Advice for my electric kart + spreadsheet

[ElecKart]

Hello, I'm new here. Here's my introduction post: http://www.diygokarts.com/vb/showthread.php?t=39063.
As you can see in the post, I've read the stickies about electric motors, and itsid's post was very helpful because of the formulas.
The specs I thought are:

• 150 kg max mass (will this be enough including batteries and a 75 kg max pilot (security margin)?)
• 5º max slope (can this be reduced? I'm afraid that if I put 0, it won't be powerful enough)
• 15 m/s (54 km/h) to 20 m/s (72 km/h) max speed
• 2 m/s² acceleration
• 30 min or more run time.

I've already calculated the motor for my kart according to the formulas, and I made a spreadsheet in Numbers (Apple's alternative to Excel) to simplify the calculations, and I wanted to share it with you. In the shared folder, there's both Numbers and Excel (exported from Numbers, please excuse the possible errors that could arise from that), in English and in Spanish, and also a Google Sheet in Drive (create a copy for editing): https://drive.google.com/open?id=1N_A_7eofMqMy7VYfz469jrdbda-kwJTe.

Now, the problem is that it gives me about 11.3 to 16.7 kW, depending on speed, which is way too much! I'd need some pretty big batteries for 30 min run time! Any tips to make my specs more feasible?

 

[itsid]

16 kW sounds like too much to me (just from the guts)

I think the main issue with your calculations is, that you calculated the power needs..
for an accelerated uphill run at roughly the targeted speed... could that be?

If so, pick wisely.. top speed and cruising speed are two very different things.
(should be at least if not on a race track )
Then.. top speed on level road (0 incline) and inclined road again ask for very different power,
depending on the targeted speed of course.

So, here's what I'd suggest you to do:

take three different numbers..
1) acceleration up the incline [should be fairly large and account for the steepest hill you want to/need to tackle]
2) top speed on level ground [my very last formula is probably what you are looking for]
3) a mix (30% of targeted incline, 60% of targeted top speed.. cruising [no additional acceleration]

Now...
the third value gives you the best idea of the overall powerdraw
if you settled for batteries already, use their capacity to calculate driving time,
or if you must reach 30min of driving time, then calculate the needed capacity from that.

The first two give you clues about the maximum power..
I'm thinking 4-5kW should do.. if you want some extra grunt to get slightly higher speed up an inclination maybe 7kW.. at max should fit your needs on most if not all occasions
(again from my guts w/o doing the math now]

frankly I wonder how you even reached 16kW with just a 150kg setup and less than 100km/h of speed...

Still for a 30min runtime that's at least in the realms of 1800-2000 Wh to be calculated with sufficient safety margin say 40Ah for a 48V battery pack ..[means 4pc of 40Ah 12V batteries]
not too bad is it?
for a 36V batpack you'd need around 55Ah batteries [3 pieces of course]
and so on and so forth.
certainly possible.


Again, without looking at your spreadsheet
I'm fairly certain you have a flaw in your calculations somewhere...
tst with the examples I gave in that thread and see if your spreadsheet yields the same results to test

'sid

 

[ElecKart]

16 kW sounds like too much to me (just from the guts)

I think the main issue with your calculations is, that you calculated the power needs..
for an accelerated uphill run at roughly the targeted speed... could that be?
If so, pick wisely.. top speed and cruising speed are two very different things.
(should be at least if not on a race track )
Then.. top speed on level road (0 incline) and inclined road again ask for very different power,
depending on the targeted speed of course.

So, here's what I'd suggest you to do:

take three different numbers..
1) acceleration up the incline [should be fairly large and account for the steepest hill you want to/need to tackle]
2) top speed on level ground [my very last formula is probably what you are looking for]
3) a mix (30% of targeted incline, 60% of targeted top speed.. cruising [no additional acceleration]

The first two give you clues about the maximum power..
I'm thinking 4-5kW should do.. if you want some extra grunt to get slightly higher speed up an inclination maybe 7kW.. at max should fit your needs on most if not all occasions
(again from my guts w/o doing the math now]

frankly I wonder how you even reached 16kW with just a 150kg setup and less than 100km/h of speed...

Yep, that was the problem. I assumed that I still had to accelerate once I reached top speed and that I wanted top speed in a 5º hill. Typical math misinterpretation mistake, I suppose.

Now that I corrected it to 0º and 0 m/s² at top speed, it gives 4.5 kW electrical, with a 0.8 efficiency factor (3.6 mechanical), just what you said (your gut feelings are pretty good). The same with uphill: 7.7 kW electrical and 6.2 kW mechanical. Maybe I'm worrying too much about the max slope, because kart circuits should be pretty level (I'd say 3º it's enough).

Now...
the third value gives you the best idea of the overall powerdraw
if you settled for batteries already, use their capacity to calculate driving time,
or if you must reach 30min of driving time, then calculate the needed capacity from that.

Still for a 30min runtime that's at least in the realms of 1800-2000 Wh to be calculated with sufficient safety margin say 40Ah for a 48V battery pack ..[means 4pc of 40Ah 12V batteries]
not too bad is it?
for a 36V batpack you'd need around 55Ah batteries [3 pieces of course]
and so on and so forth.
certainly possible.

I'm still in the predesign phase, so I still haven't bought anything yet. 2 kWh would be good for this motor at full power for 30 min, so I'll probably use an even smaller pack because it'll rarely be at full power and the batteries will probably be cheaper and lighter that I thought. Good news!

 

[itsid]

LOl well it's called TOP speed, because you cannot go any faster
(no positive acceleration left w/o going downhill that is )
And while you might need to calculate top speed and acceleration for uphill tracks
(say you want to compete on the Nordschleife *giggle*)
it's rather uncommon and mostly not necessary and gives you a much easier task to accomplish if you simply ignore such worst cases and dial in for either.

yeah.. kart race tracks are typically rather flat...
also keep in mind that we talk degrees
not inclination percentage (numbers you'd find on a road sign)..
kart tracks are closer to 3% inclination not 3° of slope IMHO

Anyways, glad you got some easier achievable results with your new calculations

'sid