Lipo Cell Drop Off

[Eventorizon]

I am not quite sure how to phrase the question so I will give the scenario.

I am looking at the following system in C3 as an upgrade in the future which I can design in.

Power: 2X 6S 2200mAh 40C Lipos in Series = 12S, 80C & 2200mAh capacity

Control: 1x Castle Creations Phoenix Edge 160 HV ESC (50V & 160A)

Motor: 1x Align 750MX 530Kv.

If my maths is right I can average 44A over a 3 minute fight and that would drain the batteries totally. Of course you can't actually use all the power in a pack due to voltage drop off.

So...

Is an average of 44A over 3 minutes even possible with a 2.9kg weapon?
(Runs on a 3:1 reduction giving (44.4*530)/3 = 7844 lossless)

And how many amps can I drain from the Lipos before I hit the 3.2V cut off built into the ESC?

The lipos would be these: http://electriflyer.co.uk/gens-ace-6...00mah-40c-new/

160A ESC from here: http://www.castlecreations.com/produ...x-edge-hv.html

[grant_ploughbot]

You want to ideally make sure that your not using more than 80% capacity from your packs each cycle otherwise you start to effect the life span and performance of your packs. Current wise totally depends on how well your system is built and how free it is. I think something along the lines of 3300 packs would be more suited.

[Eventorizon]

If I remember right LS4 uses 2 2600mAh 5S for its weapon. As you know, more volts = less amps. I suspect you are right but I dont have the weight or the space (Unless I got a custom Lipo which would be stupid!)

[Max]

I don't think more volts necessarily means less amps, as with more volts the more will spin faster so more resistance so more amps.
But if you are using a different motor which is designed to deliver same power as LS4's at a higher voltage then as you say less amps should be drawn.

[grant_ploughbot]

That all depends on your gearing as to if you will pull more current or not than LS4. LS only has a low RPM at the disc in comparison and so draws quite a low current to keep spinning. It's hard to tell without testing.
The higher voltage/low current works if your not pulling more power. As the motors your using can give huge amounts of power it's purely down to your gearing/build and air resistance of your beater.
A bar spinner can halve its constant current draw by adding an aerodynamic edge, it's suprising how such small things make such. A give difference.

[maddox10]

In Valkiri 2 the difference between 6S and 4S is 120A startup and 40A cont and 90A startup and 30A cont.
The increase in RPM is a lot less than you would expect.

[Eventorizon]

In Valkiri 2 the difference between 6S and 4S is 120A startup and 40A cont and 90A startup and 30A cont.
The increase in RPM is a lot less than you would expect.

So in that example continuous was a 3rd of the start up no matter the voltage, interesting. Do you have the 2 speeds to compare?

The two things I am balancing are do I go 8S with a standard ESC or 12S with a HV ESC. If the top speed was achievable then the 12S would spin 50% faster but I have no idea where the two will level out. The higher voltage will spin faster but the air resistance will go up. I am thinking there is no way to really know unless I try.

[Ellis]

The potential continuous draw figure drops as, in a hard battle, you could have a few startups and a stall or two's worth in amps to get out of that maximum 44a average draw figure. If you can get more than 2.2ah, I would.

Also, is it right that C rating doubles by putting two packs in series? That doesn't feel like it should be the case? Are you certain that is actually how the math works? If not, you have a maximum of 88a continuous from those packs. At 12s the 750MX can pull beyond 160a. Looking at your battery link I see no mention at all of burst C rating. If the burst isn't great, and the C rating doesn't double when you series the batteries, you could potentially be beyond the power of the batteries in both discharge and capacity.

Someone with the knowledge, I can see how discharge would double if you parallel two packs, but not quite how that would work if they're in series... enlightenment please!

[maddox10]

In Valkiri 2 it was a 3-1 ratio, what is consistent with most electric motors that are loaded to their rated max.

RPM of an electric motor is commanded by construction, input voltage and load.
In the spinners, aerodynamics take a serious bit of power above a certain speed, just as in cars.

I don't have direct figures, the rpm counter we used ain't to accurate, but the 4Krpm of the 4S test wasn't scaled directly to the 6S test. Maybe we reached 5Krpm.

[Eventorizon]

Looking at your battery link I see no mention at all of burst C rating. If the burst isn't great, and the C rating doesn't double when you series the batteries, you could potentially be beyond the power of the batteries in both discharge and capacity.

It is in the battery description: 40C they also do a 30 & 55C version.

If each pack can supply 40*2.2=88A then both will supply 176A. All that changes is the voltage or the capacity depending on how they are wired. ie 176A @ 22.2V 4.4Ah or 176A @ 44.4V 2.2Ah.

I have the weight for 2 of these... http://www.gensace.de/lipo-battery/f...tery-pack.html

I am pretty sure that would do it.

EDIT: With Davids help we looked at the readout on Binkys disc. Startup was 100A but only for half a second. The Idol was around 20A. The 2 5S 2650mAh lipos only used half their capacity in the loosers melee. I know the air resistance is less but it is far larger and 600g heavier.