custom designing an ebike system

[oldskoolhead]

hi all,
just a quick few questions as i would like to convert my mountain bike to electric and want a low powered motor but also want to be able to cover long distances,

now let me just fill you in on my experiences ive had with my sakura s207 as i tried to parallel a second battery to it and fried the controller as i wasnt aware that the amp hours of the battery need to match the amp rating of the controller so this experience has led me to be more cautious when i plan to design a street legal but long range ebike.

so lets say i am looking at a 36v 250-350w motor and i want to be able to power it with say a 40ah lifepo4 battery, firstly the battery needs a bms but does the bms need to be 40 amps cont. power ie do i match the rating of the bms to the battery or do i get a bms rated at say for instance 10 amps as 350w at 36v is just under 10 amps which is all i need for the nominal motor power, or would i need to get a bms rated at the motors peak power as i know my 200w sakura with its standard controller was drawing in excess of 650w as the 36v 15ah sla's covered about 12-13 mile @20mph max speed so burning 540w/h in 45 mins max so i would expect a 350w motor could draw 800w under load or there abouts so maybe id need a 30a bms?

then we get on to the controller do i then need a 40a controller to match the 40a battery or does the bms act as a current limiter to which value is yet to be established (10a? 30a? 40a?) and i only need a controller rated to the bms value?

then after all this if say my system needs to be 40a all the way through would that then suggest i would need a 1500w motor and be forced to get a programmable controller to limit the power or will my 250-350w motor just draw the power it requires without getting overloaded?

sorry to ask so many questions its just everything i thought i knew about electrics came into question when i fried my controller lol and finding definative answers to these assumably simple questions isnt as easy as you might imagine, i do know my sakura is now running on a 500w controller and using less power than it did before and is probably slower as well which is confusing seen as its supposed to be 200w street legal (though that fact is questionable anyway as it did do 20mph) but im assuming the motor would draw 200w with no load and this is the denominator according to the law (either that or sakura are just plain flouting it) any info would be much appreciated and thankyou in advance

 

[Drunkskunk]

You have the right idea about the BMS. Your BMS needs to be rated the same or higher than your controller. You really only need a BMS with Lithium batteries, and even then, it's not manditory. But it is a good Idea.

But your idea on the battery rating has some errors.

A battery is rated in Amp Hours.(AH) an amp hour is it the number of amps the battery can expend in an hour. 40 amp hours means 40 amps for 1 hour. Or, It can also mean 1 amp for 40 hours. Roughly speaking, an Amp hour is a measure of capacity.

A controller is rated in Amps, not amp hours. They are usualy rated by their peak amp rating, although some give a peak and constant rating. In general, a 10 amp controller will only allow 10 amps to go through, at which point it over rides the throttle input and holds the output to 10 amps.

There is also a rating called "C", which stands for capacity. A battery's C rate is the amount of power it can put out compared to it's capacity in Amp hours (AH). A 40AH battery with a rating of 1C can put out 40 amps. (40 * 1 = 40) If that battery had a rating of 3C, it could put out 120 amps. A battery's Amp hours Times it's C rate must be atleast equil to or higher than the controller's Amp rate. For example: A 10AH battery with a 1C rate wuld burn out quickly if used with a 20A controller, as the battery could only supply 10 amps. a 10AH battery with a 2C rating would work with the 20A controller, but it would be pushing it's limits. a 10AH battery with a 2.5C rating could supply 25 amps, more than enough to work with a 20A controller.

You can use any AH (amp hour) Size battery you want with a controller without damaging the controller. 10AH, 100AH, 1millionAH, it makes no diffrence.


Basic Math:
A amps
AH amp hours
V volts
W watts
WH watt hours
C capacity

A * V = W
AH * V = WH (watt hours, the true capacity of a battery)
AH * C = A (peak output rate of a battery in amps)

 

[wesnewell]

Standard watt ratings for motors and controllers are for continuous operation. You can exceed that rating by a factor of many times for shorter periods of time. You likely fried your controller by running it at max wattage for too long. Don't confuse max amp rating with max watt rating. The max amp rating is not for continuous operation, but for the max amperage the controller can put out. As an example, I use a 72v 1500W controller with a max amp rating of 40A. I run mine at 100V, so for continuous operation, I should not exceed 15A (15x100=1500W). The motor I use is a 1000W motor so even that is over the motor rating and would eventually fail if run continuously at that wattage, but not before my battery pack was completely drained. However if I were to run it at 40A (4000W), one or the other would fail within some time frame after some time if the battery didn't run out first.
Your battery pack (and bms, should you use one) should match the max amp rating of the controller. For this don't even think of the motor for it has nothing to do with it. So if you use a 40A max controller, you want a a battery pack capable of putting out 40A continuous to safe.
Overall, think of this just like a car. You can run you car at max rpm for a few seconds, but if you try and run it at those speeds continuously, you'll blow the engine in no time at all. Same for an ebike. Now you could put a governor on both the car and the ebike to prevent blowing something up, but IMO, that not practical. You have to be the governor, just like with your car.
If you are going to use a 350W motor and 36V battery pack, then do the math for each and buy accordingly. I would not try and put more than 1500W burst or more than 400W continuous through a 350W motor. So a 40A controller and battery pack, with you controlling the power usage. If you can't control it, then a 12a controller, which will not have a lot of power.

 

[dnmun]

more misinformation.

the BMS has nothing to do with the controller. the maximum current rating for a BMS depends on the cells that the battery is made from. if you use a pack made from the generic prismatic pouch cells which are only capable of 3C or so then the BMS would have a maximum current rating of 2-3 times the Ah capacity of your pack.

if you have a higher quality, cylindrical cell like the headway then it can deliver 5C continuous and the BMS for a pack made with the headways would be 5 times the Ah capacity. and so on up to something like the A123 which can push 10C or more, and the BMS would need to be able to handle that much current.

your controller will have current limits too, built into the device to keep it from trying to handle too much current for which it is not designed.

if you don't know anything yet, you should put off designing something like a battery or power system from scratch until you are familiar with the equipment.

if you build a lifepo4 pack without a BMS like these guys have recommended, you have only yourself to blame.