Lipo short circuit currents
- Thread starter Sinewave
- Start date
Ypedal
100 TW
can you point us to a link for exactly what cell you are refering to ?
( and never dead short any lithium cell.. not a good idea ) :wink:
( and never dead short any lithium cell.. not a good idea ) :wink:
Doctorbass
100 GW
I totally agree with Ypedal.. NEVER short circuit any lipo cells.. instead try with A123 :wink: and be aware of melting/burning wire plastic skin!
To give you an aproximative idea.. let say a 10Ah LiPO with 25C capability.. their internal resistance is usually around 6miliohms.. so I=V/R.. their short circuit current should be around 3.2V / (6miliohm+ resiatance of your short wire = let say 10miliohms total)
so 3.2/0.010=320A.. and the heat dissipated by your cells is (320A*0.006=1.92V on the cell tab)*320A = 614Watts
.. and... your cell could inflate very fast!! and then.. may explode... and burst in flame!
Doc
To give you an aproximative idea.. let say a 10Ah LiPO with 25C capability.. their internal resistance is usually around 6miliohms.. so I=V/R.. their short circuit current should be around 3.2V / (6miliohm+ resiatance of your short wire = let say 10miliohms total)
so 3.2/0.010=320A.. and the heat dissipated by your cells is (320A*0.006=1.92V on the cell tab)*320A = 614Watts
.. and... your cell could inflate very fast!! and then.. may explode... and burst in flame!Doc
Doctorbass
100 GW
Sinewave said:
If you want to finf the real capacity, you need to use a device capable of.. like the CBA-II or the Megapower 960-SR that also give you one IMPORTANT parameter: the internal resistor (RI) of the cell that is a great indicator of the max current capacity of a cell.
For energy density, it's similar, but tiy will need to use a scale to the the weight and then to divise the Wh per liter..
Wh (WattHour)is obtained by multiplying the capacity(Ah) by the NOMINAL voltage of the cell.
The NOMINAL voltage is the voltage you obtain when cell loaded at 1C is precisely at 50% of it's full capacity
I hope that helped :wink:
or I suggest you to read this document:http://www.national.com/appinfo/power/files/f19.pdf
Doc
technically speakin, energy density is expressed as watt-hours per unit of volume.
watt-hours per unit mass goes by 'specific energy' & likewise 'specific power' for watts per unit of mass
at least that's how I was taught, but then I can't see a thing in these glasses.
watt-hours per unit mass goes by 'specific energy' & likewise 'specific power' for watts per unit of mass
at least that's how I was taught, but then I can't see a thing in these glasses.
Doctorbass
100 GW
Toorbough ULL-Zeveigh said:
Thanks, i'll correct that
another thing ive been wondering is...why would a high discharge 'c' rate be advantageous?...i know its so you can use a smaller pack to get the needed currentand charge time....but this would also lead to premature energy loss compared to a pack that was designed to put out constant current at a set discharge rate.
...we need constant draw dont we? not peak spikes right?...i this case wouldnt a slow discharge be better for range?
...we need constant draw dont we? not peak spikes right?...i this case wouldnt a slow discharge be better for range?
Doctorbass
100 GW
Sinewave said:
usually large Ah battery are big!!
in my case, i have 23Ah.. and every 22.2V 23Ahpack i have weight 4.7kg.. i have 4
I think you would refer to the famous peukert effect mostly found when using SLA.. on the other hand, Li-ion battery have non sificant peukert effect...
the energy loss you talk is directly proportional to the internal resistance (ESR) of the battery. the more current you use, the more heat they will dissipate to carry this current so the more energy loss in heat you will have..
Doc
Doc
dogman dan
1 PW
The lower c rate lifepo4's require the carrying of a large pack, to keep the c rate where you want it for long battery cycle life, around 1.5c or less. This means with a 700 watt motor or less, a 20 ah pack of 36 volt is a moderate sise package of 16 pounds or so. And you will have range to ride more than an hour at a time. But if you have a bigger motor, and want 72 volts, now you are carrying 32 pounds and the package is huge. So if your ride is only 20 minuites at that speed, you may only need 10 ah of battery to get there, and a high discharge rate battery allows you to get the high amps and high voltage without carrying a suitcase on the back of the bike. So if you want speed and light weight for a short ride, the high rate cells are what you need. If you want legal ebike speeds, and long range, you want the low c rate cells. You could use the high rate cells, but why spend double the money if you don't need it. Sometimes you see a bike that the owner wants as light as possible for a very short ride, like 3 miles per charge, but unless they have huge hills, I wonder why they want a motor?
since there doesnt appear to be any short circuit current specs available...how about a guess?
so do you guys think a 3.2 volt 10ah cell would be capable of 600-800 amps short circuit?
if anyone has a current discharge or power discharge chart that would be nice.
so do you guys think a 3.2 volt 10ah cell would be capable of 600-800 amps short circuit?
if anyone has a current discharge or power discharge chart that would be nice.
Link
1 MW
Sinewave said:
Because the C rate is almost directly related to the quality of the cell. Not sure why this is, but the superior cells in terms of cycle life and overall robustness tend to boast high C rates.
Sinewave said:
Probably. But cycle life would be impacted if this was done regularly. That's at the limit of even A123.
And if we're talking LiPo, I wouldn't try it. What with the risk of the things exploding and all.
Doctorbass
100 GW
Sinewave said:
600-800A... NEVER !
These lipo have not output electrode/tab able to sustain this current range!..
have you any idea of the resistance of the total short you would need to cary this current?!! in the ideal world, let suppose your battery keep it'S 3.2V.. so at 700A it's 4miliohm TOTAL resistance... so if you put a large cooper crowbar on the cell tab, the internal resistance of the cell itself(ESR) will not be as low as you need to get this current...
I would say like 150-200A.. no more!
Doc
Ypedal
100 TW
not in the least.
Ebike controllers tend to step in and cut you off at 20 to 40 amps.. as long as your battery can do this and smile.. life is good.. having a super battery that can put out 2000 amps ( Like the Capacitor bike for example .see Vid/Pic thread.. ) has no benefit.
Ebike controllers tend to step in and cut you off at 20 to 40 amps.. as long as your battery can do this and smile.. life is good.. having a super battery that can put out 2000 amps ( Like the Capacitor bike for example .see Vid/Pic thread.. ) has no benefit.
