Need some help to choose LiPo
 

Hey guys,
Im still new here. Thanks to Serum, the BL savage Guru. He's the one who encouraged me to join this great forum. Its a very good source for those who wants to run BL.

Since Im planning to convert my savage to BL, I need some help from you guys. After all the reseach, Ive decided to use Feigao 7XL and Warrior 9920 combo. I'll get these from Mike as soon as they are available. I'm also goin to beef up my drivetrain. Here are wat Ive planned so far:

1. MIP CVDs (Ive already have this on my truck)
2. 4 diff gear mod with Lightning cups or Nova Alum cups (Just need to get the cups as I already upgraded the diff gears)
3. Hot Bodies Mega Travel Drive Shaft Center
4. GPM gear box
5. NEW ERA Bulkheads (already on my truck)
6. HD 2speed upgrade
7. Monster Paradise conversion kit

Thats all I can think of right now. Please feel free to give comments and advice about my plan to bulletproof my drivetrain

Ok my question is, since i would like to get a decent runtime (20-25mins), Ive decided to get LiPo. Ive heard about Polyquest and Kokam, but not sure which one shoud I buy. Im not really looking for speed. 35-40mph is more than enough for me as Im not a good driver:o . Tower is having sale till this wednesday so I think this is the right time for me to get a good LiPo packs . These are the packs that Im interested in but not sure if they are good enough for my BL savage project:-

http://www2.towerhobbies.com/cgi-bin...?&I=LXJSJ2&P=0

http://www2.towerhobbies.com/cgi-bin...?&I=LXKBE4&P=0

http://www2.towerhobbies.com/cgi-bin...?&I=LXKRR7&P=0

Or should I spend abit more and get the PolyQuest like either one of these:-

http://www.rclipos.com/

PQ3S-3600N
PQ2S-4000N
PQ3S-4000N
PQ2S-4400N
PQ3S-4400N


Or is there any better place that I can get these kind of LiPos?

Sorry for this long question. But I really need you help. Please shed some light. All help, comments and advice are greatly appreciated.

i would go to maxamps for lipos. i would get the 6000 mah polyquest. maxamps has really good prices on them.

I am currently using the FMA BalancePro 2s2p 6400 pack that will run my brushless Jato for about 25-30 minutes in race conditions.

I also have 2 of the FMA 3s 3200 20C packs that I will run parallel for my brushless MGT (once it's all finished).

I am VERY pleased with the BalancePro system since it charges the battery so quickly; I charge my pack at 10 amps. The system is capable of charging any of the BalancePro packs with the BalancePro charger at 3C.

I have not used Polyquest packs, but I have heard nothing but good things about them.

Lastly, I have personally witnessed RC-Monster Mike using a 2s 3200 and a 3s 3200 in series for 5s in his E-Revo. This is good testament to those cells.

I dont think that the 6000 mah will fit in the battery holder in the Monster Paradise conversion kit.

wats the difference between 15C and 20C? I read some where that higher C = higher life cycles. Is that true?

And wat voltage is suitable for big heavy truck like Savage? If I get the 7.4v, that would supply 14.8v since Im goin to connect the batts in series. Is this good enough? Or should I get the 11.1v, then I would get ~24v?

On urc I think metalman had posted a link to a forum all about lipos, I am also thinking about lipos.

Sorry wat is "urc"?

"C" refers to the discharge rate. Generally, the "C" number is the constant discharge rate that the cell/pack is capable of.
For example, a 20C 3200mah pack is capable of 64 amps constant current, and bursts of higher amps. A 15C 3200mah pack is capable of 48 amps constant current, and bursts of higher amps.
"C" stands for "capacity". 20C means 20 times the packs' capacity. Therefore a 3200mah (which is 3.2amps) at 20C is 20 times 3.2, or 64 amps.

That said, a pack with a higher "C" rating is safer at higher amp draws so logically would have a longer life than a pack with a lower "C" rating used in the same higher draw application.

So batfish, i too am also thinking about lipo. I am now using 16 gp 3300's with my 7xl/9920 which is 19.2 volts, would it be ok to used two 11.1 volt lipo packs of whatever mah i choose? And what is 2s, 3s, 4s?

URC stands for UltimateRC.com

First, 2s/3s/4s, the "s" means "series". This is the amount of cells the pack has that add to the total voltage. Each cell is 3.7 volts nominal, so 2s=7.4v, 3s=11.1v, 4s=14.8v, etc... Nimh packs are cells in series, so you could draw a comparison by calling a 6-cell nimh pack a 6s pack of nimh.

Next, 2s2p, the "p" means "parallel". Unlike traditional nimh packs, lipos are commonly run in parallel in addition to series. This "p" number is the amount of cells in parallel the pack has, which adds to the total capacity. Each "p" adds the capacity equal to one individual cell capacity in the pack. This concept can get a little tough to understand for the first time, so I'll try to explain more.
For example, let's take my FMA BalancePro 2s2p 7.4v 6400mah pack. The pack is made with cells that are 3.7v (same for all lipo) and 3200mah. 2s means TWO cells in SERIES, so 3.7v*2=7.4 volts. 2p means TWO MORE cells in PARALLEL with the first two, adding capacity equal to one cell's capacity in the pack, so 3200mah+3200mah=6400mah.
Therefore, 2s=7.4volts and 2p=6400mah.

Now to your first question - 16 nimh cells is roughly equivalent to 5s lipo. Of course, you could go to 6s lipo, which would add the equivalent of about 3 more nimh cells.
When you run any lipo cells in either series or parallel you must, for safety's sake, ALWAYS use the exact same cells. Do not mix capacities or even manufacturers. Although lipo cells won't really explode and destroy a major metropolitan area (like we've been led to believe) if you treat them right, you still HAVE to treat them right.
If you want to run 5s lipo, consider buying one FMA/Kokam 2s 3200mah and one FMA/Kokam 3s 3200mah and run those two packs in series. This is what Mike does on his E-Revo, so you know it works. If you want 6s, buy two 3s packs and run them in series, but make sure you're buying the same cells in both packs.

Lastly, make sure you have a controller capable of the cell count, and strongly consider a low-voltage cutoff device to save your cells. You should never discharge a lipo cell below 2.7volts, but most people (including me) stop at 3.0volts to be safe. This means 6.0volts on a 2s pack, 9.0volts on a 3s pack, etc... An over-discharged lipo pack can be damaged, preventing it from being charged again.

I hope this information helps out in your decision.

Wow, thanks batfish. So if i went with this setup ( one FMA/Kokam 2s 3200mah and one FMA/Kokam 3s 3200mah ). I would be able to lose a lot of weight but would i gain any more run time? Also this cutoff you speak of, i thought some lipos had built in cutoffs?

Mike_D - Your setup would be a lot lighter with 5s Lipo (as opposed to 16 GP3300s). The only problem is that there is no Balance Pro 3s pack. With 5s 3200, you will not gain more capacity. The Kokam 3200 cells only output 3000-3100mah of actual usable capacity (one thing that I don't like about them). BUT, since they are lighter weight than NiMh cells, your truck won't pull as many amps, which will seem like you are being given longer runtime. But I don't think you would get more runtime than what your GP3300s offer.

Just to add a few things to Batfish's post:
-You can actually mix batteries of different capacities, but this cannot be done with series. Let's say you have a 3s 3000mah and a 3s 2000mah. You can put these two together to make a 3s 5000mah, but you can't put them in series. If anyone wants to know the reasoning behind this, I will gladly share it.

-The safety of Lipos are getting better all the time. But about 80-90% of all the fires and "puffing" of cells (like bloating, from internal gases) is from those that do no maintain/care for their batteries properly. The 3v/cell cutoff, and making sure that a pack IS balanced will decrease the safety issues. Balancing a pack is where all of the cell voltages are as close to equal when fully charged to 4.2v/cell. If one cell in a pack is .1v out of balance, this can dramatically decrease the life of the pack, as a cell like this could charge up to 4.3v, which is above the safe limit of 4.25v. Unlike NiMh cells, Lipos can't be balanced by slow charging.

In regards to what Seth is talking about, there is a website with a forum that is dedicated to discussion about batteries, and Lipos take up about 80% of that. The website is http://www.rcgroups.com/forums/forumdisplay.php?f=129 A lot can be learned there, and you can stay up-to-date on what is going on with large battery companies (like Thunder Power) because there are representatives that post on there from those companies. Also be sure to look at the threads that are stickied, as they display a lot of good info.

Because lipo packs are less weight and more power for equivalent voltage, most people do experience an increase in run time. Keep in mind that the run time increase may only be 1 minute, a few seconds, or even upwards of a few minutes. Just don't expect miracles since after all, you're going from 3300mah to 3200mah. You should definitely NOT see a DEcrease in run time.

The only lipo packs (that I'm aware of) currently available with a built-in low-voltage cutoff are the FMA/Kokam Scorpion packs. FMA also has a LVC (low-voltage cutoff) device for up to 4s lipo. A couple other devices that have been discussed on the message board are the LiSaver ( http://flydma.com/lithium/lithium.asp?path1=lithium ) and the BattSignal ( http://battsignal.com/ ). Both of those devices are capable of higher cell counts than the FMA LVC.

Thats it, thanks metalman.

How do you go about balancing the cells inside a pack? I have read about some of them have taps on the side where you can monitor a individaul cell.