Why are rc lipos so large for their relatively small capacity?

[Buggyjam 144]

I've got an external battery pack to charge iPads/usb type charge ports etc. It's about the same size as my turnigy 2s 4200 mah packs. It's shorter but a bit wider, so about the same relatively. Yet it's capacity is 16750mah. An iPad battery is 11000mah. Yes, they're wide but they're dead thin. Apart from the plastic skin, why are rc battery packs so big for relatively small capacity? 

I have an inkling that it's something to do with the C rating. I almost know that my 16750 battery pack probably can't put out 160 amps unlike an rc lipo.  I just don't know how this equates to size. I find lipos and batteries quite interesting. A bit of a mystery but interesting.

[Backlash 481]

My guess is that its to do with the C rating...

 

[Darat76 265]

I think power packs for phones and other USB chargeable devices, the battery in the pack is a 1s lipo, not 2s, as usb voltage is 5v out

as for the power packs that can jump start a car motor, they could be 2s lipo with a voltage booster, or a straight 3s lipo

[Wooders28 4723]

I believe USB chargers normally only put out 0.5A, which would make the C rating 1/32000 , not much use in our hobby! 😂

[GregM 1109]

You can't compare capacities between a USB power bank and a RC Lipp battery without keeping the battery voltage in mind.

for power banks, there's usually a single 3,7 V LiIon cell, and the capacity in mAh is stated with this voltage in mind. When upconverting this voltage to the 5V output, the resulting mAh capacity is reduced slightly.

For R/C batteries in 1/10 scale, you usually have two 3.7 V Lipo cells in series, resulting in a total voltage of 7.4 V. To get the same capacity in mAh, you need two times the physical space.

 

For a true comparison of capacities, you shouldn't use mAh, because it's dependent on voltage. For a true comparison, you should use Wh (watt hours), which is often found on professional equipment and laptop batteries.

[Prescient 671]

Different battery tech, portable chargers etc use lithium ion, RC use lithium polymer. The batteries in portable chargers are the same cells you find in laptops etc (similar to 18650/AA).

Most portable chargers put out circa 2 amps at the USB charging port.

 

[Buggyjam 144]

Ah it really is a case of comparing apples with oranges. Of course the voltage is different too, not to mention they're not lipos. Doh! Just out of interest, why do they not make lion rc batteries? Is it the chemistry doesn't lend itself to the voltages and amps needed in a small space? I've seen videos of iPads going on fire so it's not just lipos that are volatile if damaged, it's any lithium battery?

[Wooders28 4723]

30 minutes ago, Prescient said:

Most portable chargers put out circa 2 amps at the USB charging port.

 

Making it 1/8000 C ? 🙄

[Prescient 671]

3 hours ago, Buggyjam said:

Ah it really is a case of comparing apples with oranges. Of course the voltage is different too, not to mention they're not lipos. Doh! Just out of interest, why do they not make lion rc batteries? Is it the chemistry doesn't lend itself to the voltages and amps needed in a small space? I've seen videos of iPads going on fire so it's not just lipos that are volatile if damaged, it's any lithium battery?

Think of a portable drill with a brushed 540 in and 4000mAh lithium ion battery pack and an RC car with an NiMH battery and the way they are used and require power, which is probably the closest analogy.

Runtime may be similar depending on use but potentially for the capacity much longer in a drill/driver. The way power is required, torque is delivered and power is sustained under very heavy load is different though. Drills obtain their power through the reducer gearboxes enabled by higher motor rpm achieved by higher volts 10v-18v. 

Lithium ion provides consistent power delivery, capacity and current draw for devices from mobiles, laptops, tablets and power tools. The closest to lithium ion in RC for power delivery would be NiMH. Some toy grade RC products including the cheaper helicopters and quadcopters use lithium ion.

Lipo provides a much higher discharge current which gives that burst of power enabling standing back flips. 

You can leave lithium ion in stasis at almost any storage capacity, come back 6 months later to use your device and it's fine. Lipos need more care when being stored and not used/cycled. 

[nowinaminute 2815]

Power banks often have a large 1s cell or use multiple 18650 cells that sacrifice current ability for capacity.

Technically lipo and li-ion cells are the same, there used to be true lithium polymer cells being developed but not anymore, the term just began to be used to denote lithium cells made inside pouches as opposed to hard cases and it stuck. The underlying chemistry is the same, it's just that li-ion tend to be optimised for energy density over amperage.

There are li-ion cells emerging that have quite good current abilities though. Because the demand for higher current Li-ion cells has increased, thanks in part to the vaping community you can now get 18650 cells that can sustain up to 60a which isn't bad at all for their size but still not as powerful as most comparable lipo packs which are a little larger but have lower internal resistance.

There is a certain appeal to round cells though I must admit, HBX recently released a twin hammers style car that runs on two 18650 cells, nice to be able to just slot in a couple of batteries like the old AA powered toy grades. I could see them getting more popular in semi hobby grade stuff, compact and easy to slot in and out, the simplicity is appealing. If lithium batteries didn't fail so spectacularly the 18650 type cell would have probably rendered AA, C and D size batteries obsolete by now. Imagine having little 3000mah 3.7v batteries that can put out 60a and were just as happy powering your torch or boombox or drill as your rc car etc. 



 

[GregM 1109]

On 13.8.2017 at 0:34 PM, GregM said:

For a true comparison of capacities, you shouldn't use mAh, because it's dependent on voltage. For a true comparison, you should use Wh (watt hours), which is often found on professional equipment and laptop batteries.

This video should help to explain what I was talking about.

 

[Yonez 180]

Gotta love EEVBlog