What is Battery “C” Rating & Is it important for Electric Skateboards?

Electric Skateboard Battery Technical Discussion.
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Jason
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Joined: Mon Jul 31, 2017 9:54 am

What is Battery “C” Rating & Is it important for Electric Skateboards?

Post by Jason » Wed Aug 02, 2017 9:06 am

I remember Fully-Stressing about what parts to buy when I was building my first electric skateboard, mostly because I didn't have the money to make any mistakes. I wanted to make sure I bought all the correct & best suited parts for the job first time round. I was on a tight budget but I also didn't want to skimp on the important stuff.

One of the most important & expensive parts of your eSk8 build is the battery, so I started to research, I wanted the best bang for my buck based on my knowledge at the time.

Here are some of my search queries
"What is the best battery"
"Most powerful battery"
"Best battery chemistry"
"Peak power draw from electric skateboards"

The problem is when you start researching stuff like this you end up with more questions than answers.... it's worth noting that after three years of reading & testing batteries I'm still learning... so maybe I'm just a slow learner! But let me tell you there is a lot of bullshit to filter through as well.

Anyway, After a few weeks of reading I decided to buy a Lipo because I learned that they have really high discharge rates & are high in power density. Meaning per gram they hold a lot of power. So they must be good!

So how can you determine the discharge rate of a battery?

Well fortunately this is clearly written on the label. The 'C' rating of the battery below is 20C. This means the the battery can peak output at 20X its capacity.

So what is the Capacity? Thankfully it is also on the label of all batteries, this one below is 3000mAh or 3Ah. So this pack can output 20 x 3Ah making the rated peak output for this battery 60Ah. You can get Lipo batteries with much higher C rating and also batteries with much higher mAh.
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My first battery for an eboard was an 8000mAh 30C Lipo, So 30Cx8Ah = 240A output! Wow! thats alot... so my board will be much better right?.... well not necessarily

So now you know how to determine the output amount of any battery, but this information is useless unless you know what power an electric skateboard needs to function.

So how much power is actually required for an electric skateboard to move a human from a stationary position to ball tearing top speeds?

The way to work it out is to buy a watt meter and install it onto your eboard, it goes in-line between the battery & the ESC, it gives you a real-time readout of how much power is passing through it.
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It shows you peak amps & watts, also voltage. So you connect this to your board and go ride around to see how much power you are using.

BUILD TIP You really don't need one of these permanently integrated into your eSk8. The info it displays is great to know initially but once you know the performance of your board the info really doesn't change so it becomes added weight and unnecessary wire bulk, also looking down at your feet when riding is dangerous!
You will notice it will peak when starting from stationary, when you go up hills it will peak, basically when ever the motor has a load to push it will peak, But cruising around your board should only be using 15-20A of power. Actually you want low numbers you don't want high amps.

So don't go brag to your mates about how awesome your 6KW (24V X 250AMPS) electric skateboard is!

I can assure you that if your electric skateboard pulls too many amps you won't have an electric skateboard for long & your battery will likely get damaged from discharging too quickly & over heating. What you want to build is a skateboard that is power efficient when moving your mass, one that doesn't get hot even when hitting the big hills. Heat is power that got wasted!

Having an electric skateboard use too much power would be like bragging about how much black smoke pours out the muffler on your car. Its not that you have a really powerful eSk8, it's because you designed the drive train incorrectly. My design philosophy is Powerful, Efficient & Robust. These characteristics must be balanced otherwise failure will occur. Having the power & using it efficiently is two different things. So if your setup is under powered for your load (read "body weight") it wont be efficient.

So yes your motors might be rated to 2400Watts (2.4kW) Each & Your Lipo might be able to out put 240AMPs and you might have a 150A Continuous ESC.... but you don't really want to reach the peak... its way to hot there & heat is bad!.

The best eboard will have an over engineered power system with safety features built in to prevent disaster
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Specifically when it comes to batteries, if you over discharge them, or discharge them too rapidly you will drastically reduce the life. So you need to be able to limit the power that is being requested by the motors & ESC. You can do this electronically or mechanically by reconfiguring the drive train gearing ratio.

Brushless Outrunner Motors are hungry little monsters... if you keep feeding them juice when a given load on them doesn't decrease they just want more juice. This could happen if you try riding up a big hill with a setup that is under powered for your mass. This appetite motors have for more power will rapidly convert to heat and your motor melts. So how do you stop this?

There are two electronic ways of managing how much power your system uses
1. A Battery can have its own built-in battery management circuitry, commonly known as a BMS or PCB or PCM.
2. The Motor controller can have current limiting features built in.
I recommend both of the above. You might also consider using a fuse directly after the battery just in case things heat up melting the insulation on your wires & shorting out the entire system and destroying everything in a spectacular fire.

So the moral of the story is, you really don't need the highest "C" rated battery on the market. What you want is a battery that is specifically designed to easily deliver plenty of power up to the limit you require. Ideally you also want one that protects itself using a built in circuit, so if it is asked to deliver too much power it will limit the discharge. This will ensure your ESC & Motors are protected and also ensures your battery lasts a very long time.

I recommend that 60A Peak is enough for an electric skateboard. If you are constantly needing more then this amount of amps you should increase battery voltage & increase the mechanical reduction to lessen the load on the motor. Or go on a DIET!

Higher Voltage = Lower Currents
Less Load = Lower Currents
Lower Current = Less Heat.
Less Heat = Longer Life & Improved Reliability of electronics.

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