Help selecting cells for Gouach battery, please

[offGridDownUnder]

I'm a backer, and I've been notified my casing will ship soon. I backed a 48V case + cells, but presumably due to the bizzarro world we find ourselves in, I'm told I'll receive the case and must source the cells myself. Links are provided to several sources in Europe and the United States. I am in Australia.

For reference about the specifications, this page gives the recommended cell sources: Cells suppliers

I've checked several, and I don't see shipping to Australia, so no go. Within Australia, we have a very good supplier here: Lithium Battery Cells #1 Rechargeable eBike Batteries

As I post this, there are two entries which match 18650. How would I go about deciding which is better for my use? The gouach supplied choices highlight some as their recommendation, which can give some clues, but I don't know what is the basis for choice.

As much 'why' as possible would be appreciated - I do want to learn. This happens to be the next pool I'm being thrown into.

The target machine uses a Baserunner (or I could swap in a Phaserunner) and a v3 All-Axle rear. The current battery is a 48V 20Ah Cap Rouge cell. Given I paid for cells and won't get them I may have some credit, and buying a 36V insert may be a way to use that up, so discussion of 36V vs 48V won't go amiss either in your comments about how to decide "best cells".

 

[slaphappygamer]

The chart on the product page has really good info.



The max number of cells is 40. So there can only be 10S4P or 13S3P. With a 36v battery, you can go 20-22 mph max. The 48v battery can go about 25-28 mph max. Typically, more parallel groups equal more range. In this case, the number of cells are the same (whether 36 or 48 volts) and the capacity will be the same. Though I think having a slower max speed, with a 36v battery, would help reserve energy for more range.

Since you've already used a 48v battery, you know what to expect as far as top speed is concerned. Unless you like going fast, going to 36 volts MAY be a hard move.

 

[lnanek]

Max amps matters too, though. My fat tire bike needs 35A to go 30mph, so trying to go that fast on a BMS only rated for 25A would usually just trip the BMS' protections and turn off the bike.

I actually paid to reserve a Gouach as well, but haven't bothered getting it due to this. I'd have to get two and put them in parallel.

 

[docw009]

I would go with 39 cells, 13S-3P. You will be looking at either Eve or Panasonic 18650 cells on the Australian website. You probably want cells that are at least 3AH, leaving the Panasonic 30Q, 35E, and NCR GA, plus the Eve 33V. Four times the cell AH will be your battery's rated AH.

I've built batteries with 30Q, 35E, the Sanyo version of the GA, and Eve 25P's. Also purchased commercial packs several with 30Q's. The 30Q seems to be disliked by a few DIY folks for short life, but I've not had any batteries wear out. I don't run them hard though. I probably only pull 4-5 amps out of my battery when riding around,

The 35E is not advertised for high current and is only an 8A cell, although they seem to be in a lot of ebike batteries. GA (10A) and 30Q (15A) are better choices, in my opinion.

 

[neptronix]

Damn, you only have 3p and you need to drive a hub motor that likes 2000w.
I would look into molicel or whatever the highest discharge cell you can get your hands on is.

You will get lower capacity but at least you'll be able to power the motor with such a limited amount of cells in parallel.
I'd suggest a 6C continuous rated cell or higher.

 

[offGridDownUnder]

Four times the cell AH will be your battery's rated AH.

I don't understand this, and I would like to.

What characteristics lead to this?

 

[offGridDownUnder]

6C continuous rated cell

The 18650 cells listed in the Cap Rouge link don't provide this rating that I can see. Is this calculated, or something I would look up elsewhere?

If calculated, can you point me at the details?

 

[neptronix]

Usually a cell's datasheet ( google it ) will tell you this. Few websites that sell cells publish the important specifications.

See this thread for a lot of talk about high output cylindrical cells. There are a couple models of cell mentioned here that can be bought nowadays. Molicel comes first to mind.
Tabless design cylindrical cells tests

 

[docw009]

The 18650 cells listed in the Cap Rouge link don't provide this rating that I can see. Is this calculated, or something I would look up elsewhere?

If calculated, can you point me at the details?

I believe the C rating comes from the AH rating. Take a 3 amp-hour cell and C is 3. Then 6C would be 18 amps, But for the Cap Rouge inventory, in the 18650 size you're looking at

Molicel 30B 3000mah 35A peak
Molicel 28A 2800mah 35A peak

Neptronix says you have a 2000W motor? That's 40A peak, so 13A per cell is feasible, but your 25A BMS will get hot if you hold it there. Do they use a aux heat sink with it?

 

[slaphappygamer]

I wonder if the BMS can be swapped out. If you can only use 10s or 13s, that would require 2 different BMSs right?

 

[amberwolf]

I would go with 39 cells, 13S-3P. You will be looking at either Eve or Panasonic 18650 cells on the Australian website. You probably want cells that are at least 3AH, leaving the Panasonic 30Q, 35E, and NCR GA, plus the Eve 33V. Four times the cell AH will be your battery's rated AH.

I don't understand this, and I would like to.

What characteristics lead to this?

I think he meant three, because he's talking about a 3P battery. If it was a 4P battery (four parallel cells in each group), then it would be four times.

 

[amberwolf]

The 18650 cells listed in the Cap Rouge link don't provide this rating that I can see. Is this calculated, or something I would look up elsewhere?

If calculated, can you point me at the details?

The C rating sounds complicated in words, but:
--the C rating is a Current capability.
--a cell has a capacity, in Ah.
--the C rating is how many times the capacity it can deliver current...so C = A / Ah.
If a cell can deliver 10A, and is a 5Ah cell, then that's 10A / 5Ah = 2C.
If it can deliver 5A and is a 10Ah cell, then that's 5A / 10Ah = 0.5C.

Some are more complicated.

For instance, a lead-acid cell is typically capacity-rated at 1/20C, meaning you can only get it's rated capacity if you draw current from it at a max of 1/20 of that in A. If it's a 100Ah battery, you can only draw 5A from it if you want to get 100Ah from it. Above that current, you get less than 100Ah, by whatever amount it's discharge rating curve on it's spec sheet shows...but it's not uncommon for it to be down to half capacity by the time you get to 1C, or 100A drawn only gets you 50Ah out of it.


The C-rating is not important for the typical lithium cell. What's important is how many amps you can draw without too much voltage sag for your application, or generating too much heat in the cell. That is found by looking at the discharge curve on the spec sheet for the cell, or on test sites such as Flashlight information or various threads here on ES, etc. Like this one:

Test of Molicel INR18650-P28A 2800mAh (Gray)

lygte-info.dk

If a curve like that isn't available, and it doesn't have separate burst and continuous ratings, then you could guesstimate a useful current rating from the maximum current (A) rating a cell is given by using only 50% of that or less for continuous current, and using the regular A rating it has as a short (few seconds) peak rating.

So, just as an example, let's say you have a system that requires 40A continuous, and can draw 80A peak during short startups from a stop. The battery must be able to supply that without too much voltage sag, or else you don't get neough power.

Let's say you're stuck with a 3P pack for whatever reason. That means that each cell must supply 1/3 of that 40A(80A), or 14A (rounded up; always round need up, and capability down) continuously, and 28A peak.

If you use the linked cell above, just as an example, it sags a lot at 14A, dropping down about half a volt even when full (4.2v drops to 3.8v) and gets worse as it gets emptier...and it's one of the more capable cells out there. It can provide the 28A peak, but that's even more sag (and more heat in the cell from the wasted power inside it).


If you need to keep sag to a minimum, then for this cell you could keep it down to 5A per cell, but you'd need at least 8P to do that.


And just for some more examples of C-rate, 5A for this cell is ~2C (since it's a 2.8Ah cell). 14A is about 5C. 28A is about 10C.

 

[offGridDownUnder]

I wonder if the BMS can be swapped out.

They offer 36V & 48V batteries, and each has it's own version BMS.

A point I can make about this is that I while I ordered the 48V assemblage, I can take (what appears to be) the forced credit I have (as they are not supplying the cells I paid for) and instead convert to the 36V components - the Base/Phase-runner and All-Axle can work with this.

If there is a reason it's better. 48V means less amps, but the 48V battery will be 3p vs 4p in 36V mode.