Voltage gains vs weight = run time?

[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?