View Full Version : Oldie but a newbie. Questions...
10-09-2010, 12:12 AM
Okay guys..... I've been into RC cars since I was 12 and I'm now 35. Problem is..... I've been into Nitro for so long I'm clueless about electric. I understand some basics don't get me wrong, but the whole LiPo thing has changed in awhile. Are they still as dangerous as they used to be or have they gotten better? I've been out of cars now for about 2-3 years. All my buggies and truggies are collecting dust.
This new Rally though has me ready to come out of "retirement". What is the main advantage of going LiPo over NiMH's? I know run time and punch but how much longer run time? Traxxas says it'll do 50mph with the double NiMH's. Will the LiPo's make it any faster or just accelerate faster and a longer run time?
I am aware of the dangers of LiPo's. I have a Losi RTR little 1/18th I bought with a Tekin Brushless in it and it was fun to play with. A little wheelie monster. Haven't used it in years and only played with it for about 2 months.
What is the charge time of a LiPo pack? Or should I just stick with the NiMH's? I will have to buy a new charger as well. I'd just rather have a new one than my old one I used for my receiver packs.
Thanks for any and all advice. Can't wait for this thing to come out. I go to a bunch of drag racing events and this will be fun to play with in the pits during down time.:D
10-09-2010, 12:53 AM
LiPO technology definitely has it's pros and cons. They can be dangerous, as any battery can, but newer technology has improved the odds against a catastrophic failure. Newer ESCs have the ability to detect the low voltage point of most LiPO battery configurations. Some of these have an auto feature while others allow you to fine tune the voltage cut-off.
You will need to make sure you don't discharge the battery passed it's safe limit. Another potential problem is making sure you store the batteries properly so they don't drop below their minimum voltage.
LiPO batteries have higher discharge rates, making them superior to NIMH for performance. They have a much flatter discharge profile, making power almost linear until the pack is at it's safe discharge point. LiPO batteries can also be cycled hundreds of times with very little decrease in performance. They don't require cycling or discharging. They also don't require a cool down of any kind before you start charging them again. They don't suffer when cycled more than few times in a short amount of time. These batteries also weigh considerably less, which in turn relates back to increased performance in virtually all areas. Another benefit of LiPO batteries are the ability for the form factor to vary dramatically when compared to NIMH. You can usually have a much higher voltage pack in the same space.
LiPO batteries do however, require balancing. Some packs stay balanced extremely well, while others might need to be balanced regularly.
I don't know enough about LiPO technology to know why they might yield longer run times. It could be as result of the reduced weight causing less work for the motor. I do know they can be made in giant capacities when compared to NiMH.
Newer LiPO batteries can be charged at much higher amperage. Some batteries go above 5C charge rates. These days we're probably more limited by the output of our chargers than the batteries. Overall, if you're charging at 1C on any battery it will likely take the same time no matter how much capacity it has.
In conclusion, LiPO batteries require more effort from the user, but in my opinion offer so much more than NiMH it's completely worth it.
10-09-2010, 01:07 AM
Thanks for the reply Melon. One reason I've been into Nitro for all these years is the re-fill and go factor. I did the whole electric thing since the days of my first Grasshopper.:lol: I think having an on-road car again could be fun. I had quite a few Traxxas products and they've always been good to me.
Now... talk to me about packs. What's the difference between say a 7.4V and an 11.1? Speed, power, or run time? Can you do 2 LiPO packs with these? 2 2S's or 2 3S's?
Sorry for all the questions but I have a modding habit. I can leave ANYTHING I own alone or stock. I'm a speed freak. I understand the pinion should be changed? That correct?
After years of not having a local track, one has finally be opened again and I'm kinda itching to get back into racing. I might even buy a new engine for my Mugen Truggy who knows. They have an off-road and a on-road track there. That was one of the reasons I got out. Our track closed. That and I've got 3 boys 7, 6, and 2.
If you've got a link to a site that explains everything, I'll take that instead of you blistering your fingers. Thanks again.
10-09-2010, 01:45 AM
I used to love Nitro, but got tired of tuning and cleanup.
LiPO cells are nominal 3.7V each. So making a 2S pack you'd have 7.4V nominal. By adding another cell, making a 3S pack, you'd now have 11.1V pack. And so on... Increasing the voltage can have amazing results in speed and power. It's like strapping in a super charger. However, there is always a cost. Typically it's much harder on the drive-train and creates heat at a much more rapid pace.
One thing I've never thought about was the effects of runtime based on voltage. I would imagine that it would probably discharge slightly faster than a lower voltage pack of the same capacity, simply because of heat and increased resistance. That might be a good one for someone with more experience to chime in, or a google search:)
There are configurations where you run a lower kV motor and a higher voltage pack. In the end you can have the same RPM but since we're using a much higher voltage we've reduced the current being drawn. This makes for a more efficient system. Typically you'll see this in 1/8 electric cars and trucks.
The limitation of the VXL Rally seems to be it's 3S limit. You have a couple of options. You can run two NiMH packs in series, or run a single LiPO 3S pack. I've read that a single 3S LiPO still manages to outperform the 2 NiMH packs in series. However, I can't speak from experience. Another neat option is running two LiPO packs in parallel. This will allow the same voltage, but practically doubles you're run time.
Since you're into modding, I've seen some people replace the VXL ESC with something that can handle more voltage like the Mamba Max Pro. I'm not sure running on 4S would even be manageable by the best of drivers;)
I'm sure there is a site that does a much better and more accurate job of explaining this stuff. Unfortunately I don't know one off the top of my head.
Hopefully you and the boys will get to that new track and start doing some racing:)
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