SMC Lipos 7.4 5000mah
10-18-2007, 06:35 AM
#571
There are clearly two groups of racers. Those with the fat wallets and want to prove theirs is fatter then others with a little bit of skill mixed in. Then there are those that want to see how they stack up to others with roughly the same equipment. Not saying one of right and one is wrong but it seems clear to me that how things shake out.
10-18-2007, 08:08 AM
#572
At this point, I'd be pleased with a higher voltage cell, because I'm getting my "stickers blown off" by guys running NiMH still, and I'm gonna have to go back to keep it in the ballpark. But, down the road if/when we're all using LiPo and on even ground, it sure would be nice to use a cell that lasted a long time over one that provided the most voltage.
Anyway, this conversation really goes beyond SMC or any single company, and gets more to the core of how things may go with LiPo down the road. With the current situation of people using cells from wherever they can, it truly does feel like the wild west. Whatever happens, I hope the cell choices are limited in the end, and favor longevity over voltage.
10-18-2007, 09:23 AM
#576
I still can`t prove either one is faster than the other ....
Would of loved to prove there is a big different`s or advantage with either , I still can`t ...
10-18-2007, 09:28 AM
#577
I don't think a Lipo pack can compete with a sub-c pack as the upfront voltage is not there. A good sub-c pack will average out in the 7.40 range at 35 amps which means you will give up 2 tenths of a volt which is important in stock and 19t. In mod I don't think this would be as big of an issue as a faster motor could be used to compensate for the voltage.
One way that could make lipos equal would be to allow them to run at a lighter weight but then chassis balancing becomes an issue unless your using a Losi sedan.
Unless Lipos evolve and get stronger then they will never overtake sub-c in competitive racing so your arguements that our packs have more power at the expense of life expectancy makes no sense as the current Lipos can't compete with sub-c cells so if you want to be competitive you need to use sub-c cells.
One way that could make lipos equal would be to allow them to run at a lighter weight but then chassis balancing becomes an issue unless your using a Losi sedan.
Unless Lipos evolve and get stronger then they will never overtake sub-c in competitive racing so your arguements that our packs have more power at the expense of life expectancy makes no sense as the current Lipos can't compete with sub-c cells so if you want to be competitive you need to use sub-c cells.
10-18-2007, 09:37 AM
#578
Unless Lipos evolve and get stronger then they will never overtake sub-c in competitive racing so your arguements that our packs have more power at the expense of life expectancy makes no sense as the current Lipos can't compete with sub-c cells so if you want to be competitive you need to use sub-c cells.
I just meant against other LiPo from other manufacturers. If, with LiPo, it truly comes down to a tradeoff between voltage and cycle life, it would be unfortunate, as I could see that turning into even more of a wallet war than we have now. My hope is simply that whatever the future holds for LiPo, we find a nice compromise between the two.
I see it as a very similar situation to rubber tires. There are amazingly fast tires available out there, but they almost always last for one or two runs and then they're done. Then, you've got tires designed for "spec" racing like the Jaco Rubberz that might not be the fastest around, but they last for a lot longer. If everybody is on the same tire, speed is completely arbitrary, so you enjoy the long lasting tire and don't worry about speed since everybody is on the same page. I think spec tire rubber racing is beautiful for that. I see the future of LiPo racing being very much the same, which is why I'm hoping for some kind of spec battery for future LiPo racing. Clearly this is a long way off, still.
10-18-2007, 09:40 AM
#579
Danny,
Supposedly one of the benefits of LiPo is that it holds its voltage under load better than NiMH. I'd be interested to know at about what load the voltage of the LiPo packs is the same or higher than a good NiMH. I'm wondering if you'd only see that come into play in mod racing, or if it might even show up with milder motors, like 10.5/19T.
Supposedly one of the benefits of LiPo is that it holds its voltage under load better than NiMH. I'd be interested to know at about what load the voltage of the LiPo packs is the same or higher than a good NiMH. I'm wondering if you'd only see that come into play in mod racing, or if it might even show up with milder motors, like 10.5/19T.
10-18-2007, 10:24 AM
#580
The IR is slightly lower on a Lipo so yes under load voltage drop should be abit less but the difference in IR isn't enough to make a big difference in my opinion.
I have no racing experience with lipos versus sub-c cells so for thos who have tried it they need to tell us the difference.
As far as cycle life goes I'm no Lipo expert but it seems like manufacturers are focusing on capacity dropoff after a certain amount of cycles. This is o.k. but since IR and voltage is what makes power I think this should be the key as it you take a 4800 or 5000 Lipo pack and lose 20% after 1000 cycles the packs will still be in the 380-420 range at 35 amps which is more then enough runtime to do any stock or 19t race.
I don't think we have the answers to how IR and voltage change as the pack matures. We will find this out once competitive racing starts using Lipos as racers will want to race packs that have low IR and high voltage and if there packs dropoff they will buy new ones just like they do with sub-c.
I have no racing experience with lipos versus sub-c cells so for thos who have tried it they need to tell us the difference.
As far as cycle life goes I'm no Lipo expert but it seems like manufacturers are focusing on capacity dropoff after a certain amount of cycles. This is o.k. but since IR and voltage is what makes power I think this should be the key as it you take a 4800 or 5000 Lipo pack and lose 20% after 1000 cycles the packs will still be in the 380-420 range at 35 amps which is more then enough runtime to do any stock or 19t race.
I don't think we have the answers to how IR and voltage change as the pack matures. We will find this out once competitive racing starts using Lipos as racers will want to race packs that have low IR and high voltage and if there packs dropoff they will buy new ones just like they do with sub-c.
10-18-2007, 10:30 AM
#581
If you take the Kokam info posted on the Orion website they say there cells will hold up to 80% of its rated capacity at a 8 C discharge after 60 cycles. So it's safe to say our pack will still have 80% of it's rated capacity after 60 cycles if we use an 8 C discharge rate like Kokam.
Please reread the team-orion Kokam presentation.
http://press.team-orion.com/press/av...am_quality.pdf
The 80% capacity at 8C at 60 cycles is what Kokam is saying about an unnamed competitor on page 11.
On page 12 - Kokam presents the cycle life data on their cell.
Keep in mind this data is already outdated. The current Kokam chemistry has over double the cycle life of the data shown on page 12.
Best Regards
-Ling
10-18-2007, 10:42 AM
#582
We need to erase our brains on how we calculate IR when it comes to LiPo vs NiMh. The NiMh method is too simplied for Lipo testing
DC Internal resistance is nothing more than voltage change under different loads. The lower the IR, the less voltage change. The IR calculation is highly dependent on the loads tested.
The equation is a simply (V2-V1)/(I2-I1). Where V2 is the secondary voltage measurement under a different load, I2.
For example for a 35 amp discharge - the IR calculation would be done be either increasing or decreasing the load and measuring the voltage. My old Tekin DIS-350 both increases and decreases the load (good) during the calculation. The ice charger only decreases the load (not as accurate). I'm not sure what the GFX does.
The IR for a lipo pack may be close to a Nimh pack when the 2nd load is small. But if you do the IR calculation at much higher loads then the IR for Lipo is MUCH less.
When you RC car accelerates out of a corner and it's pulling 50-60 amps, the lipo cell will has less IR and thus a ton more voltage.
Another difference is the change in IR throughout the discharge curve. In RC, we only measure IR 60 seconds into the discharge. The IR changes quite a bit through the whole discharge process.
Basically - you can compare nimh vs lipo on just an average voltage at 1 fixed current and one IR number.
Sorry, but batteries just got more complicated.
10-18-2007, 10:42 AM
#583
Danny,
Please reread the team-orion Kokam presentation.
http://press.team-orion.com/press/av...am_quality.pdf
The 80% capacity at 8C at 60 cycles is what Kokam is saying about an unnamed competitor on page 11.
On page 12 - Kokam presents the cycle life data on their cell.
Keep in mind this data is already outdated. The current Kokam chemistry has over double the cycle life of the data shown on page 12.
Best Regards
-Ling
Please reread the team-orion Kokam presentation.
http://press.team-orion.com/press/av...am_quality.pdf
The 80% capacity at 8C at 60 cycles is what Kokam is saying about an unnamed competitor on page 11.
On page 12 - Kokam presents the cycle life data on their cell.
Keep in mind this data is already outdated. The current Kokam chemistry has over double the cycle life of the data shown on page 12.
Best Regards
-Ling
10-18-2007, 10:51 AM
#584
Danny and everyone else,
We need to erase our brains on how we calculate IR when it comes to LiPo vs NiMh. The NiMh method is too simplied for Lipo testing
DC Internal resistance is nothing more than voltage change under different loads. The lower the IR, the less voltage change. The IR calculation is highly dependent on the loads tested.
The equation is a simply (V2-V1)/(I2-I1). Where V2 is the secondary voltage measurement under a different load, I2.
For example for a 35 amp discharge - the IR calculation would be done be either increasing or decreasing the load and measuring the voltage. My old Tekin DIS-350 both increases and decreases the load (good) during the calculation. The ice charger only decreases the load (not as accurate). I'm not sure what the GFX does.
The IR for a lipo pack may be close to a Nimh pack when the 2nd load is small. But if you do the IR calculation at much higher loads then the IR for Lipo is MUCH less.
When you RC car accelerates out of a corner and it's pulling 50-60 amps, the lipo cell will has less IR and thus a ton more voltage.
Another difference is the change in IR throughout the discharge curve. In RC, we only measure IR 60 seconds into the discharge. The IR changes quite a bit through the whole discharge process.
Basically - you can compare nimh vs lipo on just an average voltage at 1 fixed current and one IR number.
Sorry, but batteries just got more complicated.
We need to erase our brains on how we calculate IR when it comes to LiPo vs NiMh. The NiMh method is too simplied for Lipo testing
DC Internal resistance is nothing more than voltage change under different loads. The lower the IR, the less voltage change. The IR calculation is highly dependent on the loads tested.
The equation is a simply (V2-V1)/(I2-I1). Where V2 is the secondary voltage measurement under a different load, I2.
For example for a 35 amp discharge - the IR calculation would be done be either increasing or decreasing the load and measuring the voltage. My old Tekin DIS-350 both increases and decreases the load (good) during the calculation. The ice charger only decreases the load (not as accurate). I'm not sure what the GFX does.
The IR for a lipo pack may be close to a Nimh pack when the 2nd load is small. But if you do the IR calculation at much higher loads then the IR for Lipo is MUCH less.
When you RC car accelerates out of a corner and it's pulling 50-60 amps, the lipo cell will has less IR and thus a ton more voltage.
Another difference is the change in IR throughout the discharge curve. In RC, we only measure IR 60 seconds into the discharge. The IR changes quite a bit through the whole discharge process.
Basically - you can compare nimh vs lipo on just an average voltage at 1 fixed current and one IR number.
Sorry, but batteries just got more complicated.
heck, this is your daytime job, might as well use it for your hobby as well. 
10-18-2007, 10:57 AM
#585
Linger: So basically your saying that a Lipo pack will have better acceleration than sub-c cells so this means a Lipo should be as fast if not faster in stock and 19t.
Since you have allot of knowledge about Lipos what do you think about IR increasing over use ? I think this is more important than losing 5,10 or 20% of the capacity which is what most companies use when testing cycle life.
Thanks for your info.
Since you have allot of knowledge about Lipos what do you think about IR increasing over use ? I think this is more important than losing 5,10 or 20% of the capacity which is what most companies use when testing cycle life.
Thanks for your info.