Really silly Li-po question...

[Planespotter]

Hi everybody, I have a quick question about Li-po capacity. I always read about the super long run times with Lipo batteries, but the advertised amperages arent much different than good old nimhs. Is there something that I am missing? Is a 5000 mah Lipo somehow different than a 5000 mah nimh (voltage notwithstanding)? Thanks in advance!

[Scottmisfits]

Internal resistance on a LiPo pack is a lot less than on a NiMh or NiCd pack. Because of this, the battery is working a lot less, more effecient, to get you the same levels of power. Because it's not working as hard, it's not drawing as much juice so it will last longer.

That's very basic. Someone else might chime in with more tech stuff but that's, in effect, what's happening.

[rmdesignworks]

Copied from rchobbies.org about lipo's.....

Understanding LiPos
Lipo batteries are very different from previous generation batteries and understanding how they work, and especially how to charge them, is the key to getting the best performance.

Series
Individual Lithium Polymer cells have a nominal voltage of 3.7 volts (vs. 1.2volts per cell for Ni-Cds). Cells are wired in series to give the following pack voltages:

1 cell = 3.7 volts
2 cells in series = 7.4 volts
3 cells in series = 11.1 volts

Parallel
Unlike Ni-Cd and Ni-MH cells that self-discharge when wired in parallel, LiPo cells can be hooked up, charged and discharged in parallel with no detrimental effect. Wiring two LiPo cells in parallel doubles the capacity (more run time), plus an important advantage of wiring in parallel is that each cell only sees half the total current.

3S 2P
A battery pack that has three cells in series (giving 11.1 volts) and 2 of these 3-cell packs are wired in parallel is commonly referred to as a 3S, 2P (3 series, 2 parallel).

C Rating
LiPo cells are also commonly given a C or current rating. This is the maximum average recommended discharge current for the cell. For example, the Thunder Power 1900mAh packs have a 6C rating. To determine the maximum recommended discharge rate multiply the capacity times the C rating. 1900mAh x 6C = 11,400. So the maximum recommended discharge rate would be 11,400mA or 11.4 amps. If your application has a higher amp draw, remember that LiPo cells can be wired in parallel, and with 2 cells in parallel each cell sees half the total current. With 3 cells in parallel, each cell see one third the current.

By wiring packs in a combination of series to get the voltage and parallel to achieve the capacity and individual cell current to an acceptable level, LiPo cells can be used to power nearly every type and size of model.

[Terranaut]

Not yet mentioned is that LiPo batteries have a higher energy density, meaning a battery will be smaller, and lighter, than other types with the same capacity. The model aircraft and heli people adopted LiPo's quickly for this reason. In flying models weight is critical. By using LiPo's in place of NiMH or NiCad you can have much higher mAH capacity for the same weight, and therefore much longer run times.

Cheers.

[oldhippie]

Lipos will also hold voltage more consistently, not dropping off much during the entire runtime. So; more efficient, higher voltage and lighter = longer runtimes and more power with less weight!

[Chris_The_Battery_Man]

There are a few mis-conceptions about lipos. Here are the facts:

-Lipos DO NOT have better runtime because of their chemistry. So if you have a 1000mah lipo and Nimh, they will both get the same runtime. Mah is your gas tank. It is a unit of measurement over time. It stands for Minni-Amp hours. Which means, how many milli-amps, in an hour, will the pack get under a certain load.


-Runtimes can be effected by efficiency though. Generally, that is only noted in the weight department. They do maintain a better voltage curve under a load, than a Nimh, thus causing higher RPM's relatively speaking. But in most cases, this will actually cause a HIGHER LOAD on the pack, because the motor will use all it's resources to achieve a given RPM. Unless you gear down.

-Lipos are generally more energy dense. So you get way more power in a lighter, smaller, package. GENERALLY. There are cases, where a good Nimh makes more sense.


It is hard to compare 2 chemistries to one another, and that is not an apples to apples comparison. Without voltage being the same, you cant compare them. Lipo is the most powerful, efficient, and simply highest performing chemistry out of a single cell in the RC world. That is why it is almost always the best choice for a battery chemistry.


But I am not sure what you mean by the advertised amperages. Nimh's can never hold a candle to the amount of current a GOOD lipo can take.

[el_tonto]

Originally Posted by Chris_The_Battery_Man

There are a few mis-conceptions about lipos. Here are the facts:

-Lipos DO NOT have better runtime because of their chemistry. So if you have a 1000mah lipo and Nimh, they will both get the same runtime. Mah is your gas tank. It is a unit of measurement over time. It stands for Minni-Amp hours. Which means, how many milli-amps, in an hour, will the pack get under a certain load.

Not quite. ONLY under light load, where the effective load resistance is much greater than the cell's internal resistance, will this be true. mAh s not the best measure of capacity - Delivered watt-hours (Wh) is, and that is a function of both load and cell internal resistance. Under high-load conditions where the cell's internal resistance is significant, the same mAh might be delivered, but at a lower voltage due to sag. Delivered Wh, therefore, is less with the NiMH. Or to put it another way, more mAh is required with the NiMH batts to deliver the same Wh that you'd see from using a LiPo.

Originally Posted by Chris_The_Battery_Man

...They do maintain a better voltage curve under a load, than a Nimh, thus causing higher RPM's relatively speaking. But in most cases, this will actually cause a HIGHER LOAD on the pack

No it won't! Higher RPM would result in greater back-EMF. Leading to a smaller difference between supply voltage and back-EMF that results in less current draw, not more (which is why gearing down leads to lower draw and longer runtimes). Slightly more might be required to overcome the ever-so-slight increase in drag from the higher RPM, but that is a tiny fraction of the power required to move the rig.

[Chris_The_Battery_Man]

Originally Posted by el_tonto

Not quite. ONLY under light load, where the effective load resistance is much greater than the cell's internal resistance, will this be true. mAh s not the best measure of capacity - Delivered watt-hours (Wh) is, and that is a function of both load and cell internal resistance. Under high-load conditions where the cell's internal resistance is significant, the same mAh might be delivered, but at a lower voltage due to sag. Delivered Wh, therefore, is less with the NiMH. Or to put it another way, more mAh is required with the NiMH batts to deliver the same Wh that you'd see from using a LiPo.



No it won't! Higher RPM would result in greater back-EMF. Leading to a smaller difference between supply voltage and back-EMF that results in less current draw, not more (which is why gearing down leads to lower draw and longer runtimes). Slightly more might be required to overcome the ever-so-slight increase in drag from the higher RPM, but that is a tiny fraction of the power required to move the rig.

I was just speaking relatively. Generally, most people don't gear down from what I have seen, after the have volted up. Gearing down after volting up is always ideal from what I have been told, even in the go-fast cars. I am no expert, but this is what I am told by one of the worlds best electric boat racers.

Even though I have been MAKING batteries for 8 years, there is still a bit of a learning curve for the more technical stuff that I don't have to use on a day to day basis. So sorry if I mis spoke. Just trying to get the general idea out there.

I have only noticed better runtimes, in situations where I increase voltage, but still only use the lower 50-75% of the voltage range. Never noticed a relative difference in runtime, by changing chemistry in similar voltage packs. And based on my in formal "Learned from the job" experience, thats what makes most sense. From my understanding, it is hard to gauge any of this without a constant load. ie, watt hours, amps, or Mah. Or am I wrong?

[sococrawler]

Originally Posted by Chris_The_Battery_Man

I was just speaking relatively. Generally, most people don't gear down from what I have seen, after the have volted up. Gearing down after volting up is always ideal from what I have been told, even in the go-fast cars. I am no expert, but this is what I am told by one of the worlds best electric boat racers.

Even though I have been MAKING batteries for 8 years, there is still a bit of a learning curve for the more technical stuff that I don't have to use on a day to day basis. So sorry if I mis spoke. Just trying to get the general idea out there.

I have only noticed better runtimes, in situations where I increase voltage, but still only use the lower 50-75% of the voltage range. Never noticed a relative difference in runtime, by changing chemistry in similar voltage packs. And based on my in formal "Learned from the job" experience, thats what makes most sense. From my understanding, it is hard to gauge any of this without a constant load. ie, watt hours, amps, or Mah. Or am I wrong?

BLAH BLAH BLAH I don't care i just want my 1700's back chris, IMO those are the best lipo's you ever made me! now after a 1 1/2 yrs i need more!!!!!!
thanks for the info guy's now i'm even learning more than just "charge em this way" no charge em that way!

[vonclod]

i believe in some apps(extreme case) a lipo could actually drain faster than a nmh simply due to the big increase in amps that can be drawn if needed or required by the system..in a lipo you can drain that tank alot faster..comparing a lipo powered whatever vs a nmh whatever..im not sure what c rateing a good nmh is these days maybee 15-20 c for a short bit..some lipo,s now will deliver 40-50c and if that kind of draw is there the lipo could drain pretty fast but with much greater performance than the nmh powered whatever, the nmh will just struggle to keep up and get hot, maybee burn up ..another thing to consider is lipo is about half the weight..a 4600 nmh weighs about 460 grams or a pound give or take and a 5000 soft case lipo weighs roughly half that..i did a comparison at one time

[Planespotter]

So the increase in runtime comes from volting up and then using less throttle to drive the same speed?

[Terranaut]

Originally Posted by Planespotter

So the increase in runtime comes from volting up and then using less throttle to drive the same speed?

Well yes, to some degree. But the real improvement comes when you gear down after volting up. The lower gearing reduces load on the motor so your throttle will drop even further, and because your motor spins faster it is more efficient and therefore pulls less current. That's where the extra run time comes from.

The nice thing is the higher voltage more than accounts for the lower gearing so you don't lose wheel speed. And the higher volts combined with the lower gearing gives much more torque at the wheels. Way to go!

Cheers.

[Scottmisfits]

So how does this work in the touring cars? I went to Lipo, a smaller pack than my nimh packs, and geared up. Not only did I gain speed, lose weight but I gained runtime. Now, gaining a little bit of run time would be understandable. But this was when the Orion 3200mAh hardcase first came out so the battery tech was decent but not what it is now. Yes the car was lighter but I still had to add a bunch so it really wasn't that much lighter. And I went from 4200-4600mAh NiMh's to the 3200mAh Orion. The increase in runtime was from about 7 minutes at a race pace to almost 17. This was in stock racing so you don't gear the car to dump at the end of the race, you gear it to motor temp so I couldn't gear up any more.

I'm not posting this to be a know it all or anything. Since what I had known is wrong apparently, I want to know what it is. Cause what you guys said is completely opposite of my findings.

[Terranaut]

Originally Posted by Scottmisfits

I'm not posting this to be a know it all or anything. Since what I had known is wrong apparently, I want to know what it is. Cause what you guys said is completely opposite of my findings.

Not that anyone is wrong but put simply, crawling is completely opposite to touring cars.

If you geared your tourer to go 3MPH at full throttle, and then drove it at an average speed of 1/2MPH you would probably get 100 minutes runtime, much like my crawler. Know what I mean?

Cheers.

[Drakeremoray]

I moved to Lipo's on everything I own and have not looked back. For crawling though you may want NIMH if you want some weight up front on your rig if your battery is mounted on the axel. With the new 5xC charge rate though of some LIPo's they work great for my Heli and Plane...After running 100+ min on my crawler I am in no real hurry to recharge and get back out.

[uncivil]

One thing I may have missed was the ability to balance the lipo. This is something that will help keep those voltages up. Would it be considered along the same lines as a discharger? When I races onroad I had an integy discharger that would do the individual cells.

[Scottmisfits]

Originally Posted by Terranaut

Not that anyone is wrong but put simply, crawling is completely opposite to touring cars.

If you geared your tourer to go 3MPH at full throttle, and then drove it at an average speed of 1/2MPH you would probably get 100 minutes runtime, much like my crawler. Know what I mean?

Cheers.

I know exactly what you mean with that. That's an easy concept to grasp. What I was talking about was strictly in my TC which goes against what has been posted here. What you're talking about is not anything even close to what I posted though.

[Old Sloppy]

Originally Posted by Terranaut

Not yet mentioned is that LiPo batteries will be smaller, and lighter, than other types with the same capacity.
Cheers.

Lipo batteries are aproximately 40% lighter than Nimh of similar capacitiy.

Lipo batteries are the exact same volume as Nimh of similar capacitiy.

Harry

[bjs1785]

Im no expert, by FAR! All I know is once I switched to lipo, I will never go back.

I find longer run times slightly, when comparing the same size... but the major improvement is halfway through the run time, I am still getting full power. With an NIMH, Halfway through, I felt decreased power. When I was getting close to decharged... I might as well be decharged.

Also, there is alot more PUNCH!

My review is not a technical review. haha

[Terranaut]

Originally Posted by Scottmisfits

I know exactly what you mean with that. That's an easy concept to grasp. What I was talking about was strictly in my TC which goes against what has been posted here. What you're talking about is not anything even close to what I posted though.

From your previous post, you went to LiPo, geared up and got longer runs. This suggests that your new LiPo had a better discharge capability than your previous battery. Which means it held a higher voltage under load which in turn lowers current draw somewhat and rusults in better motor performance and longer runs. Seems to fit with what was posted earlier, what's you take on it?

Originally Posted by Old Sloppy

Lipo batteries are aproximately 40% lighter than Nimh of similar capacitiy.

Lipo batteries are the exact same volume as Nimh of similar capacitiy.

Harry

Regards weight, LiPo cells are in a plastic jacket, NiMH are in a metal can.

Depending on discharge capability (C rating), form factor and mAH capacity LiPo cell volume/capacity can be quite variable. When cells are assembled into a battery pack LiPo's benefit from the rectangular form, whereas NiMH cylindrical cells have wasted volume within a pack. Again, it's variable. I have no argument about volumes though, it's not an issue for me. I'm happy with my LiPo's.

Cheers.