Check my math here please.... Battery sizing question

[turt1e]

Just want to double check if I have my math straight here as I'm trying to decide just how much battery I need for my kart project.

I'm thinking of going with the 6s1p 8000 mah Zippie batteries on HobbyKing and buying enough to make 12s2p for 16000 mah or 16AH pack.

So with this pack I could expect to theoretically be able to support one of the following average amp drains:

16 amps for 1 hour
32 amps for 30 minutes
64 amps for 15 minutes
128 amps for 7.5 minutes

That sound correct?

 

[Wheazel]

Correct.
I have the same pack. Got the batteries just a week ago.

This pack contains guesstimated 700Wh. 50V x 16Ah = 800 - voltage dropoff during discharge.

 

[John in CR]

Leave yourself 1/3 cushion, because the last thing you want to do with lipo is 100% discharge. That means cut those times back to useful times of:
40min
20min
10min
5min
respectively

Then charge to 90-95% capacity and discharge to only 75-80% DOD.

 

[turt1e]

Thanks guys. For some reason I kept second guessing myself last night when trying to work out the calculations. I've gotten so used to my current power setups in my RC planes that I don't really have to think about it anymore when I'm trying to find a battery for those.

I do leave myself a cushion with the lipos I use in my RC planes. Usually I go by how many mah hours I put back into my pack and I try not use more than 80% of the mah rating. In fact I just purchased 8 3s1p 2200 GEN Aces batteries and I've been dialing in the flight times I can pull from those packs. My plane pulls 36 amp max amps with my prop/motor combo according to my watts-up meter. i've set my flight times to 7 minutes and I've been putting back into the pack according to my charger 1300 mah. So with this I can back this out the same way and find out what my average amp draw is for the way I fly?

1.3 amps for 1 hour
2.6 amps for 30 minutes
5.2 amps for 15 minutes
10.4 amps for 7.5 minutes

So if this is correct, it looks like my setup is drawing an average of 10.4 amps over the course of my 7 minutes of flying. So in this example I could bump up my flights on these packs another 1 minute or 2 and still stay within 80% D0D. Which is about what I've been doing for the past 6 years of flying with Lipos. I try to never pull out more than 80% of the total mah rating.

Now, if only I could find some average real use amp draws for the Mars/ETEK motors. It would make selecting batteries for my project a lot easier.

 

[Pure]

1.5 Ah per mile you need to travel, is a good rule of thumb for most commuter ebike builds. This estimate allows for WOT runs and times when you are just cruising using a low amount of amps.

 

[Hillhater]

1.5 Ah per mile you need to travel, is a good rule of thumb for most commuter ebike builds. This estimate allows for WOT runs and times when you are just cruising using a low amount of amps.

Yes fine, ..but he is not building a commuter ebike...!

...I'm trying to decide just how much battery I need for my kart project.

according to this , http://kellycontroller.com/mot/downloads/ME0909_Performance_Data.pdf
your motor can pull 90A continuous and max of 15kw kW,..@ 300A
.. assuming you will average half that max..IE 7.5kW, your average current draw would be approx 150A.
So, at that rate your 800Whr pack could last roughly 6 mins, or say 5 min at 80% DOD

You have plenty of battery Ahr, and if those zippys are 30c they should be ok in 2p configuration even for the 300A peak ! :wink:
But i suspect you will be limiting that max amperage anyway ??

 

[dogman dan]

Wow, you must commute fast Pure. My rules of thumb for 25 mph travel, including 1000 feet of climb along the way on a long ride, are 1 ah per mile for 36v, and .75 ah per mile for 48v. On a windless day, that will get you there with about 80% dod.

Useless data for this discussion.

On a kart track, you will pull max amps for about 10 seconds, then coast 10 seconds, pull max for about ten seconds then coast. Mabye 3 second cycles, but the idea is you will spend at least half the lap braking and coasting.

Hard to estimate, but I'd say guess what the max amps is, and what the lap time is expected to be, then cut the time in half. Then calculate that max amps times half a lap. That should get you in the ballpark.

My experince shows that this kind of max amps, then off, max amps then off riding is really harsh on batteries. So when calulating how big the pack needs to be, cut the advertized c rate in half. So you may want to just add ah, which makes the pack duration less important than whether you have enough capacity to have huge amps avalible without hammering the batts.

 

[Pure]

Wow, you must commute fast Pure. My rules of thumb for 25 mph travel, including 1000 feet of climb along the way on a long ride, are 1 ah per mile for 36v, and .75 ah per mile for 48v. On a windless day, that will get you there with about 80% dod.

I'm a big dude @ 6'2" 250 pounds with a 90 pound rig running an X5. So yeah I eat more amps than the usual ebiker. I'm also pretty sure my height is also one of the deciding factors that causes wind resistance to eat so many more amps at over 25 MPH.

Funny thing about this though. I have an adopted brother who is 5'10" 145 pounds. Having my amps limited with the CA, we both hit a top speed of 37 mph. I'm guessing that with a large enough motor, once you get going to a certain speed, weight (within a certain limit) doesn't have as much effect on amps being pulled as wind resistance does.

 

[gogo]

I'm a big dude @ 6'2" 250 pounds with a 90 pound rig running an X5. So yeah I eat more amps than the usual ebiker. I'm also pretty sure my height is also one of the deciding factors that causes wind resistance to eat so many more amps at over 25 MPH.

Funny thing about this though. I have an adopted brother who is 5'10" 145 pounds. Having my amps limited with the CA, we both hit a top speed of 37 mph. I'm guessing that with a large enough motor, once you get going to a certain speed, weight (within a certain limit) doesn't have as much effect on amps being pulled as wind resistance does.

Yes, you can run the numbers using the bike calculators:
http://www.kreuzotter.de/english/espeed.htm
http://www.mne.psu.edu/lamancusa/ProdDiss/Bicycle/bikecalc1.htm

Lighter weight is usually better, but there is always a point of diminishing returns. *Sry to the OP for getting off topic*

 

[Pure]

I'm a big dude @ 6'2" 250 pounds with a 90 pound rig running an X5. So yeah I eat more amps than the usual ebiker. I'm also pretty sure my height is also one of the deciding factors that causes wind resistance to eat so many more amps at over 25 MPH.

Funny thing about this though. I have an adopted brother who is 5'10" 145 pounds. Having my amps limited with the CA, we both hit a top speed of 37 mph. I'm guessing that with a large enough motor, once you get going to a certain speed, weight (within a certain limit) doesn't have as much effect on amps being pulled as wind resistance does.

Yes, you can run the numbers using the bike calculators:
http://www.kreuzotter.de/english/espeed.htm
http://www.mne.psu.edu/lamancusa/ProdDiss/Bicycle/bikecalc1.htm

Lighter weight is usually better, but there is always a point of diminishing returns. *Sry to the OP for getting off topic*

I must apologize, I had my numbers confused. I meant 1.5 miles per amp. That's what I get for posting after having a few drinks.