understanding amps & volts

[spiff]

coming from RC background with glow motors for many years I still find it difficult to understand the relationship between amps and volts?

whats the point buying a 3s 5amp lipo as opposed to a 3s 2.2amp, when you are limited to the volts dropping from 11.1v to 10.5v?

[barneyg]

Its not amps its amp hours.  Essentially the more amp hours it is the longer it takes to go from full to empty

[dirtyharry]

voltage = speed of electricery going past
amps = volume of electricity going past.
battery capacity = amps per hour

a battery capacity ( ie 2.2 or 5 amp as you quoted ) is actually its amp per hour capacity , normally expressed amps per hour :-ah  or milli amps per hour :-mah. 

A 2200 mah  battery can supply 2.2 amps for one hour , or 4.4 amps for half an hour , or 8.8 amps for 15 mins etc

a 5000mah battery can supply 5 amps for one hour , or 10 amps for half an hour , or 20 amps for 15 mins, 40 amps for 7.5 mins etc


so the 5 amp battery ( 5 ah) will last more than twice as long as your 2.2 battery , but will also be twice as heavy.
hope that helps

[Biffa]

One thing not to forget, is an increase in mAh also increases weight so there becomes a point when the weight out ways the gain in flight time.

[kugabooga]

A super simple way to look at it is that the mah = fuel , more fuel = more weight.

maybe the Volts could be looked at as the octane rating of the fuel 
3s = Asda own make unleaded
4s = Asda premium
6s = Shell Optimax

well it makes sense in my mind .

[HowDoIfly]

Watt? 

[kilby]

To use a plumbing analogy

Volts can be viewed as the speed the water flows through a pipe
Amps reflects the diameter of the pipe

Volts hurt, Amps kill

Watts are a measurement of work done

[teslahed]

I tried to explain the basics here a while ago now;

http://www.multi-rotor.co.uk/index.php?topic=1050.0

[spiff]

ok am I right in thinking that since the voltage drop stays the same eg. 11.1v to 10.5 - having a bigger capacity battery means that voltage drop takes longer? ergo longer flight time?

[spiff]

I tried to explain the basics here a while ago now;

http://www.multi-rotor.co.uk/index.php?topic=1050.0

In your write up you say the critical voltage is 2.7v per cell? so I can lower my alarm to 3.0v/cell and still be safe?

[Biffa]

ok am I right in thinking that since the voltage drop stays the same eg. 11.1v to 10.5 - having a bigger capacity battery means that voltage drop takes longer? ergo longer flight time?

Capacity is the best way to work out flight time and you can do this by simply timing your flights and monitoring what mAh gets put back in the pack when you charge (making sure you use no more than 80% capacity).

Voltage can sag up and down with flying and vary with time so is (for me) a less reliable indication of battery performance.

[teslahed]

In your write up you say the critical voltage is 2.7v per cell? so I can lower my alarm to 3.0v/cell and still be safe?

2.7 is critical resting voltage per cell. If you have a lipo on the bench and it';s lower than 2.7 per cell - you might be able to recover but there';s no guarantee the lipo will be good to use again.

Under load the voltage will sag then recover when you stop drawing amps. So if you set your lipo alarm to 2.7 volts when you land you';ll probably be reading closer to 3.0 volts per cell.

If you haven';t got a current meter that gives you real time mAh used as you fly then the best way to work things out is to fly ';normally'; for 5 mins starting with a full lipo. Then land after exactly 5 mins, recharge your lipo and look to see how many mAh the charger says it puts back in. If you take 1000mAh to fly for 5 mins and fly with a 2500mAh battery then you can be reasonably sure you';ll be safe to fly for 10 mins with a 2.5 min reserve.

If you always land with your lipos at least 20% full then they will last longer and you wont have to buy replacements all the time.

Voltage can sag up and down with flying and vary with time so is (for me) a less reliable indication of battery performance.

My experiences are similar. Voltage doesn';t give you a linear response. It gives you an ';oh dear my lipo is about to die, better land immediately'; warning that your aircraft is about to fall out of the sky.

[guest325]

I think another factor to add to this is the ';C'; rating of the battery - this is a guide to how many amps the battery can give without being overstressed (NOT GOOD - CAN RESULT IN MAGIC SMOKE AND FIRE).
An example - a 3S2200mah 25C battery can provide 2200x25 ma = 55A, a 3S5000mah 25C battery can provide 5000x25 ma = 125A
So as you can see if your rig has 4 x 20A esc';s you potentially need 80A capacity off the battery so a 25C 3S2200 will not do, it needs to be at least 80/2.2 = 3.6 ah or 3600mah.
So if your model feels sluggish it may mean you don';t have a high enough ';C'; rating or big enough battery.
The other thing that has to be remembered is bigger batteries are heavier therefor need more power to overcome the weight, this can mean that the whole scenario becomes self defeating - the best policy (for me) is to keep things as light as possible and look for a battery size / ';C'; rating that gives a good compromise between performance and duration.

[spiff]

2.7 is critical resting voltage per cell. If you have a lipo on the bench and it';s lower than 2.7 per cell - you might be able to recover but there';s no guarantee the lipo will be good to use again.

Under load the voltage will sag then recover when you stop drawing amps. So if you set your lipo alarm to 2.7 volts when you land you';ll probably be reading closer to 3.0 volts per cell.

If you haven';t got a current meter that gives you real time mAh used as you fly then the best way to work things out is to fly ';normally'; for 5 mins starting with a full lipo. Then land after exactly 5 mins, recharge your lipo and look to see how many mAh the charger says it puts back in. If you take 1000mAh to fly for 5 mins and fly with a 2500mAh battery then you can be reasonably sure you';ll be safe to fly for 10 mins with a 2.5 min reserve.

If you always land with your lipos at least 20% full then they will last longer and you wont have to buy replacements all the time.

My experiences are similar. Voltage doesn';t give you a linear response. It gives you an ';oh dear my lipo is about to die, better land immediately'; warning that your aircraft is about to fall out of the sky.

this is what I got recently. have not used it yet. I got 2 new gen ace 3s 5amp batteries that I will be testing out on my quad soon.

http://www.hobbyking.com/hobbyking/store/__41637__H_King_Cellmeter_7_Lipo_Life_Li_ion_Nimh_Nicd_Digital_Battery_Checker_With_Balance_Function.html

[spiff]

I think another factor to add to this is the ';C'; rating of the battery - this is a guide to how many amps the battery can give without being overstressed (NOT GOOD - CAN RESULT IN MAGIC SMOKE AND FIRE).
An example - a 3S2200mah 25C battery can provide 2200x25 ma = 55A, a 3S5000mah 25C battery can provide 5000x25 ma = 125A
So as you can see if your rig has 4 x 20A esc';s you potentially need 80A capacity off the battery so a 25C 3S2200 will not do, it needs to be at least 80/2.2 = 3.6 ah or 3600mah.
So if your model feels sluggish it may mean you don';t have a high enough ';C'; rating or big enough battery.
The other thing that has to be remembered is bigger batteries are heavier therefor need more power to overcome the weight, this can mean that the whole scenario becomes self defeating - the best policy (for me) is to keep things as light as possible and look for a battery size / ';C'; rating that gives a good compromise between performance and duration.

it';s a std DJI F450 with std motors & ESC - think they are 15 amps each I think. 60 amps total

[guest325]

it';s a std DJI F450 with std motors & ESC - think they are 15 amps each I think. 60 amps total

According to this
http://www.eyeflying.com/build-your-own/quadcopter-kits-bits/dji-f450-flamewheel-arf-kit-inc-frame-esc-s-motors-and-props.html?gclid=CMqWje2-7b8CFerpwgodc3wALg
they are 30A esc';s standard so you would need 5000mah 25C or higher C rated of lower capacity (pro rata) batteries if you want full power potential.

[teslahed]

The ESCs may be rated to a maximum of 30 amps each but each ESC will draw as much current as the motors request up to the point where the ESCs melt.

So if the motors are rated at 15 amps each and the ESCs are 30 amps each you';ll still only draw 15 amps max per motor / ESC combo and you';ll have that 50% headroom on each ESCs power rating just making doubly sure you don';t burn one out.

But mentioning C rating is worth doing. If all else is equal a higher C rated battery will experience less voltage sag under load and should give better flight performance in the air.

[spiff]

so the bottom line here then is to make sure your battery capacity exceeds the demands of the motors & ESC? and keep the weight down

[teslahed]

Yup - Make sure the battery C rating is higher than the total current draw. Use maths or a watt meter to double check. Get this wrong and your quad might fall out of the sky with a hot puffy lipo that has to then be binned.

Make sure the ESCs have headroom so that you';ll never be drawing their maximum rating. 30 amp ESCs for 15 amp motors is fine.

Try to keep the weight down as much as possible without hurting flight times with low capacity batteries too much.

[guest325]

I';ve found that the best power to weight ratio batteries are zippy compacts, others may not agree with me but that';s my experience.