What am i doing wrong? Help please

[goldphive]

Hello everyone,

I am new to this whole scene and have recently finished an electric skateboard build using an ebike hub motor. it works great but i feel like i can get more run time out of my batteries.

what i am currently running is 2x 5S 5.0Ah batteries in series (10S 5.0ah) through a VESC 4.12 board and into a Leafmotor 36v 750W ebike hub motor.

I have done everything to my knowledge thus far in working with Vesc Tool to get my board running, but i worry that i could damage some components because i dont know a whole lot about electronics unfortunately. Currently i can only get about 15 minutes of run time out of my board before it shuts down. im not sure if the vesc is over heating, or if something else is going on. i checked my batteries after the board shut down and the cells were at 3.8v each. i have the vesc programmed to stop when the batteries reach 3.6v.

 

[BVH]

If your cutoff is occurring at 3.6 per cell under load and then ending up at 3.8 per cell resting, trying lowering the cutoff by .1 Volt per cell and you will get a bit more run time and hopefully end up with a resting volts per cell of 3.7. YOu could probably go even lower to a cut off of 3.4 and end up resting at 3.6.

 

[goldphive]

ive checked all the battery settings and they seem to be ok, i cant figure out why the board shuts off before my batteries hit 3.6v

 

[Voltron]

What BVH was saying is the voltage will sag from the load while you're riding to under 3.6v, but then bounce back up to 3.8v as soon as the motor stops.

 

[goldphive]

hmm, ok il take it for anther run and see if i get the same result. im nervous about how ive set VESC-tool up because im new to all this and worry about breaking components. is there any way to get a second set of eyes on how ive got it set up?

 

[Voltron]

I don't know vesc settings, but I know saggy cells... It wouldn't be unusual to see maybe .4 or .5 sag under high load, esp as it's a pretty small pack. When a pack is the same voltage but more amp hour, it usually has more cells in each group, so they share the load, and don't sag as much. Small ah pack plus high load = sag.
With stepping the cutoff down a tenth of a volt or so at a time like BVH suggested, you'll be able to see if it runs a little longer without endangering much.
Going as long as 3.3 or 3.2 wouldn't be crazy low, if they're regular lithium 18650 cells, esp as they'll rebound up a little after cutoff.

 

[goldphive]

the batteries im using are flat pack 5S 5AH batteries,

https://hobbyking.com/en_us/turnigy-5000mah-5s-30c-lipo-pack-xt-90.html?___store=en_us

is 5Ah considered low and cause sag?

 

[BVH]

You've only got 185 Watt hours of capacity and you can use only about 80% of that safely so you're down to 148 Watt Hours usable capacity. If you're steadily feeding 500 Watts into your motor, you would use up that 148 Watt hours in about 18 minutes. Do you know how many Watts you're feeding the motor on average?

On the surface of it, it looks like you are getting the run time you should be getting. I just think you need more capacity if you can fit it onboard.

 

[flat tire]

Pics of the hub motor skateboard?

I agree you have barely any battery capacity. Sounds like you may need more.

 

[goldphive]

You've only got 185 Watt hours of capacity and you can use only about 80% of that safely so you're down to 148 Watt Hours usable capacity. If you're steadily feeding 500 Watts into your motor, you would use up that 148 Watt hours in about 18 minutes. Do you know how many Watts you're feeding the motor on average?

On the surface of it, it looks like you are getting the run time you should be getting. I just think you need more capacity if you can fit it onboard.

Can you show me the calculation you used to find out how much watt hours of capacity just so i can get an understanding of it please, i thought that running 2 18.5v batteries in series would give me more than 185 Watt hours. Also, im not sure where to find how many Watts i am feeding the motor on average

 

[goldphive]

Pics of the hub motor skateboard?

I agree you have barely any battery capacity. Sounds like you may need more.

 

[Voltron]

That looks badass...

And watt hour calculation is easy.. It's the volts times the amp hours. So at full charge of 42v, times 5ah is theoretically 210 watt hours. When you series the packs up, it's doubling the voltage, but it's still 5ah.

 

[Voltron]

As for the watts, with your setup, maybe something like one of these from a RC shop.

 

[flat tire]

So at full charge of 42v, times 5ah is theoretically.

Can we start banning members with thousands of posts who mislead people on fundamentals they should know? Absolutely inexcusable.

The only correct voltage for discharge is nominal voltage or overall discharge voltage, taking into account the battery voltage during the entire discharge as it falls. For rc lipo the correct value is 3.7. So 3.7 nominal *5 series count*5 ah*2 batteries = 185 Wh.

As the batteries age and if you maintain a safe level of discharge you may see only 120-150Wh actually available.

Your board looks like it can consume plenty of power. Bigger battery needed.

 

[goldphive]

Thanks for the info! I will look into one of those Watt meters. the board is an absolute blast to ride! incredibly fast in my opinion. i dont have a mph reading yet but hope to figure out the top speed soon. I wasnt sure if the batteries would be enough when i started this, looks like either doubling up on the same batteries will be my next purchase, or if anyone knows of a good battery that would fit well on this board, that would be great!

 

[Voltron]

That is why I wrote theoretically... I almost added "or about 180wh if you go by the nominal voltage." Thanks for stepping up... And pretty quick with the ban hammer... Social distancing getting to you?

 

[BVH]

Do you have a gps device - a cycling computer or other device that will give you a speed reading? I use "Speedometer" on my Android phone.

 

[john61ct]

Best to just stick to Ah units for consumption stats and storage capacity,

unless you have a real need to derive Wh for comparison between batteries / circuits at different **nominal** voltages.

 

[BVH]

Comparing identical Amp hour battery packs of different nominal Voltages is an oranges to apples capacity comparison. Watt hours is the great equalizer.

 

[Voltron]

Have you considered backpack battery? I use one on a couple of my mountain bikes... It's a really noticable difference getting the weight off, and some people don't like the underboard ones on a longboard because of changing the flex during carving. Some of the fancy production ones have integrated spine protector and temperature activated fans if the battery is getting hot.