BMS size

[betisbikes]

Hello.

Would I be correct in thinking that if I have a 3000 watt motor and a 52 volt battery then according to the formula Power = Voltage x Amps - 3000 watts divided by 52 volts = 57.69 amps that the battery bms would need to be 60 amp or above ? or are there other factors that need to be considered ? If I tried to use a battery that only had a 50 amp bms, would the bms cut the battery out at some point when riding ?

Thanks. Ian

 

[Peter.Bridge]

It is the controller that supplies power to the motor. The battery and BMS need to be rated to the maximum controller output current + safety margin. Remember a nominal rated 52 volt battery will be 58.8v when full.

The motor rated power is pretty random and not connected to anything concrete. There are motors rated 250w that use 48v * 24Amp controllers

Usually the maximum power is a lot higher than the rated power

 

[betisbikes]

It is the controller that supplies power to the motor. The battery and BMS need to be rated to the maximum controller output current + safety margin. Remember a nominal rated 52 volt battery will be 58.8v when full.

The motor rated power is pretty random and not connected to anything concrete. There are motors rated 250w that use 48v * 24Amp controllers

Usually the maximum power is a lot higher than the rated power

Thanks for your reply. I have just got hold of a bike with a 3kw back wheel and an 80 amp controller but it has no battery. I have a 52 volt battery that i could try on it and also a 52 volt battery that I stripped down which had 21700 cells and a 50 amp bms. I am trying to build this as cheaply as possible, hence asking about the bms rating for the battery. I do know about cell discharge rates, so I won't be putting a bms in a battery that has to high a rate for the cells inside. Thanks. Ian

 

[Tony1951]

Thanks for your reply. I have just got hold of a bike with a 3kw back wheel and an 80 amp controller but it has no battery. I have a 52 volt battery that i could try on it and also a 52 volt battery that I stripped down which had 21700 cells and a 50 amp bms. I am trying to build this as cheaply as possible, hence asking about the bms rating for the battery. I do know about cell discharge rates, so I won't be putting a bms in a battery that has to high a rate for the cells inside. Thanks. Ian

3Kw?

I'd have thought this would suit you much better and it would most likely cost you a hell of a lot less. What is more, nobody is going to take it off you and crush it.

New price under £1900, 125 miles to the gallon and about 8.3 kw. You can ride it all day at 55mph.

 

[saneagle]

Thanks for your reply. I have just got hold of a bike with a 3kw back wheel and an 80 amp controller but it has no battery. I have a 52 volt battery that i could try on it and also a 52 volt battery that I stripped down which had 21700 cells and a 50 amp bms. I am trying to build this as cheaply as possible, hence asking about the bms rating for the battery. I do know about cell discharge rates, so I won't be putting a bms in a battery that has to high a rate for the cells inside. Thanks. Ian

You need a 100 amp BMS and cells than can deliver double that to be safe and reliable. If you overdrive the cells, they'll catch fire and explode, and you'll be in a lot of trouble. If you run any ebike battery at 50 amps, it'll get dangerously hot. I can see that this project is not going to end well for you, especially as you don't understand about current, internal resistance and cell specifications.

As a general rule for ebikes, you look at the individual cell specification for max current, multiply it by the number in parallel, then halve it to get the maximum current that the BMS should allow. Next take 20% off to get the max current the controller should allow.

Say the maximum current rating of your cells was 20A and you have a 4 x 14 configuration. The maximum BMs current should be 20 X 4 ÷ 2 = 40A and the controller should be 32A.