Dauntless said:
Okay, once you beat everybody in the quarter mile, how you gettin' home with a dead battery?
Best line I've read in a long time ! I laugh so hard
And it's true, you're better off with an overbuilt battery that will last you for years with a moderatly powerfull motor/controller kit that is still quite fun to ride, than the other way around. A powerfull motor is completely useless with a dead battery. A 3000W motor/controller kit with a ~ 50 Volts battery will be pulling around 60 amps of current ( 3000 W / 50V = 60 A) when working hard. A battery that is 30A-rated (
Note : in reality, eBay seller sometime even overspec their batteries.... Maybe it's really only 20A rated or even lower) will suffer so much from pulling 60 Amps, you might kill the battery in just 50 charge cycles or even less !(overheating, and loosing capacity at a dramatic rate from charge cycle to charge cycle).
Personally, I would like a long lived battery.... Lithium batteries cost a lot of money so they better be long lived anyways, right ? I would chose a battery that can conservatively handle at least 60A for a 30amp motor/controller kit. IMHO, the battery amperage-rating is not an exact cutoff value. It's more of a value to give you a rough idea of what the battery is capable of. The more amp-capable a battery is, the less it will heat up for a given load.
Let's take an example. You have a ebike with a 30amp kit (motor and controller) that is intended to run on 48V nominal.
If you buy a
48V 21Ah battery made of 78 Panasonic NCR18650B cells (each cells is 3500 mAh capacity - MAX 4.87A discharge, battery in 13S6P configuration), that battery is then rated for a
maximum of 29A discharge and will not last a long life if stressed at 30A load all the time.
NOT A GOOD CHOICE FOR YOUR TYPICAL 30A SETUP ! EVEN WORST FOR A 60A motor/controller setup.
If you buy a
48V 20.3Ah battery made of 91 Panasonic NCR18650PF cells (each cells is 2900 mAh capacity - MAX 10A discharge, battery in 13S7P configuration), that battery is then rated for a
maximum of 70A discharge, it will probably last an average good life at 30A.... I WOULD NOT CONSIDER THIS FOR A 60A motor/controller though.
If you buy a
48V 21Ah battery made of 91 Samsung INR18650-30Q cells (each cells is 3000 mAh capacity - MAX 15A discharge, battery in 13S7P configuration), that battery is then rated for a
maximum of 105A discharge, it will probably last an very long good life at 30A....
If you buy a
48V 21Ah battery made of 91 LG Chem INR18650HG2 cells (each cells is 3000 mAh capacity - MAX 20A discharge, battery in 13S7P configuration), that battery is then rated for a
maximum of 140A discharge, it will probably last a very long good life at 30A....
If you buy a
48V 21Ah battery made of 130 Sony US18650 VTC4 cells (each cells is 2100 mAh capacity - MAX 30A discharge, battery in 13S10P configuration), that battery is then rated for a
maximum of 300A discharge, it will probably last ten years at 30A :wink: .... But a 130 cells battery gets a bit heavy.
Theses guidelines are very rough and crude estimates. To get to know batteries more (and I encourage you to do so), you have to study their discharge curves (volts versus mAh) at different discharge rate (ex: 2A, 5A, 10A, 20A, etc...), their temperature rise curves at different loads (too high temps will dramatically shorten cycle life ... a cell that reaches 80°C all the time will most likely die very prematurely), their cycle life curves. Also look for low internal resistance cells/batteries, as they will heat up a lot less and live longer when submitted to high amp loads. For choosing good lithium cells, look for their datasheets (mostly available on internet via PDF files) and discharge curves.
This is another great tool for curves : http://lygte-info.dk/review/batteries2012/Common18650comparator.php
eBay batteries might be made of cheap 18650 cells. Sometimes they are even made of very cheap, low amp rated, hign internal resistance, rewraped (or even rewraped fake) recycled used laptop batteries. You dont want that crap for a battery, as it will probably boil or transform into magma if you put that on a 30A eBike.