Notices
RC Car General Discussions This forum is for all general discussions related to radio control cars. Check forums below for more specific categories if applicable.

Batteries???

Old 12-08-2010, 08:05 PM
  #1  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default Batteries???

Does a 7 cell really have much more power than a 6??? I mean its only 1.2v, right? The only reason I was asking is because I can get a 5000mah 6 cell, or maybe even two, but I can only afford one 7 cell 4200mah. Is it worth getting one battery with a tad more power but less run time, or maybe even getting two 6 cells with more runtime but a little less power? Please don't say I should go Lipo, unless you want to pay for it. By the way, I would mostly be running them on a VXL system, or a little on an XL-5. A 6 cell should be plenty of power on VXL, right???
Old 12-08-2010, 08:57 PM
  #2  
burnineyes
Senior Member
 
Join Date: Jul 2007
Location: Seminole, FL
Posts: 430
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

A 7-cell pack, assuming everything else is equal, will generally give you about 5MPH and improved acceleration. Or if you drop the pinion size to match top speed you can gain a little runtime. If you can get 2 6-cell packs of equal or better quality that would definitely be the way to go, unless you already have a few packs to keep you up and running. But unless you are getting these packs at some major discount, it sounds like you have not done your research. Most average Nimh packs normally cost more than a good quality lipo from HobbyKing, a high quality Nimh pack is usually double the cost of a good lipo.
Old 12-09-2010, 05:15 AM
  #3  
Argess
 
Argess's Avatar
 
Join Date: Jan 2006
Location: Pleasantville, NS, CANADA
Posts: 2,355
Likes: 0
Received 1 Like on 1 Post
Default RE: Batteries???

That was a better answer than I can give as you gave a number relating to performance.....5 mph increase. But here's what I would have said anyway:

Power goes up by the voltage squared: P=V^2/R where R is the resistance, or in simple terms, the motor.

So for say a motor provides 1/2 Hp (373 watts) with a 6 volt battery, using the above forumla (and this is rough as the whole motor/esc isn't quite as simple as a purely resistive load), we get R = 0.097 ohms

Using this figure and re-calculating for 7.2 volts, we get 537 watts, or 0.72 Hp.

That's a 44% increase in power.

That seems high, and it's probably because I treated the motor/ESC as a simple resistive load, but it's an indication.

But let's go further. We know for top speed, the HP required is proportional to the Velocity cubed.

Therfore, if your vehicle does 39 mph with the 6 volts battery, it will do 44 mph with a 7.2 volt battery. And that is a 5 mph increase.

If your vehicle only goes 20 mph with a 6 volts battery, it will now go 22.6 mph with a 7.2 volt battery. And that's a 2.6 mph increase.
Old 12-09-2010, 06:25 AM
  #4  
AdamHeard
Member
 
Join Date: May 2010
Location: San Luis Obispo, CA
Posts: 49
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

There is a simpler method for getting a ballpark guess of what increased voltage will do.

Power = Torque x speed.

Compare the two voltages at 50% free speed. (that's 1/2 free speed x 1/2 stall torque).

Both the free speed and stall torque linearly increase with voltage.

So, if at V1, Torque = T, and speed = S, then at V2, torque = V2/V1T and speed = V2/V1S

for a switch from 7.2 to 8.4 V, this works out to a new power of 1.36 the original. This is at half free speed, but the ratio would be true anywhere on the curve.

This also shows that double voltage quadruples power, which is a pretty cool fact. Increasing voltage certainly does not increase power linearly like many people think,



Your calculation with velocity cubed is a bit rough because you don't get constant power out of the motors, and drag isn't the only factor contributing to top speed.

ORIGINAL: Argess

That was a better answer than I can give as you gave a number relating to performance.....5 mph increase. But here's what I would have said anyway:

Power goes up by the voltage squared: P=V^2/R where R is the resistance, or in simple terms, the motor.

So for say a motor provides 1/2 Hp (373 watts) with a 6 volt battery, using the above forumla (and this is rough as the whole motor/esc isn't quite as simple as a purely resistive load), we get R = 0.097 ohms

Using this figure and re-calculating for 7.2 volts, we get 537 watts, or 0.72 Hp.

That's a 44% increase in power.

That seems high, and it's probably because I treated the motor/ESC as a simple resistive load, but it's an indication.

But let's go further. We know for top speed, the HP required is proportional to the Velocity cubed.

Therfore, if your vehicle does 39 mph with the 6 volts battery, it will do 44 mph with a 7.2 volt battery. And that is a 5 mph increase.

If your vehicle only goes 20 mph with a 6 volts battery, it will now go 22.6 mph with a 7.2 volt battery. And that's a 2.6 mph increase.
Old 12-09-2010, 07:37 AM
  #5  
The_Shark
Senior Member
 
The_Shark's Avatar
 
Join Date: Jun 2007
Location: Plainfield, IL
Posts: 7,163
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

one word- lipo
Old 12-09-2010, 07:57 AM
  #6  
Argess
 
Argess's Avatar
 
Join Date: Jan 2006
Location: Pleasantville, NS, CANADA
Posts: 2,355
Likes: 0
Received 1 Like on 1 Post
Default RE: Batteries???


ORIGINAL: AdamHeard
Your calculation with velocity cubed is a bit rough because you don't get constant power out of the motors, and drag isn't the only factor contributing to top speed.
Yes, it was very rough and didn't allow for efficiency amongst a whole lot of other things. But it yielded numbers that appear realistic.....LOL.
Old 12-09-2010, 09:14 AM
  #7  
DaveG55
Senior Member
 
DaveG55's Avatar
 
Join Date: Jan 2005
Location: Stockbridge, GA
Posts: 8,275
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???


ORIGINAL: burnineyes
But unless you are getting these packs at some major discount, it sounds like you have not done your research. Most average Nimh packs normally cost more than a good quality lipo from HobbyKing, a high quality Nimh pack is usually double the cost of a good lipo.
It's true lipo packs can be less expensive than really good Nimhs. But that does not take into account the other accessories needed to go lipo. If you are set up for Nimh then you'll need to get, at the minimum, a new lipo capable charger and possably a ballancer. Those things can make the switch a more expensive proposition than just buying different batterys.
Still, if it is at all possable, save up for a while longer and make the switch to lipo. The advantages make it well worth the wait.
Old 12-09-2010, 12:52 PM
  #8  
cooldriver385
Senior Member
 
Join Date: Jul 2008
Location: Loxahatchee, FL
Posts: 1,058
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

A good lipo from HobbyKing is about 20-30 dollars. An amazing Accucel 6 charger that also will charge your Nimhs will be 25 bucks. It has balance jacks, so no balancer is needed. A power supply, you can buy one from HobbyKing, or buy a PC power supply, thats 15 bucks. A soldering iron, thats 10-20 bucks. Dad might have one. Some connectors to solder on to your battery and charger, thats 5-10 bucks.

The ease of lipo learning, thats priceless.
[link=http://www.youtube.com/watch?v=sGgGH9pxAGA]Squirrel's Lipo University Vid 1[/link]
[link=http://www.youtube.com/watch?v=QSNYKgnn11A]Squirrel's Lipo University Vid 2[/link]
Old 12-09-2010, 02:33 PM
  #9  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

see the thing is i can get a 6 cell 5000mah for about the same as a 7 cell 3300mah. i would think the 6 cell would be enough power for me since i'm goin brushless and i'm used to an XL-5. plus, i would get like 8-10 more minutes of runtime i would estimate
Old 12-09-2010, 03:04 PM
  #10  
rc person 577
Senior Member
 
rc person 577's Avatar
 
Join Date: Mar 2010
Location: Homer, NY
Posts: 390
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

OP, What can you get the NIMH batts for? Like stated earlier lipos and a charger could be as much or a little more than the 6 cell 5000.

Have you done the research to find the price difference?

Just a thought
Old 12-09-2010, 03:07 PM
  #11  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

i already have a NIMH quick charger and i can get the 5000mah for 30 bucks off ebay. u can't buy a lipo for that!
Old 12-09-2010, 05:50 PM
  #12  
The_Shark
Senior Member
 
The_Shark's Avatar
 
Join Date: Jun 2007
Location: Plainfield, IL
Posts: 7,163
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???


ORIGINAL: thehuntingterminator

i already have a NIMH quick charger and i can get the 5000mah for 30 bucks off ebay. u can't buy a lipo for that!
since i started playing around with lipo battries, i wont go back to nimh.

lighter, faster charging, higher outputs ect. prices have come down so much from a few years ago.

i just got two name brand 3s lipos for $20 brand new! you can also get a 2-3s carger for $20.
Old 12-09-2010, 07:11 PM
  #13  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

find me a link for a 2s at 20 bucks and i'll look into lipos!
Old 12-09-2010, 07:32 PM
  #14  
The_Shark
Senior Member
 
The_Shark's Avatar
 
Join Date: Jun 2007
Location: Plainfield, IL
Posts: 7,163
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???


ORIGINAL: thehuntingterminator

find me a link for a 2s at 20 bucks and i'll look into lipos!
well, for a smaller pack (up to 3000mah) its not hard to find, however here is one for a little more than $30 [link=http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10305]link[/link]

nimh can only deliver so much proformance, where as this lipo would scream.
Old 12-09-2010, 07:37 PM
  #15  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

i would love to have lipos, but they seem like so much effort, and really right now i would rather go brushless on everything first. good link though
Old 12-09-2010, 07:42 PM
  #16  
The_Shark
Senior Member
 
The_Shark's Avatar
 
Join Date: Jun 2007
Location: Plainfield, IL
Posts: 7,163
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

it really isnt that big a deal, its your money, but in my opinion Lipo is the best upgrade for the money.
Old 12-09-2010, 07:43 PM
  #17  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

it very well might be, but when your brushed titan 550 goes out every month or two, its better to go brushelss
Old 12-09-2010, 07:49 PM
  #18  
The_Shark
Senior Member
 
The_Shark's Avatar
 
Join Date: Jun 2007
Location: Plainfield, IL
Posts: 7,163
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

tis why you go after market on the motors, stock cans are disposible, real ones have replacible brushes.

also, it is possible to go cheap and install a helie outrunner brushless motor for $10-$50 and use either a hobbywing or a plane esc for $10- $50 just look it up on youtube. im going to build a outrunner rc18t soon.
Old 12-09-2010, 07:51 PM
  #19  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

yeah but i want a traxxas VXl waterproof warranted system. i got a used bandit vxl for Xmas anyway
Old 12-09-2010, 07:55 PM
  #20  
The_Shark
Senior Member
 
The_Shark's Avatar
 
Join Date: Jun 2007
Location: Plainfield, IL
Posts: 7,163
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

thats cool, i loved my bandit, mine was old school with a slider resister as a esc

so if your getting a brushless car as a gift why cant you get a lipo set? you cant afford it? just sell some stuff you dont need, thats what i did when i was too young to work, i always bought and sold.

in no way am i trying to be a smart ass, it may seem that way as text makes you sound like a jerk
Old 12-09-2010, 08:12 PM
  #21  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

ha, ok and thanx for the help. i'm not buying the bandit, my parents are for me, and that's pretty much all my budget for Xmas. so whats your opinion: 6 cell 5000mah or 7 cell 3300mah??? both run on vxl, no lipos. what you think???
Old 12-10-2010, 03:01 AM
  #22  
cumquat
Senior Member
 
cumquat's Avatar
 
Join Date: Mar 2008
Location: kalamazoo, MI
Posts: 6,812
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???


ORIGINAL: The_Shark


ORIGINAL: thehuntingterminator

find me a link for a 2s at 20 bucks and i'll look into lipos!
well, for a smaller pack (up to 3000mah) its not hard to find, however here is one for a little more than $30 [link=http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10305]link[/link]

nimh can only deliver so much proformance, where as this lipo would scream.
or you could get 2 of these http://hobbyking.com/hobbyking/store...idProduct=9951 which will be the same price in the end but you will have 2 packs that could be wired in parallel for a 6000 mah pack.
Old 12-10-2010, 03:03 AM
  #23  
cumquat
Senior Member
 
cumquat's Avatar
 
Join Date: Mar 2008
Location: kalamazoo, MI
Posts: 6,812
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

go for the 2 6 cells. if lipo/brushless is what everybody says it is then you will wreck your "new" bandit in 5 minutes flat, especially if you have snow/ice (that is, if you can even get traction).
Old 12-10-2010, 04:47 AM
  #24  
thehuntingterminator
Senior Member
Thread Starter
 
Join Date: Jul 2010
Location: Tyler TX
Posts: 3,549
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

why would i wreck it??? i've been driving my rustler for almost a year now. lipo just seems like alotta work
Old 12-10-2010, 05:01 AM
  #25  
cumquat
Senior Member
 
cumquat's Avatar
 
Join Date: Mar 2008
Location: kalamazoo, MI
Posts: 6,812
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Batteries???

i thought the bandit was your first rc... i guess i was wrong. either way, i still say go 6 cell.

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.