking about using the LiPo/regulator approach.

Fred

tasesq

Ok - great - thanks for all th einfo gents! Always prompt and useful replies!