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Old 10-28-2011, 04:17 PM   #1
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Default Less mah battery for racing?

Hello,

I race on an indoor astro track during the winter season and would like to have the lightest cars I can for 6 minutes races. It's a tight and technical track so I'm thinking of buying lipos with less capacity to save weight.

Here are my setups:

- 1/8 buggy 4wd -
JQ THE Car converted to brushless (RCMonster conversion)
RX8 ESC
T8 1400kv motor

- 1/10 SCT 2wd -
SC10 Factory Team
Robitronic 3.5 ESC
LRP X11 10.5 motor

I will need 2S lipos for the SC10 and 5S lipos for THE car. Can you guys help me to determine what capacity of lipos and C rating I'll need for both cars to achieve a minimum of 6 minutes runtime?

In 1/8 use about 250mah per minute on this track so theorically a 1800mah lipos would be enough for the 6 minutes race. But would it give enough amps then?
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Last edited by Pulse_; 10-28-2011 at 04:37 PM.
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Old 10-28-2011, 05:05 PM   #2
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Originally Posted by Pulse_ View Post
Hello,

I race on an indoor astro track during the winter season and would like to have the lightest cars I can for 6 minutes races. It's a tight and technical track so I'm thinking of buying lipos with less capacity to save weight.

Here are my setups:

- 1/8 buggy 4wd -
JQ THE Car converted to brushless (RCMonster conversion)
RX8 ESC
T8 1400kv motor

- 1/10 SCT 2wd -
SC10 Factory Team
Robitronic 3.5 ESC
LRP X11 10.5 motor

I will need 2S lipos for the SC10 and 5S lipos for THE car. Can you guys help me to determine what capacity of lipos and C rating I'll need for both cars to achieve a minimum of 6 minutes runtime?

In 1/8 use about 250mah per minute on this track so theorically a 1800mah lipos would be enough for the 6 minutes race. But would it give enough amps then?
I've found that to be the problem. I thought about doing something similar - even 10 min mains are easily doable with a 2650mah. If I drive smooth and dont hamfist the throttle I get close to 200mah/min.

But....a 2650 @ 20C is just not enough. Since you are running 6S, that might help.

If you are serious about doing this, then I'd recommend a powermeter, and measure your amp draw, do some math, and add a little % for cushion.

Remember, they say to typically only use 80% of the capacity, I've puffed a few lipos before I knew how to properly take care of them and ran them down to lipo cutoff nearly every time - they didnt last too long.

What I kinda realized, is it was just not feasable to save much weight in that area. Even a 3500mah 30C is borderline. My 1/8th pulled 102A on pavement, 86A around a track (pretty loose at the time). Remember, C rating affects voltage drop as well - so just getting enough amps is 1/2 the battle. Although there is no standard, C rating typically affects internal resistence as well.
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Old 10-28-2011, 05:22 PM   #3
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I've found that to be the problem. I thought about doing something similar - even 10 min mains are easily doable with a 2650mah. If I drive smooth and dont hamfist the throttle I get close to 200mah/min.

But....a 2650 @ 20C is just not enough. Since you are running 6S, that might help.

If you are serious about doing this, then I'd recommend a powermeter, and measure your amp draw, do some math, and add a little % for cushion.

Remember, they say to typically only use 80% of the capacity, I've puffed a few lipos before I knew how to properly take care of them and ran them down to lipo cutoff nearly every time - they didnt last too long.

What I kinda realized, is it was just not feasable to save much weight in that area. Even a 3500mah 30C is borderline. My 1/8th pulled 102A on pavement, 86A around a track (pretty loose at the time). Remember, C rating affects voltage drop as well - so just getting enough amps is 1/2 the battle. Although there is no standard, C rating typically affects internal resistence as well.
Ok so if I understand you I can only use low capacity lipos if I compensate with a high C rating?

The thing gets even more complicated when you compare the 25C discharge rate of cheap lipos to Hyperion 25C for example. I highly doubt all the C ratings are realistic as there is no official way to measure this value.

I tried 6S 3300mah 25C Hyperion G3 ( 82,5 amps) with a 1350kv and 1400kv T8 and never got any problem, but I would like to know where is the limit..

Considering I'm a smooth driver my runtimes are way too long for the race so I was thinking this way:
less capacity needed -> less weight -> longer runtimes -> even less capacity needed and so on.... BUT I need to make sure I have enough amps

What amperage is the lowest amperage I can go to stay on the "safe side" in 1/8?
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Old 10-28-2011, 05:36 PM   #4
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Yup - and the problem is that they dont really make high C rating low MAH packs. Not sure if thats due to no demand, or the makeup of the higher MAH cells. They make 5000mah 50C's, but no 2500mah 50C's...


The limit is usually a grey area, unfortunately. You can probably run a lower MAH/C rating pack for awhile and not see any immediate problems, but I'd suspect that pack wouldnt last long if you took nearly all the capacity out of it, and also used it closer to its C rating ceiling. Remember, AVERAGE amp draw is only going to be 30 amps or so, you'll see spikes perhaps in the 80's/90's when racing, but 90% of the time, it will be much lower than that.

Thats why to avoid that grey area, I just use a substantially larger pack than whats required.

Like I said, if you wanna get technical, get a meter: http://www.hobbypartz.com/aeo-p0-watt-meter.html

Thats the one I use. It will tell you how much amps your system draws. So, set it up like you would to race, do a few laps, and pull it off the track and if you see 75.3A - then....you know thats the max you ran and can try to gauge things based off it.

The lowest I run on my RX8/1900 is the turnigy 4500 30-40C.
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Old 10-28-2011, 06:06 PM   #5
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Thanks for your feedback, I'll try the wattmeter. There are way to many variables to have one single answer so I'll test it out . I've seen many guys say 120 amps is a minimum, I ran with jut 82 amps on big and fast outdoor tracks with no problems at all so I'm doubting
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Old 10-28-2011, 06:07 PM   #6
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honestly whats the weight difference between a 2s 5000mah and a 2s 3000mah pack? Even dropping down to a 1800 or so (maybe repurpose a heli or plane mini pack?) will it be that big a difference?

Looking at tower at 2 lipo packs
http://www3.towerhobbies.com/cgi-bin...?&I=LXBPGR&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXAVHP&P=7

going from a 3000mah to a 6500mah (same 25c) yeilds no weight loss (830g). Might want to rethink the weight loss through battery program.

Edit: even looking at HK.com the difference between a 2s 3000 and a 5000+ yeilds a weight difference of only 90-110g. Going to a 2s2p even only adds about 100g or so.
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Old 10-28-2011, 06:12 PM   #7
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only real way to lower the weight and keep decent amps would be to get a shorty pack, most good ones are 4k 50c for around $50-55

i have been thinking about this as a way to cut weight. (about 80 grams)
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Old 10-29-2011, 04:25 AM   #8
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It might not make a huge difference in 1/10 but it is significant in 1/8.

The trouble is that I would have to measure the amps my system is using on this particular track with my particular car/driving style to see what minimum level of amperage I would need.

I know my system was safe in Hyperion 6S 3300 25C (82,5 amps) but that's it and the C ratings are different from brands to brands.

I'm not looking for a fast car, it's an indoor track that is meant to be run with 1/10 car but I don't want my system to get hot or damage my stuff by making a mistake or forgetting something
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Last edited by Pulse_; 10-29-2011 at 04:39 AM.
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Old 10-29-2011, 08:08 AM   #9
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Wouldnt turnigy nano tech be alright?
This is maybe to low MAH but for example: http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html
The C ratings on some of em seem real decent
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Old 10-29-2011, 09:49 AM   #10
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Wouldnt turnigy nano tech be alright?
This is maybe to low MAH but for example: http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html
The C ratings on some of em seem real decent
The problem is: are they true 65C?
If only they had a standard for C ratings...

I had only 82.5 amps with my 6S 3300 mah 25C G3 lipos and no issue. Now I have 100 amps on the same indoor technical track with my 5S 4000mah 25C nanotech and I fried my RX8 in 3 laps. It might be something else than a problem of lipo but who knows?

What components will be damaged running a too low C rated lipo?
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