LiFePo4 battery pack: A question for the experts

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Gaber
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LiFePo4 battery pack: A question for the experts

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I am re-posting a question in it's own thread so that perhaps some battery guys will be able to help me. The original post is here:

http://visforvoltage.org/forum/6140-brainstorming-commuter-bike#comment-35989
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I want to know what kind of current I can continuously draw from a set of 15 X (3.2V, 40Ah) set in series to create a 48V pack:

http://www.electricmotorsport.com/store/ems_ev_parts_batteries_thunder_sky.php

In particular, can I draw enough power to drive the EMC-R motor, which does 150A at 48V continuous:

http://www.electricmotorsport.com/store/ems_ev_parts_motors_etek-r.php

The spec sheet from the battery website makes me think they are too weak (each cell does 3A continuous, 10A peak)!

http://www.thunder-sky.com/pdf/TS-LFP40.pdf

But this website says they can do 120A peak. What am I to believe?

http://www.evpower.com.au/-Thunder-Sky-Lithium-Ion-Batteries-.html

Thanks!

-Gabe

antiscab
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Re: LiFePo4 battery pack: A question for the experts

i draw 50A continuous from my 15 TS 40AH cells.
i get 3vpc at 50A.
at 50A continuous, my pack has lost just shy of 20% of its capacity since new (850 cycles on 50%dod and 15 months ago)

150A might be a bit much for continuous, but this you will have to test (probably destructively).
i do draw between 150A and 200A peak on acceleration (voltage sags to 2.4vpc when i do)

i would suggest using larger cells if you need 150A continuous.
you will also need more cells if you need 48v@150A (at 50A, i only get 45v out of my 15 cells)

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km

Gaber
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Re: LiFePo4 battery pack: A question for the experts

Hey Matt, thanks for the info. Can you tell me about the bike you are powering?

The problem with using larger cells is that I will still need 15 of them to achieve 48V with a simple series setup. This is too much $$ and probably overkill for what I want to achieve in terms of range (I have 11 mi daily commute, so I'm looking to get a 20 mile range or so).

Hmmm. I think I was reading the spec sheet from Thunder wrong. Max continuous current = 3 CA X 40Ah = 120 Amps. Impulse amps = 10 CA X 40Ah = 400 Amps. However, the continuous average recommended by www.evpower.com.au is 0.8 CA X 40Ah = 32 Amps:

http://www.evpower.com.au/IMG/pdf/Thundersky_guidelines.pdf

(BTW, CA = cranking amps = Amps/Amphr capacity??? confusing!)

I think the question is: Will I will be doing 32 Amps on average with that EMC-R motor (link above)? This is not something I'm sure how to calculate just yet...it doesn't seem too outrageous!

antiscab
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Re: LiFePo4 battery pack: A question for the experts

Hi Gaber,

i run an emax:
emax sport

for my commute (at 40mph continuous) my 19 x 40AH cells gets me 24 miles range.
my commute is only 8 miles, and i charge at work

i would suggest getting at least 16 cells to get a real 48v under load (1C)

the reason rod recommends 0.8C continuous, is because thats the highest hes tested at (and can reliabley say will work).
ive tested at 1.5C continuous, and run them at 2C continuous without issue.
there may be some service life reducction in continuous use at higher discharge rates, but you wont know for sure until you test.

in reality, you wont see much more than 200A (5C) out of these TS 40AH cells.
when the cells are hot, you might see more.

as for working out how much your motor will draw, this will be determined more by your load than anything else.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km

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