Discharge capability of batteries? Low discharge LiFe vs high discharge 7.4 Lipo.

[Asomodai]

Hey gents. 

 

Upon conclusion of my ARX project, I can no longer use my big 1600MaH LiFe with the stock not extended due to the mosfet size. Its a single tooth short stroked setup with a low strength M110 spring and 16TPA High torque SHS Rocket Green. 

 

I started looking at smaller LiFe batteries and I am getting a bit confused at some of the terms used by Component-Shop. 

 

Looking at this particular battery https://www.componentshop.co.uk/9-9v-750mah-20c-mini-lifepo4-battery.html

 

Its only a 750MaH one so will likely need two. 

 

It states it has a discharge capability of 15A on continuous and 30A on Burst. This seems quite low as my standard 7.4 Lipo is 36.25A continuous and 72.5A Burst. My usual LiFe is 40A Burst and 80A Continuous, though fully charged this causes overspin on the ARX so is not really suitable. 

 

So which would give me the best trigger response and performance, the 9.9v life with lower discharge capability or the 7.4v lipo? 

 

If the LiFe is going to be worse, is there any reason not to go for a low discharge rated 11.1v?

[E21A]

If you’ve got a mosfet then why not go for a 30/60 11.1 lipo? 

[Asomodai]

9 minutes ago, E21A said:

If you’ve got a mosfet then why not go for a 30/60 11.1 lipo? 

 

I am not confident of my ability to rebuild a gearbox to handle it at this moment... Also worried about overspin. 

[Lozart]

The C rate of a battery is relative to it's maximum capacity, hence your old 1600mAh battery has a much higher discharge current than the 750mAh one (even if they're marked with the same C rating).

I'd say you need to do the calculations - work out what your desired discharge current is and work backwards to see if you can even get a battery with that characteristic in the physical size you need. I suspect you won't.

[Asomodai]

17 minutes ago, Lozart said:

The C rate of a battery is relative to it's maximum capacity, hence your old 1600mAh battery has a much higher discharge current than the 750mAh one (even if they're marked with the same C rating).

I'd say you need to do the calculations - work out what your desired discharge current is and work backwards to see if you can even get a battery with that characteristic in the physical size you need. I suspect you won't.

 

The figures I gave are the A rate, what's the difference between A and C? What does a high A/C rate achieve? The motor spinning quicker? 

[Lozart]

Just now, Asomodai said:

 

The figures I gave are the A rate, what's the difference between A and C? What does a high A/C rate achieve? The motor spinning quicker? 

 

The C rate is the discharge rate of the battery as a function of its capacity, a 1000mAh 10C battery will discharge up to 10A, a 1000mAh 20C battery will discharge up to 20A. mAh is a function of capacity over time - a 1000mAh battery will discharge at 1000mA (or 1A) for an hour, a 2000mAh battery will discharge at 1A for 2 hours (or 2A for one hour). In (very) simple terms, a higher voltage for a given current will spin the motor more quickly, a higher current for a given voltage will give it more torque (hence the difference in thickness of windings in high torque motors as opposed to high speed motors). Bearing in mind of course that the discharge current of a battery is entirely dependant on the resistance in the circuit it's attached to. If the motor you're driving can only take a maximum of 10A, connecting a 20A battery to it will make NO DIFFERENCE.

[Samurai]

15 Amp is not enough. Maybe enough for a well built low power AEG.

 

Edit: about the required amperage. The problem comes from the motor needing more than the current (Amps) the battery can supply. When it can't, the voltage drops and with it, the speed of the motor = trigger response and ROF. If the battery can supply the required current, the voltage will drop only a small amount compared to the voltage you measure on the battery without load.

So if you have a high voltage battery, like a 11.1V lipo, if it has low C rating then it will have a huge voltage drop, and behaves like a lower voltage battery.

(Not really because voltage drop takes time, hence the higher burst C rating, but let's not dive that deep.)

[Asomodai]

16 minutes ago, Lozart said:

 

The C rate is the discharge rate of the battery as a function of its capacity, a 1000mAh 10C battery will discharge up to 10A, a 1000mAh 20C battery will discharge up to 20A. mAh is a function of capacity over time - a 1000mAh battery will discharge at 1000mA (or 1A) for an hour, a 2000mAh battery will discharge at 1A for 2 hours (or 2A for one hour). In (very) simple terms, a higher voltage for a given current will spin the motor more quickly, a higher current for a given voltage will give it more torque (hence the difference in thickness of windings in high torque motors as opposed to high speed motors). Bearing in mind of course that the discharge current of a battery is entirely dependant on the resistance in the circuit it's attached to. If the motor you're driving can only take a maximum of 10A, connecting a 20A battery to it will make NO DIFFERENCE.

 

16 minutes ago, Samurai said:

15 Amp is not enough. Maybe enough for a well built low power AEG.

 

Edit: about the required amperage. The problem comes from the motor needing more than the current (Amps) the battery can supply. When it can't, the voltage drops and with it, the speed of the motor = trigger response and ROF. If the battery can supply the required current, the voltage will drop only a small amount compared to the voltage you measure on the battery without load.

So if you have a high voltage battery, like a 11.1V lipo, if it has low C rating then it will have a huge voltage drop, and behaves like a lower voltage battery.

(Not really because voltage drop takes time, hence the higher burst C rating, but let's not dive that deep.)

 

Thanks for the explanation folks! I do suspect I would either have to find a higher C rated 7.4v or a low C rated 11.1v rather then go with the LiFe. I'll have to look into it some more! 

I appreciate the time it took to explain to my noggin! 

[Sitting Duck]

2 hours ago, Asomodai said:

If the LiFe is going to be worse, is there any reason not to go for a low discharge rated 11.1v?

 

Only if you like rebuilding gearboxes

 

Basic gun fires about 12~13rps on 7.4v with a 18~20k stock motor

you dropped in a SHS/RA motor that runs at 28~30k - sort of 50% increase

well not quite 50% in reality as motor speeds are unloaded at 8.4v (so they say or lead to believe)

 

So you won't quite hit 20rps perhaps BUT with a rewire 16awg, deans blah blah will give you 10% maybe 15% max

(depends on dire stock wiring pansy small tamiya blah blah blah but yeah add on say 12% for rewire)

So yeah you are now on 20rps, deffo just broke 20rps imho on 7.4v all dean'd up etc....

 

Now add on a similar 11.1v with extra cell - well it is a bit more than 50% say 55% imho over the same 7.4v

(the gun is already cycling & in motion, so the extra cell is 101% extra boost hence 55% than 50%)

So now expect 20rps x 1.55 = 31 rps on a UK gun, yeah I wouldn't push it that fast on just 1 SS setup

I mean it might be OK and just get by but long term I think it is a sailing too close to the wind and will overcycle/double fire

(unless you got AB, but still on full auto it will be a bit risky)

 

20~25rps is a sensible snappy build that should last

(at 25rps most guns are starting to double fire on semi without work)

Your gun should pull only about 13 amps I reckon if done really well on std 18:1 & SHS HT (not HS motor)

measure it through a RC power tester on full auto for 5 secs say

from still the current really ramps to like double, to get the drivechain to cycle from stationary

(hence they fit a fuse double the normal operating draw)

well your gun should only draw 13amps unless shimming is crap

 

I'd try to find a nice 1500mah+ 25c 7.4v

work out what room you got to play with, may need stock extended a bit

measure with mars bar or anything what room you got

take off say 5mm each way - maybe a bit more on length for wires

check out hobbyking or whoever for dimensions and weigh up your options

(just don't buy a battery that is too tight that if it puffs very very slightly it won't fit)

((oh and double check your measurements))

 

By all means use 11.1v - but on a sluggish stock motor to get 20rps

but 11.1v on 30k motor - yeah I wouldn't, plus the 11.1v is bigger/fatter and more expensive

 

If you do a nice tweaked build and do it well most people will get by on a compact 7.4v 25c~30c

[Iceni]

looking at the form factor - 102 x 21 x 18 mm

https://www.componentshop.co.uk/7-4v-1300mah-25c-continuous-discharge-lipo-battery-101mm-long.html

101 x 20.4 x 13.3mm
32A continuous, 65A burst

It's slightly smaller in its dimensions, but with more capacity. The only gripe you might have is it's 7.4v so not a performance battery.
 

If you can find a little more length.

https://www.componentshop.co.uk/11-1v-1450mah-25c-continuous-discharge-lipo-battery.html

129 x 22 x 18mm
36.25A continuous, 72.5A burst

It's 11.1v as well. So it will be a performance battery.

[Asomodai]

21 hours ago, Sitting Duck said:

Stuff

 

14 hours ago, Iceni said:

More stuff

 

Thanks for the advice fellas! I would only be able to fit a brick style battery if it was very short, Sticks only and under 110mm. I am considering installing a different very small Mosfet to help with the space issue if it becomes a huge problem. 

 

As it stands, I did some more testing last night, this time with the fully charged LiFe battery and storage mode 11.1v lipo. It was overspinning on both like crazy, double fire on semi, the works! 

 

Did some more testing with my high 30C rated 7.4v on full charge and I am pretty happy to stick with them as the result is pretty good RPS wise. I COULD go for a lower powered LiFe, but I suspect it would perform only on par if not worse then the 7.4v. 

 

I would probably have to short stroke another tooth to use an 11.1v, at that point I would have to go with a shorter barrel setup to make up for the volume loss. 

[Sitting Duck]

I tend to use Zippy Compact's 7.4v's 25c's

(got a few 11.1v's but mostly stuff ran with 7.4v's)

 

104 x 14 x 35 dimensions goes in most guns that like a small "brick" type

https://hobbyking.com/en_us/zippy-compact-2200mah-2s-25c-lipo-pack.html

(obviously add a bit of space for wires etc.... )

 

This is what I was saying about listing up options for say batteries up to say:

114 length, 35 width 20mm thick @ 7.4v or 2 cells at 25c to 36c rangeypoohs....

 

https://hobbyking.com/en_us/batteries-chargers/batteries/lipo.html?config=52&capacity=1195-2709&discharge=25-36&length_a=0-114&height_b=0-35&width_c=0-20&wrh=1%2C7&dir=asc&order=price

 

yeah you might have to change the XT60 to deans with solder/adaptor but you get a battery that fits