Is it possible to calculate amperage drawn by the amount of mah taken out per flight?

[RcRotaryMadness]

I'm curious to know if its possible to calculate an average current draw from the following:


Fully charged LiFe pack
one 5 minute flight
recharged and put 100mah back into the pack.

5 mins / 100mah = how much amperage drawn?


In short, I need work out if a 850mah 5C LiFe pack will be ok?

850 x 5 = 4.25 constant amps

4.25a sounds very little but if a 5 minute flight is only taking 0.1 amps from the pack, then surely the current draw can't be that bad?

I am using futaba BLS451's and BLS251 on the tail.. so all low voltage.

Matt

[Mark_T]

That looks like only about 1.2A average over the flight.

[ghost2212]

As Mark says based upon what Mark says your pulling an average of 1.2A a flight. The problem is that there are likely spikes of 10A easily i would think which could pull the voltage on the LiFe down too much and affect the consistency of the servo's. If I was you i wouldn't do it, Why do you want to run such a small pack?

[Mark_T]

Pretty easy to test a pack that small on a PL8.

The 5C rating isn't a hard limit, stick it on a test discharge at 4A for 30 seconds and see what happens to the voltage, increase to 6A, test again, repeat until you hit 10A or you see a major voltage drop.

I'm always impressed at how little the BLS servos actually draw.

[RcRotaryMadness]

Pretty easy to test a pack that small on a PL8.

The 5C rating isn't a hard limit, stick it on a test discharge at 4A for 30 seconds and see what happens to the voltage, increase to 6A, test again, repeat until you hit 10A or you see a major voltage drop.

I'm always impressed at how little the BLS servos actually draw.

Thanks for the info guys and Mark thats a top idea!! I will give one a test see how well it performs.. if it can provide close to 10 amps for a brief moment then I think it will be fine.. as you say these BLS servos draw very little current!

I did have some HV Outrage servos before which were drawing about three times more..

[RcRotaryMadness]

Why do you want to run such a small pack?

For reduced weight but more for where I want to fit the pack.. instead of strapped to the side I would like it mounted inside the frame up top near the anti-rotation bracket. Needs to be compact you see..


Also how did you go about calculating 1.2 amps as the draw?

[Mark_T]

Also how did you go about calculating 1.2 amps as the draw?

100mAh would support a drain of 100mA for 1 hour

If that was used during 5 mins then you would use 12 times that in an hour.

so 100mA x 12 = 1.2A

Given various wastage in the charging process your actual consumption was less than 1.2A average.

[RcRotaryMadness]

Thank you - so the current drain overall is low but there will be some spikes... however I've just found this: Hyperion G3 LiFePO4 Packs, for Radio Tx and Rx

In the text it says these G3 LiFe packs can sustain a 60% burst for 20 seconds.

So if max constant amps is 4.25a then add another 60% on that = 6.8 amp bursts for up to 20 seconds. Still on the low side but I'd expect it be ok given the servos i'm using?

[RcRotaryMadness]

This is a 90 sized nitro fitted with bls451s, the green bar on the right shows total current draw at the pack. Mine is a 550e so a tad smaller than that!

[Mark_T]

Personally I like to test when I can in preference to believing the marketing copy

I rather lost my trust in the printing on battery packs once I started checking things for myself.

Odds are you'll be fine if the numbers they quote are real, and for what it is worth, Hyperion have always struck me as one of the more open suppliers out there when it comes to the numbers.

But since you can test a pack you don't really have to take it all on faith.

[Mark_T]

This is a 90 sized nitro fitted with bls451s, the green bar on the right shows total current draw at the pack. Mine is a 550e so a tad smaller than that!

Don't forget that test was using a flybar which will tend to result in lower servo loading compared to current FBL models.

[RcRotaryMadness]

Cheers Mark - I will do some testing with the PL8 see how the packs hold up!

[Mark_T]

Keep a close eye on pack temperature and condition and be prepared to react if it looks like it is going badly, but with LiFe it should be less risky than pushing a Lipo too far.

[Storrm]

The video states its monitoring the elevator servo, so does this mean the amp draw has to be multiplied by 3 or were the figures showing the total draw on the pack?

[Mark_T]

The video states its monitoring the elevator servo, so does this mean the amp draw has to be multiplied by 3 or were the figures showing the total draw on the pack?

They say they are showing the elevator servo load and the current for the total pack, so no, I'd read that as the total drain from all servos.

The figures shown would be consistent with other reports on the same servos so is believable.