Creating 18650 battery pack?

So how would one go about creating a battery pack with 18650 batteries and getting a bms to manage the pack, then installing it into the vehicle? Because this seems like the way to go. They seem cheap enough. Cheap ish.. and the car is old and needs a new pack. Where can I get a bms that can manage that many cells and 360v? And at least 24kwh? And how the f*** do I get it into the case that's there. Seems heavy and like I'll need some jacks and something with wheels to get the thing out after pulling the fuse in the back seats and undoing the wiring connectors.

I wouldn't bother. Used packs are easy enough to find and unless you really, really know what you're doing, soldering that many 18650 cells together would be a disaster. The cells need to be balanced and unless you can find an off-the-shelf BMS that will handle that much power and that many cells, trying to make your own is a nice electrical engineering project, not a DIY activity.

You should never solder lithium batteries, or any other kind that is damaged by heat. Spot welding is the preferred permanent attachment, as it produces much less heat over a large area on the cell. But yes, trying to make a Leaf pack from scratch, from small form factor cells, is not a good idea.

Where can I get a 360v bms for making a pack? Was gonna see if I can stick one in the trunk.

https://www.mynissanleaf.com/viewtopic.php?t=17470 is the BMS for a gen 1 Leaf. https://www.mynissanleaf.com/viewtopic.php?p=380865#p380865 shows what it physically looks like w/o its casing.

Anyone know of a high voltage contactor that will work for turning off the 360v? I need one between each cell pack so I can charge them with 72v chargers. Need 110/240v ones so I can wire it to the charge port. I think that should work they have independent bms units I think. I just need dpdt relays so I can use the always off position for when the battery is turned on, somehow link it to the ignition being on, and the always on position for Charging it with a 240v plug. And a second high voltage relay to surn on/off the charger. at 110 and 240v. This should get 28kw i think and if I can get a good waterproof connector I can disassemble it and take it out of the car and charge it inside where it's warm. Or in a few mini barbeque grills just in case.


https://www.ebay.com/itm/383276816832?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=5-67Qmx9QE-&sssrc=2349624&ssuid=pdt6GyGETvi&var=&widget_ver=artemis&media=COPY

https://www.diyelectriccar.com/

Using lithium batteries to create a custom battery pack for your vehicle is indeed a feasible option, albeit with some technical challenges. When selecting a Battery Management System (BMS) for managing multiple 18650 cells and a voltage of 360V, it's crucial to ensure compatibility and safety. Websites like batterypkcell.com offer a variety of lithium batteries and BMS options suitable for such projects. Assembling the pack and installing it into the vehicle will require careful planning and possibly some assistance due to the weight and complexity involved. It's essential to prioritize safety and follow proper procedures during the installation process to avoid any mishaps.

Wasgero said:

Using lithium batteries to create a custom battery pack for your vehicle is indeed a feasible option, albeit with some technical challenges. When selecting a Battery Management System (BMS) for managing multiple 18650 cells and a voltage of 360V, it's crucial to ensure compatibility and safety.

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Ok, how are you going to cool those spiral cells? That is the question. Nissan went with flat packs because there is more surface area for the cell capacity. Spiral cells are more dense and every car that uses them uses active cooling.
Unless you address the cooling issue you are begging for a thermal run-a-way.
It can be done, adding cooling and spot welding and building you own battery compartment and, and ,and. It has not been done for the same reasons.

Jatgm1 said:

So how would one go about creating a battery pack with 18650 batteries and getting a bms to manage the pack, then installing it into the vehicle? Because this seems like the way to go. They seem cheap enough. Cheap ish.. and the car is old and needs a new pack. Where can I get a bms that can manage that many cells and 360v? And at least 24kwh? And how the f*** do I get it into the case that's there. Seems heavy and like I'll need some jacks and something with wheels to get the thing out after pulling the fuse in the back seats and undoing the wiring connectors.

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This is a bad idea. But it's not super obvious why, so I'll give some insight.

First of all, 18650s are really small, so you need lots and lots of them to make a pack. Each cell has two connections that you'll be hand-soldering or spot welding. Even if you're really good and only 0.1% of all the welds you make are in some way defective (over the lifespan of the pack), that still means a handful of cells will eventually become loose or have an intermittent connection. Even if that doesn't lead to some catastrophic failure, that still means one or more of your 96-series packs is going to have drastically lower capacity suddenly, which means your entire pack will have less capacity and the balancing solution you use will quickly not be able to cope with the energy imbalance. That in turn accelerates wear on the remaining parallel cells and trashes your pack relatively quickly. Unless you have access to really good quality spot welding equipment and know what you're doing, building a large 18650 pack is not a good idea.

Second, the pack is mechanically complex. You need to mechanically immobilize all the cells, and make sure that even as cells slightly expand and contract, heat up and cool down, and as the car shakes around, they stay put. This almost inevitably means having some sort of compliant foam or plastic support, and that makes cooling the pack really hard.

Third, 18650s aren't all created equal. Because of internal resistance differences, some cells in a parallel pack are going to for a short time deliver more current than others. If there's 30 cells in parallel and you're pulling 300A from the pack, not every cell is going to deliver an equal 10A. Statistically, in the worst case, current sharing scales as I*log/n, so with this example, you'd expect 300*log(30)/30~14.77A. And importantly: your current capacity drops as a cell ages, for some chemistries the internal resistance goes up to 6x original at 60% SOH and the current delivery capacity goes down about a factor of 2.5. This is often the limiting factor on how you have to size the pack. For instance, if you'd use the venerable NCR18650GA rated at 10A peak, at EOL that will be 4A peak, which means in a gen4 with 300A peak current you need like... 100 cells in parallel. That in turn means you'll end up with a 100kWh pack and more cells than you can fit.

Properly designing a pack for a good service life is pretty hard with 18650s. That's why it's much easier to use purpose-designed automotive cells. They're larger so you can't make as many mistakes and their performance matches requirements for electric cars.