How much of battery power I need to drive a motor

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
110
Dear Sir/Madam,

I have a motor with 250W 24V, I buy a LiIon battery 24V10Ah, this battery is formed by 18650 cell in 6S5P structure. I install it to my motor car, if without loading, the wheel run very fast, when running with loading, it is very very slow. What kind of battery I should select to make my motor car running if I only have this one motor?

How do I calculate?

Best regards,

Kelvin.
 

sagor

Joined Mar 10, 2019
116
Look at the basic numbers. A 10Ah battery is rated as such usually at 1/20C, meaning it is designed to provide 0.5A for 20 hours and maintain that 24V voltage. There are differences based on load surges and such, but that is a good "rule of thumb" to work by.
Now, a 250W motor will draw about 11A under load. Your 10Ah battery most likely cannot maintain that kind of load and keep its voltage at 24V. The internal battery resistance may be too high to provide that kind of current to that motor.
I suspect you would need a much larger battery to run a 250W motor under load.
To draw 10A for a motor steady state, I would "estimate" at least a 100Ah battery would be a good choice, though some smaller ratings may be ok, depending on steady load or intermittent loads.
 

oz93666

Joined Sep 7, 2010
672
Look at the basic numbers. A 10Ah battery is rated as such usually at 1/20C, meaning it is designed to provide 0.5A for 20 hours
Wow ... where did you get those figures from ??? Even the slowest discharge 18650s are happy at 2C (can be discharged in 30 secs) .... some are OK at 30C and higher ....

Lets suppose this battery is made up from basic 18650s ... it will be comfortable discharging at 20A , that makes 480W ... If new this battery should be easily able to run your 250W motor ... 6s is rather low voltage , normally 7s is used for 24V ... the voltage of lithium varies widely depending on state of charge 4.2 to 3.3 V so 7s gives 29.4 to 23.1 ... that's open circuit , with your motor connected it will drop a few more volts .
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
110
Wow ... where did you get those figures from ??? Even the slowest discharge 18650s are happy at 2C (can be discharged in 30 secs) .... some are OK at 30C and higher ....

Lets suppose this battery is made up from basic 18650s ... it will be comfortable discharging at 20A , that makes 480W ... If new this battery should be easily able to run your 250W motor ... 6s is rather low voltage , normally 7s is used for 24V ... the voltage of lithium varies widely depending on state of charge 4.2 to 3.3 V so 7s gives 29.4 to 23.1 ... that's open circuit , with your motor connected it will drop a few more volts .
Thanks a lot. After I research more information about the concept of C-rate, does it mean a 2C-rate of 18650 (e.g. 10Ah) can provider larger power than a 1C-rate of 18650 (e.g. also 10Ah) within a short time?
 

oz93666

Joined Sep 7, 2010
672
Thanks a lot. After I research more information about the concept of C-rate, does it mean a 2C-rate of 18650 (e.g. 10Ah) can provider larger power than a 1C-rate of 18650 (e.g. also 10Ah) within a short time?
C stands for Capacity ...so if your battery or cell has a 10Ahr capacity , if manufacturer says "maximum discharge 2C " then 2xC is allowable ... 2 x 10 = 20A of course this is just the maximum recommended to stop the cell/battery being damaged...

Some 18650 cells have a 30C maximum allowable discharge !! that means a 2500mAhr cell could deliver 75A without damage ... it would be flat in 2 mins. delivering 75A
 

SimpleJoe

Joined Mar 22, 2016
34
I have a motor with 250W 24V,
I install it to my motor car
Just bringing up the mechanical side of this, how big is this car? If its a model/Radio Controlled car this should be fine but if it is anything bigger I'm surprised it is even moving at all. With a quick search on electric skateboards most have a motor roughly around 2000W and upwards and a skate board is a stripped down as you can make it. What is your situation?
 
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