help, lifepo4, internal resistance vs size, parallel..??

[cycleops612]

All too hard for me, i have searched, no joy

Issues re a good cheap durable pouch cell lifepo4 (LFP) utility batteries?

please consider in cheap ass 2-350w mode, not 750w+ US mode .

Batteries are definitely THE expensive achilles heel of ebike/evs. For the frugal majority on the planet, 350 watts max is often plenty to get around, & 24v 15a can manage that.

He can buy fewer cells, or maybe spend the savings on bigger/better cells for range rather than raw power. A poor rickshaw driver e.g.

EU rules, BTW insist on ~250w max.

So, How do the relevant characteristics of otherwise identical pouch lifepo4 cells, differ due to their size? Internal resistance e.g.

Are bigger cells better/worse in any way?

Recharging issues can be critical to rickshaw productivity. Maybe larger cells can absorb more charge, discerningly applied, with less damage/heat.

Bar the obvious current reasons, could there be advantages to using same wh packs, with less voltage but bigger cells?

Methinks a big advantage of pouches is the simplicity of relatively huge cells vs scores of complexly connected canister cells. So why not extend this logic to, "a bigger lfp pouch beats a smaller lfp pouch"?

Then, we have the converse, related practice, of makers like Ping, using smaller parallel cells for higher c-rates. (their 15 ah packs are made of groups of 3x 5ah cells i think)

How does this tie in with internal resistance etc. as above. Are there downsides? Longevity?

As the quip goes, sorry its so long, i didnt have time to make it short.

I think my query comes down to the specs of internal resistance of differing cells, and the effects of this.

Economics can still trump tech tho. If larger cells are cheaper and more durable pro rata, they are still a buy, lo volt warts and all.

Fewer cells = simpler more servicable battery. Tracing and replacing failed cells is much simpler.

since i am at it, a dimension that can matter a lot in a pack, is that it is narrow enough to fit ideally between the riders legs, yet pedal ok.

this dimension is dictated by the number of pouch cells laid flat in the conventional brick pak. More cells = thicker.

 

[999zip999]

A richshaw how about a pic.

 

[dogman dan]

Well,, the cheapest per mile, longest lasting battery I ever owned was the ping I bought in 2008. no way you see that price now, partly because the cells are better now.

But a rickshaw driver needs 350wh? honestly,, I think you need to double, or even quadruple that, unless he works a 2 hour shift.

Start with a 24v 20 ah,, then if you can add another. Even with a lower wattage load,, a 20 ah pack will experience a lower discharge rate than a 10, and thus last longer. Then another one on charge while you are working might be needed to get through a busy day.

You can find better cells than the ping,, better c rate, lower resistance. But lower your discharge rate enough, and you don't have to have quite such low resistance.

Bigger battery is how you make them last,, always.

 

[cycleops612]

well, i suppose I have a part answer


i compared the 20ah 3.2v lfp cell here with the specs on the sister product, 60 ah cell.

http://www.batteryspace.com/lifepo4-prismatic-module-3-2v-20-ah-10c-rate-64-wh-6-0---un38-3-passed-dgr.aspx

and

http://www.batteryspace.com/lifepo4-prismatic-module-3-2v-60-ah-10c-rate-192-wh-----un38-3-passed-dgr.aspx

(is a link to respective spec sheets there)

sample of one, but Lo:

The 60ah cell has a </= 2.0m ohm internal resistance & the 20ah cell has </= 2.5m ohm.
100ah cell is lower again, 1.8m ohm

Less internal resistance in larger cells seems significant. Thats my question mainly answered.

But just saying, if a guy is poor and has modest max power requirements, of say 350w, i am not convinced he could never ever be better off with a 500wh 24v battery, than a dearer 48v 500wh battery, purely in terms of the (all important) battery - half the cells which are twice as large. Maybe bigger cells is a poor mans way of enjoying some of the benefits of a "bigger battery" - less strain on it.

u would think each Wh was cheaper in larger mainstream cell sizes?

 

[cycleops612]

Well,, the cheapest per mile, longest lasting battery I ever owned was the ping I bought in 2008. no way you see that price now, partly because the cells are better now.

But a rickshaw driver needs 350wh? honestly,, I think you need to double, or even quadruple that, unless he works a 2 hour shift.

Start with a 24v 20 ah,, then if you can add another. Even with a lower wattage load,, a 20 ah pack will experience a lower discharge rate than a 10, and thus last longer. Then another one on charge while you are working might be needed to get through a busy day.

You can find better cells than the ping,, better c rate, lower resistance. But lower your discharge rate enough, and you don't have to have quite such low resistance.

Bigger battery is how you make them last,, always.

yes, i agree, I wish ping luck, but i would spend the premium he charges, for hi C rates on a bigger standard lfp pack.

Bigger is better doesnt change the fact that bigger is heavier and dearer, and at some point, is no longer an effective option.

The rickshaw was just an example. my job is a bit easier than that thankfully.

The mid-drive my1016z2 motor (sadly, with brushes), e.g can VERY cheaply be had with; a 9:1 reduction box, choice of chain sprockets and even an axle assembly, so my guess is, 350w would work fine with a little manual assist, even w/ 28" wheels.

http://www.ebay.com/itm/24V-350W-Electric-Motor-W-Gear-9T-Sprocket-24-Volt-350-Watt-MY1016Z3-U-ST11-/310663455118?hash=item4854fc218e:g:LXwAAMXQ-KRRgwvZ&vxp=mtr

http://www.ebay.com/itm/Electric-Scooter-13-Tooth-Sprocket-Motor-Engine-Parts-Motor-Pinion-Gear-MY1016Z-/112014903605?hash=item1a149bc935:gmUAAOSwVFlULdZf

If cheap 24v controllers can handle 20/25amps ok, then fine, 480w 24v motors are no problem if it suits better.

A better example may have been the hordes of motorbike taxiS in asian cities. Since they seem to spend much of their time hanging around a depot (charging point) waiting for fares anyway, it would be great if they were electric, not least, for the taxi bike owners - almost free to run and service. Sort of a 2 seater bicycle, hold the pedals (more trouble than they are worth). Labour is cheap, so fussing with swap batteries is doable economically.

Pulse charging i must read up on. Sounds interesting.

the below is quite natty, 300w btw & a damn sight heavier than a rickshaw. It could well be exactly the motor/rig described above.

http://accessary2.sale.fustat.com/pz50870f5-3-wheels-electric-vehicle-thcl-10a.html

 

[999zip999]

Oh that's a rickshaw you need a big high quality battery. Lots of protential weight what type of riding Hills ect.