Motor selection advice

[Josh42]

I'm building my first go-kart and have been reading as much as possible to make the best choices I can for my own goals. This forum and its members have been an incredible resource to that end.

For space and weight reasons (not to mention cost), I'm using 20Ah batteries (https://cdn.shopify.com/s/files/1/2364/9089/files/EXP12200-Specs.pdf). I realize the unfortunate limitations this may give me in run time but have come to terms with that. To mitigate current draw with a bit more performance, I've opted for a 72V setup, though.

I'm now between two different motor configurations: either an MY1020-style 3000W with a paired 50A controller (https://www.amazon.com/gp/product/B07KR2FN89) or the Golden 3kW (HPM3000B) with a Kelly KLS-S controller. Obviously the Golden setup will be more expensive than the MY1020, but it's far better quality/performance, so that might be worth it to me if I can get away with it.

My concern is the current draw on the 20Ah batteries. I'm having a hard time finding reliable information about max discharge rates on AGM batteries. It seems like ~2C for the MY1020 should be tolerable, but beyond that, I'm not as confident. The 72V Golden 3kW could draw ~4C from my battery setup. This leads me to a few questions:

1. Is a 4C draw okay for, say, 20-second bursts on a 10% duty cycle?
2. Will AGM batteries supply whatever is asked of them as long as temperature and 50% discharge limits are respected?
3. Are there rules of thumb to abide by when sizing motors with batteries? (and are these rules based on science?)

Let me know your thoughts. I appreciate any info you can provide. And be gentle - I'm still trying to learn.

Thanks!

 

[Functional Artist]

Howdy

I haven't used a 72V set up...yet
...but,
I do have a 60V 2,000W brushless motor on my Aerial Atom kart
...powered by (5) Mighty Max 12V 15AH SLA's
https://www.diygokarts.com/vb/showthread.php?t=40167&page=13

Them (5) little SLA's power it
...but the voltage drop is significant or HUGE

Check out the meter showing a (~5V) drop in the video
...& when that happens, the batt pack, that started off showing ~63V, is "now" in essence, a 58V pack

https://www.youtube.com/watch?v=FSQ0qhfWa9g

 

[Josh42]

Thanks, FA - I remember seeing that in your build thread. My first impression was, "D***, that's impressive to pull 60A from that controller!" I realize it's a peak draw, just didn't expect the internals of that controller to support it at all. I've seen data curves for the 48V 2000W version of that motor, and also the 72V 3000W one I'm considering (both attached below). The 48V shows a peak draw of ~75A at 4000rpm to achieve its 6.9Nm of torque, so it makes sense that you're seeing 60A with a higher voltage. The 72V motor is showing a peak draw of 47A, but only achieves 5.4Nm of torque. I was hoping to mitigate this weaker torque with a better overall gear ratio. But still, the 25Nm at ~0rpm of the Golden 3kW motor is really appealing!

So anyway, you're pulling ~4C from those batteries and you're seeing a momentary 5V drop under load. That's definitely significant, but the voltage rebounds when the load decreases, so it seems like a relatively normal behavior, particularly with lower capacity batteries. I'm no battery expert, but I'd wager you'd see the same behavior on a similarly-sized LiPo pack in that situation.

• Have you noticed any long-term effects on the batteries from this voltage drop?
• What kind of run times are you usually getting with your Atom kart?

BTW, that is a fantastic kart, and I thoroughly enjoyed following your progress on the build thread. Kudos, and thank you for sharing your experience!