was i thinking about making an electric skateboard and i was wondering if i could connect one 18.5v 5000mah 40c with a 18.5v 5000mah 30c together

 

Yes you can ... there are two ways of "connecting them together " ... in parallel ,that's connecting +ve of one battery to +ve of the other ...and -ve to -ve make sure they have nearly the same voltage when you make this connection ... this will give one big pack with 18.5V output ....

The other way is in series which gives a voltage of 37V , you probably don't want that ....

The different discharge (C) rates are only the maximum permissible without harming the battery , if you put two in parallel they will share the load , and will be much happier and cooler ....The max discharge rate of this big battery is limited by the lowest C rating ... it can discharge at 30c ... but the capacity has doubled so that's 300A max .... over 5KW you will never need that

 

thanks so much, i am going to connect the two lipos in series in order to achieve the 37v needed to run the motors that will power the electric skateboard.

 

thanks so much, i am going to connect the two lipos in series in order to achieve the 37v needed to run the motors that will power the electric skateboard.

That will work fine also , since the capacities are the same (5000mA) ... The C ratings don't come into it , because your skateboard will never demand such high currents..

I assume you have protection circuitry on the batteries to keep cells in balance and prevent discharging too low

 

A guy in my neighbourhood rides an electric skateboard. He goes very fast even uphill. It looks like it has wireless remote control. It makes a loud high pitched sound.