Building a really big pack, could use some advice.

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jimbo69ny   1 W

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Building a really big pack, could use some advice.

Post by jimbo69ny » Feb 10 2018 1:01am

Hey guys and gals,

I'm new here and I just thought I'd say hello. I stumbled across this site while searching for 18650 cells. I just registered and I look forward to reading through your past threads.

My name is Jim and I am building a range extending battery pack for my 2014 Toyota Rav 4 EV. I have a thread started on myrav4ev.com. If your curious you can check it out here. http://www.myrav4ev.com/forum/viewtopic ... 8c0bad1880

Some recommendations that I am looking for...
I am going to need a good spot welder
I am going to need around 2,852cells. Preferably Panasonic 3400 mah cells
Recommendations on good fusible links for my 18650 cells.

Thanks guys! Feel free to let me know if you have any questions regarding my project.
Last edited by jimbo69ny on Feb 12 2018 12:47am, edited 1 time in total.

DAND214   1 GW

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Re: New here

Post by DAND214 » Feb 10 2018 10:13am

Welcome Jim. I'm sure you will find all you need here.
That's a lot of cells!

Dan

jimbo69ny   1 W

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Re: New here

Post by jimbo69ny » Feb 11 2018 2:53pm

Yeah it is. Its going to be a lot of work and MONEY. :(

Are there other people here working on EVs and large packs or is it mostly smaller packs and bike people?

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amberwolf   100 GW

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Re: New here

Post by amberwolf » Feb 12 2018 12:33am

You can look in the Large EV section, and the Motorcycle section, for some of the larger stuff.

BTW, you might want to change the subject line of the first post of this thread to something about your project, so more people will look at it.

Few people look at "new here" type of posts (quite a few spammers use that line ;) ).


Personally, I think you'd want to look at used Tesla packs, and use whole modules from them as-is. They're already encased and fused. :)

jimbo69ny   1 W

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Re: New here

Post by jimbo69ny » Feb 12 2018 12:45am

Tesla modules are large and wont fit where I want to put them. They are beautiful modules but I dont want to lose my entire rear cargo area. There is a hidden lower rear cargo area on the Rav 4 and that is the only place that I am willing to sacrifice. I use my car for work and I need the space.

If I used Tesla modules I would I would need 16 just to get to the voltage I need for my Rav 4. I could only fit 5 in the lower cargo area, the other 11 would have to take up the upper cargo area. They are $1400 each so that would be $22,400.

That is not the correct path to go down.

jimbo69ny   1 W

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Re: Building a really big pack, could use some advice.

Post by jimbo69ny » Feb 12 2018 12:49am

I just posted this thread. I need help finding the best series/parallel method for my modules.

https://endless-sphere.com/forums/viewt ... 7#p1357547

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amberwolf   100 GW

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Re: Building a really big pack, could use some advice.

Post by amberwolf » Feb 12 2018 1:08am

There's a few threads about 18650 pack layouts that would help you; I'd recommend the "common pack mistakes how to avoid" thread as a good one to start with.

After that, there's a lot of threads with 18650 in the title that you can look for, and then get an idea from the thread titles which ones to look at first for pack design (probably too many to read them all). Sorry I don't remember the names of any of them besides the one above. :(




Regarding the used tesla modules, unless it's the capacity of the individual cells that are the problem, it could be the cheapest way to get genuine good cells, if you keep an eye out for modules someone local to you is selling because they bought a whole pack but only need part of it, and are having trouble finding a buyer. It'd save you shipping costs that way (just time to break it down and recover the cells, if you don't want to use the modules).

If it's the voltage of the modules, you could rewire them (cutting busbars, reconnecting sections in series vs parallel); I don't know how much of a PITA that would be.


Based on the little I've read recently I'd guess that genuine new brand-name cells with higher capacities are probably going to cost $3-5 each once you add shipping/handling costs. So I guess that is cheaper than you're seeing cells from Tesla packs for (to get the size pack you need).

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amberwolf   100 GW

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Re: Building a really big pack, could use some advice.

Post by amberwolf » Feb 12 2018 1:14am

I definitely understand about not wanting to lose the cargo area--presently around a third or more of my cargobox on SB Cruiser is full of battery; until I build an under-trike box for it and rearrange the cells into a "thin layer" instead of a box-shape, I'm stuck with that. :/

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spinningmagnets   100 GW

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Re: Building a really big pack, could use some advice.

Post by spinningmagnets » Feb 12 2018 8:50pm

These fuses are dirt cheap and I believe the best option.

https://www.youtube.com/watch?v=Mrf9nfBBs18

jimbo69ny   1 W

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Re: Building a really big pack, could use some advice.

Post by jimbo69ny » Feb 12 2018 8:58pm

You like the glass fuses? I literally just got done reading a thread from someone who thought the glass fuses were somehow causing his pack to be off balance. I guess his pack with glass fuses made his BMS work a lot but his pack with wire fuses was always balanced. Not sure if there is any truth to that.

How many amps do the glass fuses blow at?

jimbo69ny   1 W

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Re: Building a really big pack, could use some advice.

Post by jimbo69ny » Feb 12 2018 9:02pm

A new potential issue was just brought to my attention. My OEM Tesla pack uses 2600 mah 18650 cells. It’s 92s48p. I want to add a 3400 mah 18650 cells. My pack would be 92s31p and it would be in parallel to the OEM pack.

I haven’t considered resistance yet and someone told me that the 3400 mah cells would have a lower resistance so my additional pack would be used up before the OEM pack. Truth to that?

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amberwolf   100 GW

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Re: Building a really big pack, could use some advice.

Post by amberwolf » Feb 13 2018 2:13pm

If theyr'e in parallel, then they'll just recharge from the other pack whenever current isn't flowing out of it.

Even if that never happens, then they'll still both be discharging, just at different rates.

As the lower resistance pack (which won't have as much voltage drop under the same load) drains, and it's no-load voltage drops to the point where it matches the higher-resistance pack's under load, then both packs will be discharging. A bit like a continously-changing leapfrog.

Exactly how it works out depends on each pack's cells' discharge curve and the loads on them, as well as the actual pack capacity.

Similarly, charging them in parallel also depends on their charge curve, and the charger's capabilities.

As for what internal resistance each cell has, you'd need to look at that specific cells' curves for that from the manufacturer. Every cell with the same capacity does not have the same curves.

There's a lot of threads discussing paralling various chemistries and pack sizes, whcih discuss what happens in various situations. Unfortunately no single search will turn htem up, but most f them are in the battery subsection, so if you look thru the thread topic list there, you'll find most of them obvious by title. Maybe a search in that section for just threads with "parallel*" in the topic would show you many of them.

You'll find that most things have been discussed here on ES if you poke around, and you can answer your own questions without waiting for someone to come along, if you need a quicker answer. ;)
Last edited by amberwolf on Feb 13 2018 2:26pm, edited 1 time in total.

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Re: Building a really big pack, could use some advice.

Post by amberwolf » Feb 13 2018 2:24pm

jimbo69ny wrote:
Feb 12 2018 8:58pm
I guess his pack with glass fuses made his BMS work a lot but his pack with wire fuses was always balanced.
It would depend on the resistance of the fuses (and any holders he might use) and the interconnects. If they are high enough resistance to cause voltage drop at the low currents charging is done at (assuming a fuse on each 18650 cell), then there's probably an issue with how they're connected or made.
How many amps do the glass fuses blow at?
Which fuses? Each one has a marking on it to say what it's rated for. Then there is a chart from the manufacturer to show how long it takes at that current to blow.

So...some blow at half an amp, some blow at 10 amps, etc.

THen the fuses also have to be rated for the voltage they're disconnecting, or it'll just arc across the fuse anyway. (one reason high voltage fuses may contain sand)

I recommend doing some reading about fuses and how they work (Bussmann bussfuses.net has some info on their site, as do other fuse manufacturers, and there's other stuff out there on the web), as it sounds like you might not have enough of the basics of them yet to define your needs for them and be sure to use the right ones. Once you've got the requirements figured out, you'll also want to get a few and test that they'll blow when you want them to, but only then. YOu don't want them blowing while you're just driving, but you do need them to blow when something fails.

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