What batteries to buy and why?

[glassdoctor]

Try this... I got this off of a PDF file from kokam

[MetalMan]

The 30C curve looks quite good. Now we just need a graph of 5sec. bursts...

[skellyo]

IMO, these discharge graphs are the only way to objectively compare the cells without taking cost into consideration. Being able to see how a cell holds voltage under load throughout its capacity allows me to see that ideally one cell could keep a higher speed in a vehicle longer than another cell.

With cost being no object, why would I want to buy a cell that sags more under load at 50% capacity versus one that doesn't sag under load until say 80% capacity? It's only logical to want a cell that maintains a more constant performance throughout the capacity of the cell, is it not?

Edit: Took too long to post and bumped down, but this was in response to BrianG

[MetalMan]

Performance depends greatly on temperature. Those initial dips occur because the cell isn't warm enough (the IR increases as temp. decreases), and so once the cells warm up (IR decreases), then the voltage increases. If a cell is started at a higher temp. like 90 deg., it will perform much better, but might end up getting too hot at the end of a discharge, depending on usage.

[BP-Revo]

I really don't pay THAT much attention to graphs and such.

I go alot by people's experiences here. Sylvester switched from the Maxamps 8K's (what I was using) to Flightpower and said there was a huge difference and that runtime was only 10 minutes less despite going from 8000mah to just 3700.

This has prompted me to sell what I need to in order to get some fliughtpower 4900's, which should give me similar runtime to the 8k's and all the flightpower performance.

I also heard the cells that hold voltage under load better do not drain as quickly, leading to more runtime (which seems to be proven by Sylvester's experiences).

[BrianG]

I just tried to find the type of graph I'm talking about at a couple fo PC websites, but am having no luck. But basically there are 2 or 3 axis's - one for each of the quantifyable measurements: capacity, price, discharge rating, or volts under load. Then, each pack will be a dot on the graph. Then, it's easy to see where a certain pack falls in reference to other cells. It was just an idea...

[glassdoctor]

He didn't really say exactly what the runtimes were.... maybe they were 20 min vs 10 min. :D

[BP-Revo]

I doubt it. The 8K's normally give at least 30 minutes or so - don't go scaring me now GD!!! lol

I just hope that the Kokams that are on the market now can handle what I will be doing to them. With all I've spent on Lipos in the last couple of months, I could have bought two Novarossi .21s.

[glassdoctor]

http://www.elektromodellflug.de/akku...kam-parade.htm

There's some data on the Kokams... more graphs on the new 4000 cells as well as the 4800s that AAngel and I are both going to try.

The 4000 looks great....

[glassdoctor]

Also found a similar tests for several other high-end lipos...(since everyone loves graphs around here ;) ) interesting to see certain brands that use the same cells.... have peculiarly similar curves.

Flightpower
http://www.elektromodellflug.de/akku...htpowerevo.htm

Hyperion
http://www.elektromodellflug.de/akku...litestrorm.htm

Polyquest
http://www.elektromodellflug.de/akku-test/polyquest.htm

TP
http://www.elektromodellflug.de/akku-test/tp-extrem.htm

Tanic
http://www.elektromodellflug.de/akku-test/tanic-06.htm

Xcell (same as Neu packs)
http://www.elektromodellflug.de/akku-test/xcell.htm

[skellyo]

The Kokam 4000's look to be very comparable to the Flightpower 3700's (or any enerland cell they tested for that matter.) I think between the two, cost would be the decision maker if I were in the market for some more cells.

Although, the size of the Trakpowers make them very attractive since I can fit them directly in my T3 or EVO III. That and the protective casing on them are two of the most attractive features IMO. If I only had my E-Maxx and LSP-R's, it would have been a tough choice on what to buy.

[captain harlock]

Hey, skellyo, when you say Evo III, you mean Tamiya's old shaft driven tourer?

[skellyo]

Originally Posted by captain harlock

Hey, skellyo, when you say Evo III, you mean Tamiya's old shaft driven tourer?

Yep, exactly!

I got my Kokam 4800s in today. Having only had experience with "budget" packs, I was surprised to find that the Kokams have pcbs at both ends of the pack. Although I'm sure that they serve a purpose, I am now concerned that these might break.

Other than the concern about the pcbs, I do have to say that I am thoroughly impressed. These things put out some current. Of course, I don't have any measuring equipment, but I can make an observation. My truggy is running with a lot more zip with every snap of the throttle. I can't wait to get these out on the track to see how they perform. More specifically, I want to see how hot they get, as I tend to run my packs from fully charged until the lvc cuts out.