Choosing a cell type for GMAC 10T trike build

[emcdermid]

So, I'm new to this, and I think I've gone through enough details to confuse myself. I need a sanity check, or at least battery advice.

I've put together a build for my recumbent (tadpole-style) trike. It'll be a Grin GMAC10T/CA3/Phaserunner setup, with a torque sensing BB for pedal assist. Total weight of rider and trike (with motor/battery) is about 330 lbs, though I hope to eventually bring that down to 290 or less. As such, the plan is primarily pedal assist to help with hills - I'll have a throttle, but that's just to get me out of trouble/get me home if something goes wrong with my knee, etc.

On the theory of "nobody regrets too much battery" and "I don't need to fit it on a downtube", I've zeroed in on the EM3EV rectangular hard case 14s7P, which comes with a few different cell options. The choices, in increasing order of cost, along with the current ratings from EM3EV are as follows: (Edited to correct the numbers for the 25r, which I'd put down incorrectly and were low.)

25r - 17.2 AH (max continuous current 40A, max burst 55A)
PF - 20 AH (max continuous current 40A, max burst 55A, same price as the 25r)
35E - 23.8 AH (max continuous current 36A, max burst 47A)
HG2 - 20 AH (no current stats listed, though I'd expect they're probably higher?)

Using Grin's trip simulator, even a really aggressive climb (for me) up to Rocky Mountain National Park from here on the Colorado front range would stay within the GMAC's wattage range if I apply a 20A current limit, though I could probably go up to 30A if and when I do finally drop those extra pounds. And most of my trips into towns (I'm rural) wouldn't come close to that and would likely use 8-10 AH of battery, wind permitting. My wife and I would love to eventually try some light "credit card" touring, but that'd be a special occasion kind of thing.

Assuming I'm aiming to stay in the 80/20% charge range for most riding, I *think* these numbers seem to be consistent with maximizing longevity/total cycles for any of those last three choices (the PF, 35E, and HG2). If that's the case, then I think I'm probably just deciding between the PF/35E for price vs range - the HG2 configuration is quite a bit pricier, and I'm not sure I need the ability to pull that much current.

That's where I'm at, but I'm just not sure I've actually understood all of the different issues involved. Can anyone tell me if I'm on the right track, or if I've missed/misunderstood some variables?

 

[Jenming]

Your 25r numbers are low. Em3ev has them at 40a/55a, which sounds conservative, they are rated at 20a each. I have no problems pulling 40a continuous on 6p of 25r with my gmac 8t.

With the 8t I want 40a continuous. I think with the 10t you need less. So all 3 you listed and the 25r should be fine.

Have fun, and if you want to go faster play around with field weakening. 4 amps made a big difference for me speed wise.

 

[emcdermid]

Whoops, you're right - I copied down the numbers for the 22P instead (which isn't actually an option right now). I'll edit my original post to fix it, wouldn't want to pass misinformation on to anyone else.

 

[goatman]

330lbs in the rocky mountains with a bumb knee, is it hilly? if it is, id go 25r's for the amps but i dont know how many amps you really need, 40 amps/7p= 6 amps max per cell but if you need 60amps , the 25r and phaserunner can handle it but can the motor?

what size is the rear wheel?

there are quite a few members here that would jump into this thread if you added Trike to the title

one member here, Triketech, has a couple scorpion FS trikes with that set up i think.

https://endless-sphere.com/forums/viewtopic.php?f=2&t=106842&p=1565792#p1565792

http://www.triketech.com/Drivetrain/PowerAssist/HPV-MAC-V2.html

 

[emcdermid]

330lbs in the rocky mountains with a bumb knee, is it hilly?

I'm on the front range, just east of the Rockies. But yes, we get hills, and "east" is almost synonymous with "downhill".

Not really long grades unless I actually do head up into the mountains, though, and I don't see myself doing a ton of that.

if it is, id go 25r's for the amps but i dont know how many amps you really need, 40 amps/7p= 6 amps max per cell but if you need 60amps , the 25r and phaserunner can handle it but can the motor?

what size is the rear wheel?

Good point, it's a 26" wheel. I don't think I need 40 amps, much less 60 - climbing up to the national park *is* a long mountain climb, and Grin's simulator seems to think I could do it on the GMAC 10 with 20 amps.

I'm really asking to get a sense for whether I'm choosing a setup that will let me get a long working life out of the battery. I thought that at one point I'd read that "continuous current" ratings needed to be halved or more to get that kind of life, but I can't find the reference again. It's entirely possible I got my wires crossed somewhere, I kind of feel like I've been trying to sip from a firehose with all of the information out there.

there are quite a few members here that would jump into this thread if you added Trike to the title

one member here, Triketech, has a couple scorpion FS trikes with that set up i think.

Done. And yep, Triketech played no small part in convincing me to go this route vs a TSDZ2.

 

[emcdermid]

I'm on the front range, just east of the Rockies. But yes, we get hills, and "east" is almost synonymous with "downhill".

...and for anyone wondering, one of the first things I learned around here was to try to ride west *first* when choosing routes to ride. The ride back is a lot more fun that way.

 

[goatman]

do you have rear suspension? if not you could go to a 24x3.0 tire

 

[emcdermid]

do you have rear suspension? if not you could go to a 24x3.0 tire

Yes, I do; the trike is fully suspended. Didn't realize I hadn't mentioned it - it's an Azub Ti-Fly 26 that I bought (used) a while back.

I love the thing, and have no desire to change that wheel geometry.

 

[goatman]

NICE, i like the rear triangle pivot, looks strong and bolt-on drop outs :thumb:

 

[Jenming]

I'm really asking to get a sense for whether I'm choosing a setup that will let me get a long working life out of the battery. I thought that at one point I'd read that "continuous current" ratings needed to be halved or more to get that kind of life, but I can't find the reference again.

EM3EV has already put pretty conservative ratings on those batteries. Take the 25r for example. That's a "20 amp" battery, which times 7P would be 140 amps. They have it listed as 40 amp continuous 55 amp max. Note that their battery build and BMS choice is limiting the max amps. Maybe the continuous as well.

 

[emcdermid]

EM3EV has already put pretty conservative ratings on those batteries. Take the 25r for example. That's a "20 amp" battery, which times 7P would be 140 amps. They have it listed as 40 amp continuous 55 amp max.

Aha! I'd wondered why those numbers were lower than the theoretical 20x7, but had forgotten to include it on my list of things to ask. So if I understand correctly, that "use at a lower rate" advice is already baked into these numbers, whether as a matter of good practice or BMS limitations.

I had also been confused about how all this interfaces with C-rate, but apparently I just screwed up my math, because while I'd thought it was lower the (C x Ah) number for each of these exceeds em3ev's "max burst" rate.