Lipo advice

[RickRick]

Just looking into lipo's

whats the score with using crazy high power brushless motors 3.5/4.5 with a relativly low 25c 3500 lipo pack

does it just limit the current it can push out, or is there a chance of damaging the cell?

[JJ The Boat Wizzard]

Hi m8

If the motor is trying to pull 90-100amps and the battery cannot give this say a 3500 25c that can only give 25c so thats 87.5amps , the battery is at risk of 'puffing' where the cells puffup and can burst into flames if pushed too far (however this is highly unlikley) , also you wont get a motor and a spedo in a 1/10th car that pulls 100amp most spedos that say they can cope with 250A+ that is utter crap they cannot a 1/8th spedo can only just handle 150A , a 1/10th brushless motor will pull between 60-100Amp and thats 'silly motors' as you refer to them. so you dont have to worry.

Only worry if the battery is like a 2400 10c then your in trouble

Hope this helps
John

[mark christopher]

ignore the bit above about puffing and bursting into flames

the lipo will give its max, so if the speedo is needing 100amps, the pack will only deliver is 87.5 amps, very little chance of puffing or smoke as thats its rating

look for around 3600 30c or 4900 25c or above

[JJ The Boat Wizzard]

Ive seen it happen ! dont say its not possible just because you have never seen it! i have seen it happen just not in a long while and i dont expect to see it happen again especially with the new cells. I also know the technical reason why this happens but its too long winded to post i would gladly enlighten you if i saw you face to face :P

However he is right about what batterys you want to be looking for

John

[leeleefocus]

Originally Posted by mark christopher

ignore the bit above about puffing and bursting into flames

the lipo will give its max, so if the speedo is needing 100amps, the pack will only deliver is 87.5 amps, very little chance of puffing or smoke as thats its rating

look for around 3600 30c or 4900 25c or above

Your wrong there i'm afraid(Don't give advice about lipo's if you don't know what your chatting about). The pack can deliver more than it is rated at.

The rating is the safe ammount of current that the battery can deliver without damaging itself and or going below a useful voltage(voltage sag). If you pull more current from the battery then the other poster is correct there is a possibility of puffing, over heating the battery and/or damage to the cell internally.

The possibility of fire is there but you have probably got more chance winning the lottery.

What you need to know is the constant current and the peak current that your speed controller and motor combo will pull and then get a lipo that has a higher constant current rating and burst current rating than your setup.

If you treat lipo's correctly they will be one of the best purchases you make. If you abuse them there is a chance they will bite you back.

I advise you to spend a few hours reading up before you buy and watch a few of the youtube videos.

Once you get them though there is no going back.

[JJ The Boat Wizzard]

Originally Posted by leeleefocus

Your wrong there i'm afraid(Don't give advice about lipo's if you don't know what your chatting about). The pack can deliver more than it is rated at.

The rating is the safe ammount of current that the battery can deliver without damaging itself and or going below a useful voltage(voltage sag). If you pull more current from the battery then the other poster is correct there is a possibility of puffing, over heating the battery and/or damage to the cell internally.

The possibility of fire is there but you have probably got more chance winning the lottery.

What you need to know is the constant current and the peak current that your speed controller and motor combo will pull and then get a lipo that has a higher constant current rating and burst current rating than your setup.

If you treat lipo's correctly they will be one of the best purchases you make. If you abuse them there is a chance they will bite you back.

I advise you to spend a few hours reading up before you buy and watch a few of the youtube videos.

Once you get them though there is no going back.

THANK YOU FOR BACKING ME UP!

[Mike Haswell]

Remember that the C is the constant current rating - most have a higher 'burst' rating although it can be difficult to find. I have a 5,000mAh which is rated at 20C (100 amps) for continuous discharge and 30C (150 amps) for burst.
This has performed very nicely over the last year but having tried one of the latest higher rated packs then it would be much better to get one of those. Not only can it deliver more power comfortably but will be under less stress.

[RickRick]

i've already gone for a tekin 3400 23c pack, i've got a choice of 3.5 4.5 or 6.5 motors to go at too, so what would you think the most powerful motor that lipo would be safe at

[blue_pinky]

So I'm with Mark, it's a physics/chemistry thing...which true scientists could explain better I'm sure, but I'll have a go....

The motor and esc can only use the energy that is available to it from it's energy source. An esc and motor that are 'rated' to draw large currents can only do this IF the power source is capable of delivering it. The lipo cells are self limiting, i.e the cell chemistry only allows a given amount of energy to be provided to the load...the advisory numbers that are given with the battery describe the current available over a time period, constant = long time period, and peak = short period, but could also deliver a much higher current over a much shorter time period before being damaged.

Hence the reason that the 'c' rating and capacity can be used to compare a general performance difference from one battery to another.

You will not damage a battery until you drain too much energy from it...which is a power AND time calculation.

The videos you see are of people who have over discharged a battery..by drawing too much current off for too long a time...hence the damage.

A small battery on a fast motor and esc combo will only result in shorter running times at slower speeds when compared to a bigger battery on the same load.

So, for your last question of what motor to choose now that you have the battery sorted...it depends what your looking for, a faster motor will result in more speed for a shorter amount of time, a slower motor will result in slower speeds but for longer run times. And it depends on what your using the car for, racing or bashing, etc etc.

To keep it safe, always run a low voltage cut-off for your battery, this will prevent you taking too much energy from the battery (ie over discharging it) and damaging it, potentially in the way it has been described to you.

I hope this helps

[JJ The Boat Wizzard]

BLUE PINKY , no your partly wrong , your right about the video's and the over discharging (ALWAYS RUN LIPO CUTOFF) however you can also damage a lipo if you try to take say 175Amps out of a battery that should only be giving bursts and climbouts of 80-100amps.

but low C batterys are very hard to find and not used anymore so lets leave it at that , also go for whatever motor you want your battery is fine with any of thoose motors

John

[blue_pinky]

Your absolutely right...that would be taking too much energy too quickly! 175 amps would VERY quickly drain a battery and cause damage, exactly as you've described, as the battery chemistry can't deliver that level of power for very long at all.

[leeleefocus]

I find this a bit worrying that people are using lipo's and not understanding how it all works and even more worrying is they are giving other people advice.

Lipo's are given a safe discharge rate that is there so you can choose a combination of battery/motor/esc that does not draw more current than they are safely rated at.

The battery will be able to deliver more current than it's rating but you run the risk of damaging the battery.

Do some research before using lipo and read up on hard facts.

There is already alot of assumptions and opinion in this thread mixed in with a few facts.

I'm not normally this serious but lipo's are potential fire crackers

[mark christopher]

just spoke to trakpower..................My source of info, who say that the pack will NOT swell/smoke/fire, the pack will simply not deliver the power to the motor.
in a car you very raley get full amp draw, espcially with brushless. if you want to get your facts right please ask the profesionals and dont rely on you tube where any tom dick or harry can post info!!

lipo problems occur on over discharge or over charge

[leeleefocus]

Yes thats right don't rely on youtube but it's gives examples of what happens when it goes wrong.

I don't know why trakpower are saying a pack will not swell did you speak to the receptionist or something.

A pack will swell if put under to much load experienced it myself. I do agree with you about the smoke and fire virtually nil chance.

Some lithium battery's have protection circuits built into them to prevent To much current being drawn/overcharge/over discharge.

The Lipo's we are using have no such circuit in them.

The best example of pulling to much current from a lipo is to create a dead short which is the most extreme example of pulling to much current. I can almost guarantee that any lipo will either puff or melt the solder from the top of the cells and then it is ready for the bin.

I know the example was a system pulling 100A with the battery rated at a constant current of 87.5 which the system would probably run for a while but the battery life will be reduced. It may not puff but the battery is being used outside of it's safe rating so there is the possibility of damage.

The information you are getting is incorrect.

I don't know what question you put to trakpower. Try asking them if you took one of there battery's and pull more current than it's rated at what would happen. I think the advice you would get back would be not to do it.

If it was done once then the battery would likely suffer nil effects as long as it wasn't massively stressed but if it was put into a system that was pulling above it's specification constantly or on a regular basis then you will likely damage the battery.

[mark christopher]

lee
i asked Mr trakpower............ Im sponsored by Trakpower and know he is NOT a receptionist

[leeleefocus]

Sorry for taking the mickey that was out of order.

I appreciate that you are trying to post and help the original poster.

But i'm sure even trakpower would not recommend using a battery outside of it's spec.

The c rating is there so that a motor and esc combo can be chosen that does not exceed that c rating.

A lipo rated at 87.5A would likely be able to deliver 100A but it would degrade(possibly damage) the battery much faster probably. There is always the possibility that the manufacturer air's on the side of caution when giving a c rating.

There is no exact science to it because of tolerances and differences in battery's.

Just keep the c rating of the battery above that of the motor/esc combo.

[JJ The Boat Wizzard]

Well i have rung both LRP (SMD) and Flightpower and they agree with me and lee and im sure trackpower would do too if you said you were using them outside there C rating!

I think trackpower use flightpower cells anyway :S ??

John

[Ian]

Hi, Ian Watson from TrakPower here, I hope you dont mind me typing out a long boring technical post, but hopefully it will be of some information to you

C rating is a loose and relative number developed to describe the performance of a pack, there is no industry standard to what a cell should hold at this particular rating. it is generally based upon what current draw a pack can handle before the voltage drops to a certain point. Most packs out there have two C-ratings on their packs, constant and burst. Take as an example our 25C 4900 pack, a proven cell used widely in all sorts of setups. now this pack can handle 122.5A constantly, which means that you could put a 122.5A load on that pack, and it will hold a usable voltage for the majority of its discharge before dropping off near its capacity point. At 25C this will take approx. 2 minutes and 30 seconds to flatten the pack completely.

Burst discharge is on the other hand, what a battery can handle over a short period of time and keep above 3.0v per cell. In this case its roughly 250A. No questions asked, you will never reach this current draw! If you would look at a graph of discharge at this rating you would see the voltage drop off within several seconds, and will hit 3v per cell, this is what is known as a "burst", as the pack can handle this current in "burst" of several seconds without hitting its cut-off. As soon as you stop drawing the current the packs voltage will then increase and you carry on as normal.

In all reality with a car, average current draw over a race is very very low, OK it might peak off the line over over a hundred amps for a split second and as you pull out of corners, but In a strait line your car will require little effort to keep at a speed (think back to physics lessons). Even as you are pulling from a start it is unlikely you will hit the constant rating of the pack, far less its burst. Put it this way, if you flatten a 4900 is 10 minutes, that's 6C average, less than 30A average over the race!

But, what happens if you do go to this burst discharge current with a correctly set up ESC? Will it puff? will it blow up? Will you kill the pack? It's a no.

In theory, IF you hit this burst current, and keep it discharging at that rate, you will hit the 3v mark, and it will keep on going below this save level. If you keep doing this, you WILL damage the pack, it will puff, get very hot, but its unlikely to explode, unless you push it very far! It takes a lot of effort to blow up a pack, and a simple short over-discharge is unlikely to push it to a dangerous point.

99% of controllers have a cut-off in them, and if they don't i would strongly advise to use an external cut-off of some sort. If this works correctly, and you are drawing excessive current from your pack, it will hit 3v, and the ESC will stop, the voltage will go back up, and you can go again (within reason, dont do this when your pack is actually flat!). So what happens here, nothing, your pack will carry on as though nothing had happened.

Technically it is not the current draw which kills a pack, its the relationship between current draw voltage drop and heat. If you draw too much current, the voltage will drop off, below 3v per cell, and damage the pack, end of story.

To put this into real life terms, in a particular type of RC Glider, they draw on average 75-100C (yes you heard me correct there!) from their packs, for a maximum of 3 to 4 seconds. This is way over any packs "burst" rating, but as it only happens for a short period of time, the packs do not go below 3v per cell, the result, they do not kill their packs at the rate you would image them to! If they flew for more than 3 or 4 seconds at this rate then yes, the packs would get damaged.

Do we reccomend using a pack outside of spec? certainly not! but you would have to try very, very hard to get it to that point

Sorry if half of you have fallen asleep with the length of this post, i didn't intend it to be this long but lipos are a complex topic which isnt always as obvious as it seems. If anyone would like more information about this topic, and would like it in more detail, please feel free to call me on 01763 848404. I sadly dont know much about what fits in what cars (im a RC pilot, not a driver), but im happy to answer any questions regarding lipos and how they work and how they should be treated.

In regards to FlightPower and Trakpower using the same cells, thats correct, we are the same company.

Kind Regards
Ian Watson
TrakPower Lithium Polymer
Technical Support
ian@trakpower.co.uk
01763 848404

[JJ The Boat Wizzard]

Quote "Technically it is not the current draw which kills a pack, its the relationship between current draw voltage drop and heat. If you draw too much current, the voltage will drop off, below 3v per cell, and damage the pack, end of story.

To put this into real life terms, in a particular type of RC Glider, they draw on average 75-100C (yes you heard me correct there!) from their packs, for a maximum of 3 to 4 seconds. This is way over any packs "burst" rating, but as it only happens for a short period of time, the packs do not go below 3v per cell, the result, they do not kill their packs at the rate you would image them to! If they flew for more than 3 or 4 seconds at this rate then yes, the packs would get damaged. " End Quote


!That is all we have been trying to say!
Thanks for confirming that


John

[blue_pinky]

Thank you Ian, just what this thread needed!

Very much appreciated, every day is a school day as they say