discharge battery
 

hello !!

i have several qustions about battery discharge:

1) if the battery is 5000mah and have a discharge rate of 3c then the corrent
is 15 A, but what happen if the motor consumed only 10 A current ? Is the engine overheats ?, and how this affect the battery ?

2) when i choosing a bms (battery managment) for my battery pack, what should be the bms current in the case described above, acooerding to the motor consumption or the battery ?

3) when i connect several cells in series, and each cell have a discharge rate of 3c, is the pack also have only 3c discharge rate ?

thank you all
tome

1) You will never connect your batterys directly to the motor. You will have an ESC (electronic speed controller) regulating the power going to the motor. If the ESC is supplying 10A to the motor, the battery will be happy with it. I don't know about your motor so I can't say whats the max power it can handle.

2)I don't know what bms is. If thats same as ESC (regulates the amount of power going to the motor), maybe 20A.

3) Yes, your pack will also have discharge rate of 3c. What determines the power of your motor is watts (W), not amps or volts individually. Power=Voltage*Current so if you have 2 packs in series, you can get double the power compared to a single pack.

The C rate is a measure of how much current there is at any moment in time, in comparison to the amp-hours the battery contains. The term is also used in a different way to indicate how much current a battery will be happy with when pulling it continuously. A 10Ahr battery rated at 2C can produce instaneous amperages from zero to much higher than 20amps - spiking to the rating of the controller, which might be a 40 amp controller. for example. So a 2C <rated> battery would be operating at 4C at that point, above what it is happy at (20 amps). The instantaneous current is not the same as the rating of the battery, and varies as you use the bike.

regarding post # 1, if you add a BMS use one that works with the number of cells (series) you are getting. And if the BMS has a over-current cut off (not all of them do) make sure it is reasonable. It can match the rated amperage of your cells (15A) or be twice that much (30A) and still do its job. (I am assuming you will use a conrtroller a.k.a. ESC that allows a maximum amperage that is reasonable (it should be lower than the amperage cutoff on your BMS)

c = 1hr
2C = 30 min
3C = ( 1hr / 3 ) 20 minutes.

so a 3C battery is capable of discharging at a fast enough rate to drain completely in 20 minutes, according to specification.. some RC lipo cells are capable of 60C, ie : 1 minute discharge, or more.

Why does this matter ?. Well, if you plan to use the smallest possible battery that can power your bike, you need a battery capable of doing it without getting too hot.

a BMS needs to be sized big/powerfull enough to sustain the load of your motor controller *(ESC), so if you have a 20 amp controller, and plan to use 5ah ( 5000mah , same thing ), the battery needs to be capable of 4C, and the BMS needs to be capable of the same output , ie : 20 amps continuous.

If you use a high power battery on a low power motor, the battery will remain cool, last a long time, and life is good.

The motor will only get too hot if it's too small for the job, regardless of what battery you use.