LiFe max safe current draw?

[rizky_p]

Hi i have this(my sons actually) RC toy car with an aging 9.6v 700maH Ni-CD pack, the Ni-Cd pack is almost dead i planned to replaced it with 3x18650 (with tons of capacity my son will be happy). Should i go for normal Protected 18650 or LiFePo type 18650? as i heard somewhere that LiFe battery is safer and can sustain higher amp load. I have tried 3x18650(LiPo) but the batteries got warm and concerns me.

FYI. Normal amp load from the battery measure 1.5A-2A when the wheels is loaded and jumps to almost 3A when the motor start but only 0.6A when the wheels spin in the air(like upside down/no load)

Thanks

 

[Yenster]

I would recommend the protected 18650 Li-ions that don't have the extra 'short-circuit' protection. Those can put out over 7 amps.

 

[SilverFox]

Hello Rizky p,

The Black and Decker VBX A123 1100 mAh 18650 cells can withstand 30 amps, but work well at about 12 amps continuous.

Tom

 

[BentHeadTX]

Rizky,
Agree with Tom on this one, those A123 Systems lithium iron nano-phosphate cells are amazing. They can get hammered with current, won't explode and last longer than lithium-ion. That could be the reason for that cost thing

 

[rizky_p]

@Yenster: isn't it dangerous running Li-Po 18650 over 7amps? isn't that why there is a short-circuit for protection on Li-Po batteries?

@SilverFox/BentHeadTX: are those iron-phosphate you mention are LiFePo4 for short? is that what the Black and Decker batteries are made from?

thanks for the replies guys, really appreciated.

 

[Yenster]

@Yenster: isn't it dangerous running Li-Po 18650 over 7amps? isn't that why there is a short-circuit for protection on Li-Po batteries?

Sorry, I was referring to Li-ion 18650s, and also in regards to the short-circuit protection that can prematurely cut-off the battery under high-drain (low resistance) conditions. But, yeah, the short-circuit protection can be safer, but I think your 3 amp drain is pretty safe.

 

[mdocod]

for the purpose of simplicity I think you'll be much happier with a direct replacement with a ~2000mAH NIMH pack. I assume it's a standard 8xAA RC pack. (double flat configuration)...

This will save the trouble of making the pack fit, and will be safe.

if you were going to go Li-Ion I would say that you almost HAVE to go with 3xLiFeP04 cells, and really don't even have the option of going with standard cobalt oxide cells because 2 cells would be too low on the voltage (8.4V fresh off charger), while 3 would be ~12.6V fresh off charger, probably too high for the toy. 3 LiFeP04 cells fresh off the charger might sit something like 11.4V temporarily, which is about the same voltage you can expect your current NICD pack to be fresh off the charger, so it should be fine. If you went with standard li-ion, and the car crashed hard, and penetrated a cell somehow, you or your son or your house could be severely damaged. With LiFeP04 cells the level of safety is very high, similar to NIMH. ALSO, as I understand it, LiFeP04 can be series charged and the cells deal with overcharge very well, meaning that a balancing charger isn't necessarily required.

Even this cheapo brand NIMH pack will substantially outperform the stock pack IMO:
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=1575

 

[BentHeadTX]

@SilverFox/BentHeadTX: are those iron-phosphate you mention are LiFePo4 for short? is that what the Black and Decker batteries are made from?

Li (Lithium) Fe (Iron) PO4 (Phosphate)

The A123 System cells use nanotechnology so they call them "nano-phoshate" cells but they are LiFe cells. The A123 batteries I use are in the DeWalt drills which develop 1 HP peak (746 watts) considering the batteries are actually 32.5V, this leads me to believe the batteries get hit with at least a 25 amp draw. The 2.3Ah M1 cells inside work very well and I have had no problems with them.

 

[Yenster]

mdocod, your 'The Lithium Ion Incandescent Guide and Compatibility chart' is a very informative document. However, may I suggest an update to your reference to 'protected' cells as it relates to 'startup' problems. You should differentiate between protected cells that only have overcharge/discharge protection, such as these which don't have startup problems. As opposed to these which do have the issues you've described due to its additional short-circuit PCB. This way people will better understand the cause and options. Just my suggestion.

 

[mdocod]

yenster... thank you for the suggestion, however that it completely and totally off topic in this thread.... in the future please respond in that thread (the guide) or in a PM to me if you have an idea for the guide. At this time I have no intension of recommending the use of any ultrafire/trustfire/smartfire/etc cells in the guide (they have a pretty poor track record of capacity and quality issues), so I'm not going to bother going into the details those cells... With new cells emerging on the market all the time, I will try to keep an eye on possible safe alternatives that are of good quality. FYI: both of the cells you linked to are PCB protected with current limits set. Keep in mind that DX/KD are not the best at listing detailed information about all their products. In most cases the descriptions/specs are just copy/pasted from elsewhere and are often missing critical information on products. If you read through the reviews of those cells, you'll find that people have found them to be current limited.

 

[Yenster]

mdocod....sorry, I'll try to post appropiately in the future. Though to be clear here, I wasn't promoting the brand, just noting a difference in degrees of 'Protection'. And I wasn't going by specs on the sites, but by my own experience with those cells. And, yes, they both are PCB protected, but there are differences between the two, where one is current limited by low (dynamic/sag) voltage, whereas the other is a triggered 'open circuit'.

I was also thinking this information would be helpful to the OP to distinguish between the two types of protection when considering high drain battery options, since it becomes a significant factor in those scenarios (not unlike the incan startup issues).

 

[65535]

I think you would be completely happy with a 8 AA NIMH pack. can get about 2200mah nominal, and the proper voltage 11.1 volts nominal and 12.6 volts fresh posses a possible circuit damage. On the other hand, it might make the car much more powerful. I would recommend getting unprotected LG cells 2600mAh 18650 cells and getting one of these. http://pfmdistribution.com/secure/shop/item.asp?itemid=203

should make it pretty safe, and give over an hour of runtime.

 

[rizky_p]

@65535: I tried 3x18650 and yeah it make the car more powerfull than before, amazing torque compared to when using the old Ni-CD. I hope that the speed controller can stand the higher current. This toy RC i mention weight almost 3.5 kgs(without battery, it has 2 electric motors) and yet with 3 18650 the handling is fantastic. Almost feels like my HBX bonzer with 540 motors (sorry if you not in this sort of thing)

@mdocod/65535: i think i will try the LiFePo4 18650 for my son to play with since it is ALOT safer. I will not using higher capacity Ni-CD or Ni-MH since the charger that come with the toys charges at 350mA and will take forever to fully charge 2000 Ni-Mh Pack. And the 3 18650 batteries fits nicely without play inside battery compartment and the cover can be closed securely.

@anyone: thanks for helping me out on this, i learnt alot. I can be sure that my son will be very happy playing with his toys now it last for an hour. It used to be 8 minutes or less.


Cheers.