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Old 10-11-2007, 03:03 PM   #16
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Originally Posted by R1R 7humbs View Post
If I had a 7.4v pack with 8 amps of capacity should it perform as well as a 11.1v pack with 5 amps of capacity?

amps * volts = watts equation says they should be very simliar. From what I've read the 7.4v pack would heat up more than the 11.1v pack because of the nature of the current. The 7.4v and 11.1v packs are the same size +-1mm and I could use the extended run time of the 8 amp pack.

I've asked this question before (diff in voltage, without mentioning the difference in mah) I knew more about batteries and I was told the 7.4v pack won't perform as well... can anyone explain why, Thanks.
I have a bachelors in EE, so I will attempt to answer your question.

I am going to assume that when you say 7.4V with 8 amps of capacity, you are actually saying 7.4V with 8000mAh of capacity. Amps alone is not a measurement of capacity unless time is included. So, I will also assume you are talking about 11.1V pack with 5Ah or 5000mAh of capacity. (Current x Time = Capacity)

Because mAh is not a direct measurement of energy in a battery, watt-hours or joules must be used to compare these two batteries.

In watt-hours, the 7.4V 8000mah pack has 59.2kWh and the 11.1V pack 5000mAh pack has 55kWh. So in this case, loads being equal, the 7.4V pack will have a longer runtime than the 11.1V pack. (Volts x Capacity = Watt-Hours = Total Energy)

As far as performance, if both batteries are 20C, the 7.4V battery at 8000mAh can have 1184W sustained. The 11.1V battery at 5000mAh has 1110W sustained. This shows that the 7.4V battery slightly wins out in performance compared to the 11.1V battery. (Volts x Capacity x Current Rating = Watts)
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Old 10-11-2007, 03:58 PM   #17
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but what will the effect of higher voltage do to current? what will the effect of higher voltage have on the motor?............
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Old 10-12-2007, 10:27 AM   #18
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Originally Posted by mattnin View Post
"...In watt-hours, the 7.4V 8000mah pack has 59.2kWh and the 11.1V pack 5000mAh pack has 55kWh. So in this case, loads being equal, the 7.4V pack will have a longer runtime than the 11.1V pack. (Volts x Capacity = Watt-Hours = Total Energy)

As far as performance, if both batteries are 20C, the 7.4V battery at 8000mAh can have 1184W sustained. The 11.1V battery at 5000mAh has 1110W sustained. This shows that the 7.4V battery slightly wins out in performance compared to the 11.1V battery. (Volts x Capacity x Current Rating = Watts)"
Thanks and yes I meant the capacity or what it can put out over the period of 1hr. I should have said 8 amps / hr not just an 8 amp pack.

Quote:
Originally Posted by mugenlife
but what will the effect of higher voltage do to current? what will the effect of higher voltage have on the motor?............
mattnin, I'd also like to hear what you think the higher voltage will do to the current (more drive right?). Because from what I've learned... even though the 7.4v 8000mah pack can deliver more energy than the 11.1v 5000mah pack, a 7500kV motor will go faster with the 11.1v motor (right?). I guess the speed difference has to do with the inner workings of the electric motor and the change in the magnetic field due to electrical current is obviously manipulated by the number of turns or wire wraps, right?

I recently read that it's actually better to run high voltage and low amperage. Simliar to why power lines are high voltage and low amperage, so they can be thin... if it was the other way around they'd melt, right?
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Old 10-12-2007, 01:19 PM   #19
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more voltage means less current is needed to do the same work. more voltage will make the motor spin faster.....waits to get flamed and then gets ready to ask another question which wont get answered by those that will flame me
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Old 10-12-2007, 04:44 PM   #20
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Quote:
Originally Posted by R1R 7humbs View Post
I recently read that it's actually better to run high voltage and low amperage. Simliar to why power lines are high voltage and low amperage, so they can be thin... if it was the other way around they'd melt, right?
Yup this is true. In 1/8th scale electric conversions we typically use 4s lipos to keep the current and temps down as opposed to using a 3s.
(3s not needed in 10th scale, unless bashing)

simply put, given the same setups, driving conditions, etc, the higher the capacity of the batteries, the longer the runtime. You can figure estimated runtimes if you run your car for 5 minutes, then check how many maH you put back in after you've charged the pack. Divide by 5, now you know how many maH you burn per minute.
Different batteries will perform differently, so this is only a guideline.

The higher the voltage of the batteries, the faster the motor can spin.
This is why when you battieries get low, your car slows down. The voltage of the battery has dropped.

Think of maH as your gas tank. bigger number, bigger tank.
higher voltage, higher potential rpms. (higher top speed)
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Old 10-12-2007, 09:03 PM   #21
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If I may, lets turn this into practical useful information, and look at the whole system, not just the battery specs. Yes, the 2 batteries do have very similar energy storage, and your comment about using different motors is headed in the right direction. But I don't think gearing has been well explained, it can have essentially the same primary effect. You would get close to the same results by changing gearing that you would by changing motors.

Given the same motor and gearing the higher voltage will indeed at least try to spin the motor faster, but this higher power comes at the expense of higher current, thus lowering runtime. If you go down one third in either motor Kv or pinion gear (those 2 combine to produce the load that has been mentioned) then current is lowered, and we are almost back to where we started. (Yes, you go down one third to two thirds, then multiply that by the 1.5 voltage factor and you are back to 1.) And more gear with 7.4v will produce almost same primary result as the higher voltage pack.

Well, I did say almost, it is also true that the higher voltage is more efficient, less current losses. And less heat on the motor and esc IF the above adjustment is made. Just like high voltage powerlines, bingo.

Bottom line, it's a system, and the gearing is always a HUGE part of the equation. Given appropriate gearing they are very close, with a slight edge to the more efficient higher voltage set up.

And yes, this is a general discussion, other factors come into play if you are pushing things to the limit, say for example the crazy wild 8th scales mentioned. Wow, those ya gotta see. The higher power involved increases the current losses, making the higher voltage a bigger benefit. And the hotter motors have voltage limits, at some point the electric field in the motor can't be switched fast enough, the bearings have limits, the esc has limits, the batteries, torque demands on the driveline, yada yada.

Me, if I didn't care about racing rules, go for the higher voltage and gear down. OK, so maybe I wouldn't gear down a third, we are all by definition crazy idiots here after all, so I'd gear down maybe 20%, give up a little run time, and go like crazy.....

If on the other hand, if you ever wanted to race, (try it, it's addictive) the 7.4v pack may actually become legal at some point.

Hope this helps, ask questions or flame on as desired.

And if anyone really cares, I'm a practicing degreed Mechanical Engineer with 20 years of RC. Go LOSI! I have the very first run of the JR-X2, orange cracking wheels and all, among many others.

Dave
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Old 10-12-2007, 09:04 PM   #22
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My post above was referring to energy and speed, the higher voltage with lower gearing should have an acceleration advantage as well. Which would reduce runtime somewhat. Man, these systems are complicated, even for those who should know a little better.....

Dave

Last edited by Dave H; 10-12-2007 at 09:22 PM. Reason: PS
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Old 10-15-2007, 10:32 PM   #23
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Quote:
Originally Posted by R1R 7humbs View Post
If I had a 7.4v pack with 8 amps of capacity should it perform as well as a 11.1v pack with 5 amps of capacity?

amps * volts = watts equation says they should be very simliar. From what I've read the 7.4v pack would heat up more than the 11.1v pack because of the nature of the current. The 7.4v and 11.1v packs are the same size +-1mm and I could use the extended run time of the 8 amp pack.

I've asked this question before (diff in voltage, without mentioning the difference in mah) I knew more about batteries and I was told the 7.4v pack won't perform as well... can anyone explain why, Thanks.
2s 8000mAH maxamps lipo
~7v under load x (amps) 160amps = 1120 watt hours
3s 5000mAH maxamps lipo
~10.5v under load x 100amps = 1050 watt hours

That is the math for you in watt hours, now I will tell you that with the velineon you can gear it up to 30t-32t pinion and a 76t spur and get to around 70mph with the 3s but you wouldn't even want to try it with 2s as it would be way too hot, *if the 2s could provide the specs and stay with in temp itself (under 140) the motor would still break 200 in no time geared like that. You need increased effeciency of the voltage w/ the 3s if you want to run 60-70 range. What those numbers really mean is that the two batteries if running the same speed would last about the same amount of time

That velineon is a fantastic motor, I wouldn't change it for anything on the market right now. Silky smooth and one torquey sog.

hope that helps.. and with a little less attitude as well :P
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