Working on LiFePO4 36 volt pack

[Toorbough ULL-Zeveigh]

I think the main issue I'll have with the pack is the weight. 36lbs is quite heavy for something that's supposed to weigh alot less than NiMh...(snip)

'supposed to' is right, because that's a mistaken impression.
As of this moment, nimh actually has the higher specific energy figure compared to LiFe.
But that will probably change at some point as it's still early in the game & expect increments in LiFe capacity just as Nimh is still showing which is already rather mature.

 

[BiGH]

Ask what the cells are protected by. One of the reasons the cells can be heavy is that many LiFePO4 cells have a metal box around the edge of it. This weights a substantial ammount, and battery places prefer to supply batteries in this manner.

I would personally try and find a supplier that can supply cells without the protective metal box, and build a perspex style box like ypeddle has. Maybe build two 10ah packs. This means that if you don't need the full range of your bike for some reason, you can just take one pack - less weight = less power required. This is what i plan to do with 4x 36v 10ah packs - two in series 2 in parrallel to give 72v 20ah

it also makes fitting to the bike easier.

edit:

stealing solarbbq's pics again:

you can see this pack is made up of individual cells,

where the invididual cells are EACH covered in a metal cover:

http://www.users.bigpond.com/solarbbq/lithiums1.htm

 

[erdurbin]

Looks like the max discharge rate is 15C. With my setup, the most I will ever be pulling is a little under 3C. These should be plenty for my needs.

 

[Battboy]

I thought I would start a chart that folks can help fill out to provide a meaningful comparison of the new LiFePO4 Power Packs that are coming on the market soon. I entered the information on the first line for LiFeBATT, since I have all that information. I will let others do more comparisons for similar 36V/10Ah pack offerings.

The idea here is to provide a more thorough comparison than simply a Price Point, since this is a new battery chemistry and many are not familiar with what kind of specs. to ask for and to compare both Price, Quality, Power, & other important characteristics. If you can't find the spec. you can substitute "UNKNOWN" and maybe other's can help fill those in later?

Best Regards,

Don Harmon

 

[flip_normal]

meaningful comparisons could also include life of pack (charge cycles untill pack is down to 85% capacity) and time to charge the pack.

It would also be useful to see results of lab tests on charge cycles. If you can charge a pack in 20 mins then discharge it in 40mins the test would take 1,500 hours. This would take about 2 months. I'd love to know capacity after this kind of duty cycle.

 

[Battboy]

Yes, but most LiFePO4 mfg. will quote 1,500 - 2000 cycles so that's pretty much a given standard. Claims of more would be speculative.

The testing you are talking about is what we are doing starting in a few weeks here in the U.S. under an Independent Testing Laboratory's supervision & objective scrutiny.

Best,

DH

 

[flip_normal]

cheers Don, nice to know.

 

[oatnet]

Also, I missed the link on the specs: 270mmx207mmx200mm and weighs 16500g. That equals to 36.3lbs!! Ouch that's heavy. Does that sound right for a LiFe pack? Falcon EV's pack wieghs in @ 22lbs but @ twice the price.

I had noticed that too and figured it was an error. Zane called me about something this morning and I mentioned it to him... He calculated the weight of packs and converted it to lbs, then said it looked like the metric figures he was given might be inaccurate, and he is checking into it.

-JD

 

[mvadventure]

The possible error of conversion concerns me as I was under the impression that they had been doing this for quite some time now. Zane discussed the new improved chargers and BMS systems with me and it seems that if you had some of these on the bench, now or earlier, you would measure and weigh them to double check your figures. Especially when it comes to actually shipping them.

Oh well, we'll see. My order was in on 8/8/07 so three to five weeks to go.

Mike

 

[Battboy]

I use this link all the time to double check Metric dims.: http://www.worldwidemetric.com/metcal.htm

Best,

DH

 

[DeeL2003]

Man I hope the weight measurements are wrong. If it's in the 20's I might just buy them real soon.

keep us posted on what Zane finds out.

MVadventure: can't wait for you to get your pack and give us the thumbs up or down.

 

[Toorbough ULL-Zeveigh]

So it's finally starting to sink in.
I've said it b4 but apparantly need to drive the message home.

LiFePO4 are only about 15% lighter than nimh.

That is gravimetric density or wh/kg. Also they take up about 40% more volume than nimh for the same number of Watt-hours. As far as energy density goes, LiFePO4 is only average. Its strength lies in it's power density & it's lifespan. Plus it's early in it's development cycle so it's energy density can theorhetically be improved upon at least 4-fold from what's been indicated. You need to be aware what it's relative strengths & weaknesses are to make a rational calculated decision accordingly.

Also keep in mind not to assume that the generic chinese LiFe give the same performance as A123. Their lifespan & current capability are about half (painting with broad strokes) which is still pretty good. At this point you're paying for the privelege of being an early adopter. The next generation will no doubt clean up the field on all fronts. But if spiral-wound nimh, graphite based SLA, or a safe LiIon get an investment push I hope we'll have an embarassment of choice.

My approach for the time being is to go with the best $/Wh deal that will do the job. I need to repack a hub enclosure & don't want to sacrifice 40% fewer Wh to fit the restricted space. Since I don't have a requirement for high amps I've regtrettably ruled out LiFePO4 because I would luv to not to have do this again for 2000 cycles. As they say, your requirements fer sur will vary.

 

[Battboy]

LiFePO4 would improve the performance, make the bike lighter and “coolerâ€￾ since Ni-MH can get mighty hot during discharge. What a Marketing edge; an e-bike that has a 1,500->2000 Cycle life!

•C-LiFePO4 has good potential for power
–60% of capacity at 10C
–60 mAh/g at 40C has been shown

The spec. of the 10Ah Life battery:

Energy Density - 90~100Wh/kgs
Power Density - 650W~700W/kgs.

DH

 

[xyster]

That is gravimetric density or wh/kg. Also they take up about 40% more volume than nimh for the same number of Watt-hours. As far as energy density goes, LiFePO4 is only average. Its strength lies in it's power density & it's lifespan. Plus it's early in it's development cycle so it's energy density can theorhetically be improved upon at least 4-fold from what's been indicated. You need to be aware what it's relative strengths & weaknesses are to make a rational calculated decision accordingly.

LiMn has better volumetric density and can be charged with regular li-ion/lipo chargers, but a lower power density, and (perhaps) a lower cycle life.

LiMn 26650: 3.8v X 2.5ah = 9.5 wh
http://www.swift-tuning.com/EMOLI_26700A.pdf
LiFePO4 26650: 3.3v X 2.2ah = 7.3wh
http://www.a123systems.com/newsite/pdf/ANR26650M1_Datasheet_FEB2007-1.pdf

Seems to hold for the other size cells presently available too.

 

[Toorbough ULL-Zeveigh]

LiMn has better volumetric density and can be charged with regular li-ion/lipo chargers, but a lower power density, and (perhaps) a lower cycle life.

That's my overarcing point.
There is no holy grail that is best across the board.
So while we're waiting for that, in the meantime you have to carefully choose your parameters.
You can have power, capacity, weight, longevity & price.
Pick any three.

 

[eP]

You can have power, capacity, weight, longevity & price.
Pick any three.

Longevity isn't a constant. It's strongly depend on temperature and charging rate.

 

[Toorbough ULL-Zeveigh]

You can have power, capacity, weight, longevity & price.
Pick any three.

Longevity isn't a constant...

Neither are most of the rest of them! :lol:
Of those five parameters, only weight is a constant & even that can vary, but only by a small amount.

 

[erdurbin]

I called Zane last week and he said it should ship this week.
More details to come...

 

[Lowell]

While the cycle life of NiMH is less than LiFe, nobody has ever complained my NiMH powered bike is lacking in performance.

 

[fechter]

Hmmm..... the Nimh batteries in my Honda hybrid have gone through thousands of cycles and seem to be working great also.

I heard somewhere (that brain cell died) that if you never fully charge or fully discharge Nimh batteries, they will last practically forever. A BMS of some sort might help in this regard. I'd be willing to give up, say 20%, of my capacity to extend the cycle life of a battery to "practically forever".

 

[erdurbin]

I would say that the Honda batteries are kept topped off most of the time. This is not the case on an e-bike since we don't lug around a generator. A regen controller helps some, but I have only heard reports of around 15% less charging time with a regen controller.

 

[fechter]

The Honda batteries don't really stay topped off, but they never get fully discharged and rarely get fully charged (at least according to the battery level meter). Most of the time it's somewhere between 1/4 and 3/4.

 

[erdurbin]

That is interesting. That was probably the problem with the SLA's being used for cars.

 

[CGameProgrammer]

No, it's done on purpose. NiMH lasts far longer if it's always kept between 40% and 60%, so hybrids use battery packs much larger than what they need so they can keep the SoC between that narrow range. That's why the packs can last for 10 years theoretically.

Electric cars can't afford to leave so much energy unused, but this significantly shortens lifespan, which is why they weren't necessarily viable until recently.

 

[xyster]

No, it's done on purpose. NiMH lasts far longer if it's always kept between 40% and 60%, so hybrids use battery packs much larger than what they need so they can keep the SoC between that narrow range. That's why the packs can last for 10 years theoretically.

Lithium too. Both chemistries prefer to be operated and stored in a range centered on about 50% state-of-charge. %40-60%, 30%-70%, the farther outside the range one drifts on a regular basis, the less battery life one gets.