C Rating of 18650 cells in my project? Advice please.

Batteries, Chargers, and Battery Management Systems.
Post Reply
jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 12, 2018 3:38 pm

Hello,

I am adding a range extending battery pack to my 2014 Toyota Rav 4 EV. The vehicle's OEM pack was made by Tesla and uses 18650 cells. It has a 92s48p configuration. The main fuse for the OEM battery pack is 325 amps. 325 amps / 48 = 6.77 amps. That means each cell could potentially see 6.77 amps at the time the main fuse blows. I dont know what nominal amperage of the car during normal driving is.

I am adding an additional pack using 18650 cells. It will be a 92s31p configuration. I am open to suggestions on cells but I was leaning towards the Panasonic ncr18650b cell because I have heard they are reliable and they have one of the highest mah rating I have seen. I will be using cell fuses in my build.

My question is this,
I am trying to figure out if the cells in my additional pack will be able to handle the amp load of the car. I have been told that the Panasonic ncr18650b cells are 2 c with a peak amperage of 6.7 per cell. 6.7 x 31 parallel cells would give me a total of 207.7 amps.

To put it another way. If I took the 325 amp rating from the OEM battery pack's fuse. Would I divide that by 79? (48p from the OEM pack + 31p from my additional pack) If so that would give me a value of 4.11 amps per cell. That sounds much safer than 6.7 amps per cell.

Thanks!

jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

Re: C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 12, 2018 3:58 pm

I suspect that total amperage is divided between all parallel cells. Meaning if max amperage my car will take as the OEM fuse blows is 325 amps, and I have 79 total parallel cells (48p in the OEM pack and 31p in my additional pack), I think that means that I will see 4.11 amps per cell. Id love confirmation of this though. Please tell me if I am right or wrong.

If it is indeed 4.11 amps per cell I think that means I will be fine with almost any 18650 cell. But if I am wrong and 325 amps isnt divided by all 79 parallel cells I might be screwed. I cant make my additional pack any larger and keep it in the space I want to use.

Punx0r
10 GW
10 GW
Posts: 4384
Joined: May 03, 2012 8:16 am
Location: England

Re: C Rating of 18650 cells in my project? Advice please.

Post by Punx0r » Feb 12, 2018 5:29 pm

Yes, with parallel cells the total current draw is shared evenly between them.

The exception to this is where the connections to some cells have different resistance than others. E,g. if the cells at the end of a long string of parallel cells connected by wire or nickel strip that is too thin will have a higher resistance to the main battery leads and will not have as much current drawn from them, instead the load is borne by the other cells.

As you observe though, you will have a lot of cells for a modest current draw and the average load on a cell will be quite small. This does let you choose almost any cell out there and not worry too much about ensuring they are all evenly connection. A 325A fuse does imply maximum normal operating current will be significantly less. As a rough guide, the fuse is probably rated at least 150% of normal max. current.

As a sanity check, 96S x 3.8V = ~365V. If peak current is 200A then you have ~ 73kW of power (~97BHP).

Punx0r
10 GW
10 GW
Posts: 4384
Joined: May 03, 2012 8:16 am
Location: England

Re: C Rating of 18650 cells in my project? Advice please.

Post by Punx0r » Feb 12, 2018 5:34 pm

Just looked at Wikipedia and the Gen 1 RAVEV is stated as being 50kW, so that's ~130A.

However, it also says the battery is 288V 90Ah NiMH.

You need to take some caution there as NiMH can get a bit explodey if not charged or if connected in parallel with another battery properly.

jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

Re: C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 12, 2018 5:37 pm

Alright, it sounds like I was correct, that the current will be shared evenly between all cells. Thank you for explaining resistance too. I am also considering that right now as I plan my modules. I have a thread about that too. https://endless-sphere.com/forums/viewt ... 0#p1357720 If you have advice for me there I would appreciate it.

Back to this problem though, my Rav 4 EV uses an early Model S motor. It is rated at 125 kw peak. So 125,000 watts / 386 volts = 324 amps peak.

jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

Re: C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 12, 2018 5:38 pm

You looked at the specs for a Gen 1. I have a Gen 2. It was a very limited run, only 2600 were made and it was only available in California. Tesla made the vehicle for Toyota for compliance. Its basically a Model S in a Toyota Rav 4 body.

Punx0r
10 GW
10 GW
Posts: 4384
Joined: May 03, 2012 8:16 am
Location: England

Re: C Rating of 18650 cells in my project? Advice please.

Post by Punx0r » Feb 12, 2018 6:05 pm

Ah, yes , apologies, please disregard my last post.

That fuse rating is sailing surprisingly close to the wind!

jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

Re: C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 12, 2018 6:08 pm

It is!!! I have only ever heard of one vehicle ever popping a fuse. She hit the throttle on the highway and it popped. The fuse in the safety plug (a plug in all EVs that you pull when servicing) is 350 amps. I am surprised they are different but they are.

Either way, I am very glad you responded. Thank you.

If you have an opinion on my series/parallel thread I would be happy to hear your input on that too.

Thanks!

User avatar
flippy
100 W
100 W
Posts: 247
Joined: Aug 12, 2015 3:07 pm

Re: C Rating of 18650 cells in my project? Advice please.

Post by flippy » Feb 14, 2018 3:46 am

i would recommend using PF cells. they have a higher current rating and can deal with more abuse. they have a bit lower capacity at 2.9Ah wich makes for 90Ah in your planned new battery and a continous discharge rating of 310A without breaking spec.

also: PF cells are MUCH cheaper.

what are you going to do for a BMS? there are not a lot of 92S 300A+ rated boards.

jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

Re: C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 14, 2018 9:56 am

I have been speaking with Liion Wholesale and he recommended the LG MJ1 cells over the Panasonic NCR18650B cells. Panasonic are around 3340 mah nominal at 6.7 amps peak and the LG's are 3400 mah nominal at 10 amps peak. He quoted me a price of only $3.80 per LG MJ1 cell! Panasonics were $1 more!

What are PF cells? I assume that is the designation at the end of the model number. Who makes them?

I havent looked into BMS too much yet. I suspect what your saying is correct. However the BMS isnt going to be seeing 300 amps is it? Just the voltage. I saw videos online with the Batium system. I dont know if that can be used in a car but I dont see why not. I need to look into it more. If anyone has any suggestions I am all ears. Each of the 9 modules will have 10 series blocks. Then each of the modules will be in series with the one next to it. Because I need to get to 92s, I will also have 2 series modules on the right side of my box.

User avatar
flippy
100 W
100 W
Posts: 247
Joined: Aug 12, 2015 3:07 pm

Re: C Rating of 18650 cells in my project? Advice please.

Post by flippy » Feb 15, 2018 5:45 am

Panasonic NCR18650PF 2900mAh
10A discharge but here is the real kicker: they are ~2 euro a piece in bulk from nkon. price per watt is almost unbeatable.

yes, a proper board with a current shunt can measure 300A, but gettin a board that can do 92S is a different story.

personally i would simply use 3 34S programmable and even them out over the pack and hook them up but only connect it to the charger. so no driving power goes tru them, only charging power. pack protection would basically be a manual fuse. and you can hook up the mosfet gate output of each bms to a separate contactor to break the pack if one bms triggers. it would be the most economical option and more modular. just a bit more work to set up.

jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

Re: C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 15, 2018 9:30 am

Wow thats a good deal. Do you have any info on who to contact to buy them? Someone reliable hopefully?

As for the rest of it, I dont know, that seems like a lot. I think my system will be pretty simple. 92s31p in parallel with the existing 92s48p Tesla pack. Because it will be in parallel the onboard 10kwh charger will charge both packs. The OEM pack has a BMS that I could probably integrate with if I knew anything about CAN, but I dont. EIther way, I dont think it will be a problem having a separate BMS for my secondary pack. Someone from Batrium just responded and said that their system does work in cars. So that might be the way to go.

User avatar
flippy
100 W
100 W
Posts: 247
Joined: Aug 12, 2015 3:07 pm

Re: C Rating of 18650 cells in my project? Advice please.

Post by flippy » Feb 15, 2018 9:32 am

https://eu.nkon.nl/rechargeable/18650-s ... 00mah.html

92S times 11 bucks per cell monitor is going to be a really expensive ordeal. that is 1000 bucks just for the monitor boards and another few hundred for the control boards and assorted crap.
personally i dont find such a complicated bms worth the price in this use case. you start with basically a perfect balance and only need to do very light balancing. something you can do with 3 fairly cheap programmable boards. that would cost you about 300 bucks in total.

jimbo69ny
100 mW
100 mW
Posts: 45
Joined: Feb 10, 2018 12:53 am

Re: C Rating of 18650 cells in my project? Advice please.

Post by jimbo69ny » Feb 15, 2018 9:55 am

Great points. I havent looked into pricing or any other BMS' yet. $1000 to cover a $14,000 battery doesnt seem that bad. Cheaper is always better though. I will look at that once I get closer to needing the BMS.

I have another thread on this already but I am currently looking into the issues of running a newer battery pack with higher mah rating directly in parallel with the OEM pack. It seems as though resistance are going to be less on the newer cells, therefore the secondary pack would be delivering more of the amperage to the motor. It doesn't seem like that big of a deal but people keep freaking out and saying I cant do it.

One thing about me is if you tell me I cant do something, I am definitely going to do it. HAHA

User avatar
flippy
100 W
100 W
Posts: 247
Joined: Aug 12, 2015 3:07 pm

Re: C Rating of 18650 cells in my project? Advice please.

Post by flippy » Feb 15, 2018 10:56 am

dont worry, the packs will balance eachother out depending on current draw. it is a fact the new pack will push more power but the current is still about half as when you have only 1 pack so people need to stop complaining. spreading the load over 2 packs like this does wonders for the wear of the cells. in theory you would even get more power as you have a lot less voltage sag.

ps: i only get 7 grand worth of cells if you use PF's. :roll:

Post Reply