Many low-capacity or few high-capacity lipo cells?

[MadVlrus]

Hey fellas!

Simple question. I am in the process of building a 60v lipo battery for my 3000W electric scooter. That is 16 in series. Is it better to have many low capacity lipos (say 1500mAh or 2200mAh), or is it better to have fewer high capacity lipos (say 4500mAh or even 10000maH)?

It seems to me like the bigger capacity, the better. Larger lipos hold more charge/m3 and I'm assuming they will remain in balance for longer (5 batteries in parallel insetad of 15).

But perhaps I am overseeing something important here? Educated input is highly appreciated.

 

[wesnewell]

If you have a 60V system, then a 16s (59.2V) pack would make more sense to me since it would have an even number of cells. Takes less packs the larger the capacity, which makes wiring easier.

 

[monkeystador]

Why would you go for LiPos? What about LiFePo4? They last a good bit longer and are not as dangerous.
Going for bigger cells is easier wiring.

 

[MadVlrus]

If you have a 60V system, then a 16s (59.2V) pack would make more sense to me since it would have an even number of cells. Takes less packs the larger the capacity, which makes wiring easier.

I meant 16s, my bad.

Why would you go for LiPos? What about LiFePo4? They last a good bit longer and are not as dangerous.
Going for bigger cells is easier wiring.

I am not sure what makes you say LiFePo4 last longer... at best I could get 60% of capacity per same volume (calculating battery capacity and battery volume). That is the only reason. I feel like as long as I wire things properly, then lipo is superior choice.


It may seem that larger is indeed better, but for some reason larger tend to also be relatively more expensive.

 

[monkeystador]

Really? LiPo? A good LiPoFe4 cell can be last as long as 2000 cycles ( and they will keep going just with less capacity ) if you discharge on 80% of it and charge at 0.5C . The Cells are save to use and can tolerate discharges of 3C constantly and pulses of 10C. Granted for racing this isnt good, not even close to a RC LiPo. They are heavier and cost more, but last longer.
So how many cycles does your lipo do?

 

[wesnewell]

Yeah, lifepo4 can last 2000 cycles, IF, and that's a big if, you discharge them at 0.2-0.5C. Read the fine print. Discharge them at 1C and cut that in half. Discharge at 2C and cut that in half again. Discharge them over 2C and don't expect them to last as long as 20C rc lipo. And they're about twice the volume and weight of rc lipo.

 

[monkeystador]

Yeah i guess you are right there, still using the cells right the have longer durability than lipos and are safer for road use. Unless you go all the way and make sure the lipo's wont get wet or damaged.
There are many other issues with lipo's vs lifepo4 prismatic cells. Like a 40AH cell vs building a 40AH lipo pack.
Thx for pointing out the 0.5c. The 3C max constant discharge will of course reduce the lifespan of the cell more. Still your prediction of lifespan reduction is just a hyperbole. It depends on the cell type really. There are lifeypo4 out there that can do a lot more than 2000 cycles at 0.5c and 60% discharge. So i wouldn't just count them out yet.

 

[dogman dan]

Simple answer to your question. You need to get at least 5000mha packs, just to avoid carrying 10 pounds of wire to connect a bunch of smaller ones.

This applies to hobby lipo, not 19650's or other cells that tab weld to connect. You could remove a lot of wires, and solder up larger sections of hobby cells, but that is just a lot of work.

For 3000w, you need at least 15 ah of the 20c shit packs. Or 10 ah of better ones. 16s is nice, since it divides by 4. but 18s is nice if you want 6s packs.

 

[monkeystador]

Dont like to hijack. But i need a 60v 40AH pack. Thats 16s8p . And i dont trust lipos without a BMS for each . I wouldnt know where to get that. If anyone know, i would be happy to receive a PM.

 

[wesnewell]

These days BMS's for rc lipo/li-ion etc. are available all over. Here's a couple of places.
http://www.aliexpress.com/store/group/Battery-Management-System-BMS/313864_259134332.html
http://bestekpower.com/pcmbmspcbforli-ionli-polymerbatterypacks/