18650 2S2P 7.4V batt pack for RC cars

[wechnivag]

Hi,

Besides flashlights, i also mess around with RC cars, which normally use 6 subC NiMH batt packs for 7.2V.

I was thinking if it is possible to use 4 18650 batteries in a 2S2P pack to give 7.4 V. Much lighter and also higher capacity. I have the following concerns:

1) Will the Lion 18650 be damaged by high current discharge rates? I think RC cars can easily pull 20-30A peak or more during acceleration. I suppose the discharge current is improved with the 2cells in parallel.

2) Any concern with connecting the Lion cells in series, will the protection circuits be affected?

3) Can i use this charger from DX to charge the 2S2P 7.4 V pack? http://www.dealextreme.com/details.dx/sku.8798

Any CPFers tried this already?
Thanks a lot for the advice!:rock:CPF forum rox!

 

[FILIPPO]

I think 18650 can't handle 20amps...that's all

 

[IMSabbel]

Use A123 18650 or 25600 cells.
Normal LiIon i wouldnt trust past 5C even if you take huge hits in cycle-lifetime into account.
The A123 one should be able to do it. They wont have more capacity then NiMh, but still be much lighter.

 

[Fallingwater]

18650 LiFe cells (including A123) are a far better choice.

Just about the only RC things you can use standard LiIons in are slow planes, everything else requires too many amps.

 

[freedom2000]

Hi,

For RC planes I am using Lipo cells. They are rated around 15, 20C... so you can pull a lot of Amp from them.

See here if you need a link to a cheap but reliable shop.

JP

 

[eebowler]

Many people use A123 cells and Spinel (LiMn) cells for airplanes but I personally have don't think that they are all that good for vehicles.

Standard LiCo chemistry LiIon CAN NOT handle the high current draws of RC vehicles, not even 1/18 scale cars so they're out.

Spinel (EMoli) chemistry has a max discharge of 20A for a single cell or about 40A for two in parallel. This still isn't much for a 1/10 vehicle and with a 2S2P configuration you still only get a capacity of 2800mAh. You can easily find 3800mAh (20C /76A) LiPo packs available.

 

[Fallingwater]

Many people use A123 cells and Spinel (LiMn) cells for airplanes but I personally have don't think that they are all that good for vehicles.
Standard LiCo chemistry LiIon CAN NOT handle the high current draws of RC vehicles, not even 1/18 scale cars so they're out.

Not sure if these two statements are connected, but if they are, A123s are LiFe, not LiCo.

 

[eebowler]

Sorry for being incomplete.
A123 cells are Lithium nano-phosphate chemistry. These cells can output HIGH constant currents in the range of 45A with bursts of over 60A for a single cell. They are the best lithium ion chemistry for high power applications however, they still have the drawbacks of 1) lower nominal voltage and 2) lower capacity (1300mAh per cell) compared to lithium polymer cells.

Check out batteryuniversity for more information.

 

[mdocod]

I definitely have to recommend the already mentioned A123 cells for your RC application. A pair of 26650 size cells will be very similar in capacity to a typical 6 sub-C NIMH pack, but will survive much longer (more cycles), and be somewhat lighter. they are becoming very popular in RC crowds.

 

[wechnivag]

wow, thanks a lot of the replies, i'm glad i check with the experts here before going ahead and buying the batteries for a test run.

Thinking about it, what would have happened if i had gone ahead using standard Lion batteries on a car and subjected it to high current drain? Protected cells will just limit the current?

Unprotected cells will go KAPOOWW, exploding 1/10 scale fireball?

hehe.

 

[mdocod]

the protected cells would have gone open circuit a fraction of a second from you pushing the accelerator. So basically nothing would happen with the protected cells, (car might leap forward a few inches before shutting down). With unprotected cells, you'd probably experience deceivingly good results on the first run or 2, after which time the cells would begin to really bite the dust, substantially reduced capacity, increased resistance, and increased likelihood of fire/explosion. I think the cells would either explode, or just be dead weight by the 5th cycle.

 

[Fallingwater]

With unprotected cells, you'd probably experience deceivingly good results on the first run or 2, after which time the cells would begin to really bite the dust, substantially reduced capacity, increased resistance, and increased likelihood of fire/explosion. I think the cells would either explode, or just be dead weight by the 5th cycle.

At that current draw, wouldn't unprotected cells just sag way beyond acceptable voltage and not deliver any result worth speaking of?

 

[LuxLuthor]

This gives you info on safe chemistry 18650 size Lithium cells & tool packs that have the A123 or Emoli/Konion cells (pro's and con's to each chemistry), with links to the RC Groups forum.

 

[mdocod]

At that current draw, wouldn't unprotected cells just sag way beyond acceptable voltage and not deliver any result worth speaking of? -FallingWater

Depends on the specific cells in question. A 2S2P 18650 pack made from good quality 18650s would probably deliver 10-15A almost as well as a 6 cell NIMH pack. And in those first couple cycles, would probably handle the burst rates of 20+A well enough that the vehicle might seem to be performing reasonably well. The reasonable performance would diminish rapidly.

 

[bosko90]

Now also I have this idea. After so many years, I think the cells are made significant progress. What do you think about LG cell HG2 18650 3000mAh 20A - 35 Amp Peak? If I take 2 in parallel, it give me 40A and 70A peak. This means 288W and 504W peak. I think that my Tamia 540 engine have 90W. I'm on the safe side.

 

[Tachead]

Just use lithium polymer man. They are the best for high amp draw applications. That's why they are the most common chemistry for RC which in some applications can draw 250amps+

 

[bosko90]

So even for cordless drill would not be good? Sometimes are NiCd were good enough.