Voltage gains vs weight = run time?

[stum]

Has anyone bothered calculating the breakeven point for when higher voltage could actually lead to longer run times?

Let me give a few constraints for my application and why I ask the question...

• 660grams max weight (give or take)
• 14.8v-22.2v (4s,5s,6s)
• $240 usd max per pack
• Losi 8ight-t (tekno v3)

Here is the thing.. how many watt hours is needed to counteract some additional weight. Keep in mind heat is not an issue with any of the voltages and all setups will be geared for around 42mph.

A 4s 5000mAH 30c pack weighs in at 528 grams, a 5s 5000mAH 30c pack weighs in at 660grams (my max allowable)

The 4s 30c pack is about 2220 watt/hr
The 5s 30c pack is about 2775 watt/hr

The question is; how can I calculate the loss from the additional weight to see if there is any net gain in run time? 132grams is quite a bit more weight, so not only do you have more amp pulls for the entire pack but you will most likely cause more damage over time while racing due to the added weight (part wear). That and the higher cost, already have the 5s not looking as good. The 6s due to the weight limitations will be around 4000mAH thus making it equal to the 5s setup so there is no real gains (that is assuming a 30c pack). A 25c pack would be equal to the 4s 5000mAH pack in watt/hrs but would actually have less run time due to the added weight making it a very bad choice expecially considering the cost and would actually yeild the least amount of run time of any of my choices.


Thoughts?

[What's_nitro?]

Not sure if the relationship is really so linear, but you could try using a ratio of WH/m. Say your truck has a mass of 4000g. Lets also say that you KNOW that your truck will run for 20 minutes on a 2500WH pack. Now with the heavier pack (~700g) added (~4700g total) you would need a pack that stores ~2940WH.

Since: 4700g/4000g=1.175 and so 1.175*2500WH=~2940WH

Of course there are other things to consider. But assuming your driving style is constant through every lap () then all that we are playing with is inertia. So the extra WH should fill in the gap where the extra power is needed to accelerate the heavier vehicle and achieve the same run time. This also assumes that the power system draws the same power at the higher voltage, etcetera, etcetera....

Any pack within your weight limit with a higher WH than the ~2940 figure should in theory give longer run times.

I hope this made sense. It's almost 4 in the morning so I can't tell....

[VintageMA]

Please help me out on how you calculated the watt/hr rating you obtained.

For the two 5000 mAh cells you mention I get:
4S = 14.8V 14.8V x 5Ah = 74W/hr
5S = 18.5V 18.5V x 5Ah = 92.5W/hr

Personally I go for a different perspective on looking at the higher voltages and try to keep the battery pack weight and overall power about the same. You know for most lipos of the 25C and 30C that the energy density is about the same (ie: a single 5000 mAh cell would be equal to two 2500 mAh cells).

If you are running a 4S 5000mAh pack and want to go to 5S I would recommend a 5S 4000 mAh pack. Or for 6S a 3300 mAh pack.
4S - 14.8V x 5Ah = 74W/hr
5S - 18.5V x 4Ah = 74W/hr
6S - 22.2V x 3.3Ah = 73.26W/hr

All of these setups will give you about the same runtime, but the 6S may give you a little more than the 4S. The reason is that if it takes say 1000Watts to move you truck then it will take ~33% less Amps at 6S than at 4S to produce the same 1000Watts.

Your batteries will run cooler as well as your ESC and motor with less current flow. It is the voltage drop across the purely resistive load of each component that produces the heat. And with lower current flow, the load will be easier on the cells and you will see less voltage drop at the cells themselves. Because of this the cells will be able to put out a higher voltage for a longer period of time where the higher current cells heat up faster and experience more voltage drop and give out quicker.

You may see that with a 4S 5Ah pack that you are only putting back in about 4500 mA, but running the 6S setup (say on the 3300mAh cells) you may find that you put back in 3200 mA because the pack ran more efficiently.

[stum]

Originally Posted by VintageMA

Please help me out on how you calculated the watt/hr rating you obtained.

For the two 5000 mAh cells you mention I get:
4S = 14.8V 14.8V x 5Ah = 74W/hr
5S = 18.5V 18.5V x 5Ah = 92.5W/hr

First take your true or as close to true "c" rating and multiply by AH giving you your batteries capable amps, then simply Voltage x Amps is what I used.. you can't possibly figure out how long a battery will last doing volts x AH calculation because if the packs can't handle the load (amp draw) you have them on they will LVC early and never get close to that difference. Motors are capable and will pull the same peaks on any voltage so if you are going/stopping a lot you are not going to net a ton of savings on the amp draw, running wot or when you are not pushing it with a higher voltage system it will require less avg amp draw yes... but that isn't really the case more times than not when racing offroad.

Personally I go for a different perspective on looking at the higher voltages and try to keep the battery pack weight and overall power about the same. You know for most lipos of the 25C and 30C that the energy density is about the same (ie: a single 5000 mAh cell would be equal to two 2500 mAh cells).

If you are running a 4S 5000mAh pack and want to go to 5S I would recommend a 5S 4000 mAh pack. Or for 6S a 3300 mAh pack.
4S - 14.8V x 5Ah = 74W/hr
5S - 18.5V x 4Ah = 74W/hr
6S - 22.2V x 3.3Ah = 73.26W/hr

All of these setups will give you about the same runtime, but the 6S may give you a little more than the 4S. The reason is that if it takes say 1000Watts to move you truck then it will take ~33% less Amps at 6S than at 4S to produce the same 1000Watts.

Most motors we use can and do pull over 100amps cont and do so a lot, if a 6s lipo can't meet that min amp draw when it hits that 100amps pull it will drain it a ton faster than a higher rated battery (even of lower voltage) that can handle the pull. A 3300mAH pack rated for 30c is the absolute lowest you could possibly go and hope for better results as that just nudges 100amp rating. Drop in a 3300mAH pack good for 20c and it's only good for about 66amps and will dump a ton faster (hit lvc) than the 30c would and you lost any gains from your higher voltage. Again keeping in mind this is for dirt track racing w/ constant spikes, not high speed passes.

Your batteries will run cooler as well as your ESC and motor with less current flow. It is the voltage drop across the purely resistive load of each component that produces the heat. And with lower current flow, the load will be easier on the cells and you will see less voltage drop at the cells themselves. Because of this the cells will be able to put out a higher voltage for a longer period of time where the higher current cells heat up faster and experience more voltage drop and give out quicker.

Heat is not an issue with any of these options.. furthermore most lipo's operate more effeciently around 120-130 range if your system doesn't pull enough to reach these temps you may actually not get as good of results as well, something else to consider.

You may see that with a 4S 5Ah pack that you are only putting back in about 4500 mA, but running the 6S setup (say on the 3300mAh cells) you may find that you put back in 3200 mA because the pack ran more efficiently.

PS any time you don't put back in the full mAH in a 4s, 5s or 6s setup there is an easy fix.. buy better batteries, this means your system is drawing more than they are rated for. My weak 5k packs come back around 4800, my good 5k packs come back around the full 5000mAH.

[VintageMA]

I agree with some of what you say, but disagree with most of it.

Power is what moves a vehicle not volts or Amps! Power is a combination of volts x amps. If your vehicle takes 1000 Watts (arbitrary number) at peak power, it will take about 1000 watts at peak power regardless of using 4S, 5S or 10S.

You get efficiency with a HV setup because it will take much less amps at 10S than it will at 4S to produce the same power.

The power ratings you are providing C rating x pack Ah x voltage are the continous watts the pack can theoretically put out, not the actual W/hr ratings of the pack. If you drive a 30C pack at 30C full time it will last for 2 minutes.

You say that our motors can hit 100 Amps regularly and they do, yes that is true. But that is what the peak 50C or 60C rating of the pack is for - an instantaneous burst of energy and nothing more. But as you go up in voltage that peak Amp draw is much less - and that is the whole point of HV.

Since heat is not an issue in this thread as you describe I won't waste my time elaborating.

If your arguement is to go from a 4S to a 5S pack and stay with 5000mAh cells and get better runtime due to efficiency then yes - you may get a few more minutes, but at the same time you'll need more power for the same performance so it's not a true performance gain.

[lincpimp]

Most buggies can get by with a 25c 5000 4s pack. So a 25c 2500 8s pack will have the same overall effect. Most lipos from the same mfg have the same energy density, so go by weight. So if the 4s pack weighs 500 grams, a 6s pack that also weighs 500 grams will provide the same runtime, if the motor rpm and gearing are setup the same for each setup.

Most people stick with 4s as it is easier to find a solid 4s esc than a solid hv esc...

[What's_nitro?]

Originally Posted by VintageMA

Please help me out on how you calculated the watt/hr rating you obtained.

I can't believe I didn't catch that... Apparently being wayyy over-tired (at 4:00 AM) causes me to overlook things I otherwise would have caught immediately. Those ratings look much better!

However, I would like a 2500WH LiPo that fit into my RC...

Anyways do you think that ratio method would be a good way to estimate the required extra capacity? Or was I completely out of my mind when I posted that???

[JThiessen]

I dont want to hijack this thread, but it relates to a dilema I am trying to solve. My B'in law has an EMaxx with a Novak HV-4.5 and he's got a pair of Zippy 4S 4300's. He is relying on me to set it up for him, and I'd like to know from you guys, wether I should set it up as a 1P or a 2P setup. After all the reading I've done, it doesn't appear that having 9600mah is going to do anything for him except add "X" amount of run time. If that will be more than using each pack individually, me thinks its goings to a wash, with only the weight being different. Am I correct on that?

The higher voltage/lower kv setups are going to run longer and more efficient. As others have stated, as you go up in voltage you should be running lower mah packs that are going to weigh about the same as the lower voltage higher mah packs for the same runtime.

I ran 4s 5000 setup earlier this year and I get a few more minutes run time with my 5s 4000 setup (geared for the same speed) under race conditions. The 5s 4000 pack is slightly heavier, but not enough to make a difference in how the car handles. I get SUBSTANTIALLY more runtime with a 5s 5000 pack vs a 4s 5000 pack even though the 5s 5000 pack is SUBSTANTIALLY heavier.

You don't have to over complicate it. All I do is go volt & mah to get energy in pack and match that you want to run. Verify that the higher volt pack is reasonable weight and go have some fun.

14.8 * 5000 = 74,000 mah - Zippy-H Pack Weight: 529 grams
18.5 * 4000 = 74,000 mah - Zippy-H Pack Weight: 581 grams
22.2 * 3300 = 73,260 mah - Zippy-H Pack Weight: 529 grams

The 5s 4000 pack is actually a higher C rate pack. the 4s & 6s are 20-30c rated while the 5s 4000 is 25-35c rated.

The higher voltage will run a bit longer due to better efficiency even though the total energy of the pack is the same and even if the weight is slighty heavier.

[Finnster]

I don't like that C rating multiplier either, and I think it gives far too much influence to an otherwise unreliable and often distorted rating. IE, going from a 25C to 30C is a 20% increase. This is unrelatable to the difference in runtimes.

As said, adding 130g to a Truggy that weighs ~4300-4500g is only a 3% increase. I doubt you will really notice it. However if you've increased the pack size by 25%, you will most def notice the increase in runtime.

I often calc what my power consumption is for a given application, and its then freed from the voltage fo the pack. EG, your current setup, over 10 min of racing, how many mah do you consume? You can easily turn this to Wh/min. Then you determine the runtime you desire and then calc the needed Wh. Then you can determine the various capacities needed for a given voltage.

I would not assume every motor is going to pull 100A+. Its not. My 1515/2Y-8S setup never pulled that high, yet made as many watts as the low volt setup before. Additionally you do not produce as much waste heat, which is wasted runtime.
Given the same Wh of batteries, my 2Y on 10S will run longer, harder and cooler than a 1.5D on 4S as I am not wasting 100's of watts of power as heat.

The eff% should also be considered, but yeah, beyond a certain amount its not worth worrying over as there will be far more factors that have a greater influence that you can control here (such as style of driving, track layout, mud accumulation, etc etc.)

[lutach]

This can only be answered when anyone tries High Voltage. I got 30 minutes from my 25C 3700mAh 4S packs in a 8S set up in my MBX5T. I only got that though because I had to go from a 15T to 17T pinion in order to fit my Neu 1515/2Y, but with the 15T in there I was able to get almost 1hr of run time. So all the math can be done, but to find the true answer, one has to actually go HV to see it happen in front of you.

[TexasSP]

Oh what do you know about HV lutach?!


Seriously though, I came, I saw, and HV kicked LV's ass! Thanks in part to guys like Lutach, Linc, and several others I am a faithful HV convert and have seen the light for myself.

This is kind of like all those theories a certain battery assembler used to post on the TRX forums about the superiority of nimh output over lipo output regarding voltage sustain under load. Simplistic theory and formula's never tell the truth real world testing/experience will. Even controlled testing lies because nothing can duplicate properly what happens in real application.

[lutach]

Originally Posted by TexasSP

Oh what do you know about HV lutach?!


Seriously though, I came, I saw, and HV kicked LV's ass! Thanks in part to guys like Lutach, Linc, and several others I am a faithful HV convert and have seen the light for myself.

This is kind of like all those theories a certain battery assembler used to post on the TRX forums about the superiority of nimh output over lipo output regarding voltage sustain under load. Simplistic theory and formula's never tell the truth real world testing/experience will. Even controlled testing lies because nothing can duplicate properly what happens in real application.

I'm still learning. I just hope Castle makes a 20S ESC for cars and then I will show everyone how a 10A or less spike will look like lol.

[jhautz]

Originally Posted by lutach

I'm still learning. I just hope Castle makes a 20S ESC for cars and then I will show everyone how a 10A or less spike will look like lol.

Geez lutach 20s.... Just get an extention cord and plug it into the wall...

[lutach]

Originally Posted by jhautz

Geez lutach 20s.... Just get an extention cord and plug it into the wall...

Yes sir, 20S and if I'm feeling good, I'll go 22-23 small LiFEPO. I'll have to see which one will weigh less.