|
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???
|
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.
|
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. |
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. |
RE: Batteries???
one word- lipo
|
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. |
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. 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. |
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] |
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
|
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 |
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!
|
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! 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. |
RE: Batteries???
find me a link for a 2s at 20 bucks and i'll look into lipos!
|
RE: Batteries???
ORIGINAL: thehuntingterminator find me a link for a 2s at 20 bucks and i'll look into lipos! nimh can only deliver so much proformance, where as this lipo would scream. |
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
|
RE: Batteries???
it really isnt that big a deal, its your money, but in my opinion Lipo is the best upgrade for the money.
|
RE: Batteries???
it very well might be, but when your brushed titan 550 goes out every month or two, its better to go brushelss
|
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. |
RE: Batteries???
yeah but i want a traxxas VXl waterproof warranted system. i got a used bandit vxl for Xmas anyway
|
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;) |
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???
|
RE: Batteries???
ORIGINAL: The_Shark ORIGINAL: thehuntingterminator find me a link for a 2s at 20 bucks and i'll look into lipos! nimh can only deliver so much proformance, where as this lipo would scream. |
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).
|
RE: Batteries???
why would i wreck it??? i've been driving my rustler for almost a year now. lipo just seems like alotta work
|
RE: Batteries???
i thought the bandit was your first rc... i guess i was wrong. either way, i still say go 6 cell.
|
| All times are GMT -8. The time now is 01:24 PM. |
Copyright © 2026 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.