Creating a 72V, 20s 5p Battery
I wanted to run some things by the community here. Our senior design is making an EV and we would like to do a 72V system. Using the LG 18650 cells could we create a 72V system. On here there are only kits that go up to 52V but If we use a different BMS but the cells and connectors from Vruzend, would a 72V system be possible?
We are looking at using a 20s and 5p arrangement to give us 72.7V and 17.5Ah using a total of 100 cells. The BMS that we plan on using is located here: http://www.batterysupports.com/72v-84v-20s-150a-20x-36v-lithium-ion-liion-lipo-battery-bms-p-296.html . The charger that we are looking at is located here: http://www.evassemble.com/index.php?main_page=product_info&cPath=3&products_id=7 . These are examples of components that we have found in our initial research.
We are learning throughout this process and would really appreciate all input and suggestions. Thanks.
Yes, a 20s configuration would definitely work. Just make sure that whatever battery you are building can handle the max current (A) needs of your motor/controller.
That BMS looks like it should do the job for you, but I don't see a 20s li-ion charger on that page. I only see a 20s LiFePO4, which is not what you want. You need an 84V charger for 20s li-ion. You might be able to get the 21s charger and tweak the output voltage lower with a potentiometer inside the charger - check with the vendor on that.
Thanks for the feedback Micah. Sorry for my late response, We have now officially started the projected so I recently really dove into some of the details and now have some more questions.
1. Is there a limit to how many cells we can have in parallel? Using your LG 3.5 Ah cells for example, a 20s 5p battery would be 17.5 Ah, could a 20s 6p or 20s 7 or 8p configuration be possible to increase capacity to 21Ah, 24.5Ah, or 28 Ah battery respectively?
2. The max discharge limit for the for the LG cells is 10A according to the information on this website. I also read that your connectors recommend a max discharge of 7A max and only for 10 to 15 seconds. From my understanding, that would limit the max discharge to 7 A rather than 10 correct?
3. If I wanted to increase the Amperage out to say 49 A let's say, using 7A max for each connector (ie each cell) I would need a 7p system correct?
4. For the connector kit, it comes with "6 main wire clamps (for charge/discharge wires)" for a 52v system. What are these used for, how are they attached how would be needed for a 20S 7p system? If there is a link to an alternative forum post where this question is answered that would be great.
Really appreciate your help and your company is great! Our project wouldn't be possible without it!
1. There's no theoretical limit, its just that the more cells in parallel, the incrementally more difficult it can be to push the caps on all of them at the same time. So we recommend starting with 5 or 6 in parallel, then if you find it comfortable, add more as you see fit.
2. The first version of our kits are actually rated for 3.5A continuous or 5A peak. We should be ready with a second version of our kit in another month though that will be rated for much higher power, definitely over 10 A continuous.
3. Yep, your math checks out. But I'd stick to lower current if possible.
4. These are placed on the first and last terminal of the pack, and its where the wire connects to the battery pack. I'd recommend checking out the video on the Tech Center page to see how they are installed. http://vruzend.com/tech-center/
I've been digging deeper into our requirements and looking at what our power requirements will actually be. Right now we are looking at needing a battery pack capable of supplying 40-50A continuous and 60 amps peak. This leads me to a few other questions:
1. Do you have an update or official specs on the upgraded kit for the battery terminals? Will they be rated to over 10 amps continuous? Will the kit be a similar price?
2. Given our high current requirements, we would be looking at 7 or 8 cells in parallel assuming each cell could put out 7A continuous and 10 amps peak for 5 seconds or so. I've been reading your book and am wondering if the bus bars that come in these kits are able to handle these high levels of current or should we be looking into stacking several bus bars onto each other to increase their current carrying capacity? What are different ways that we can account for the high current flowing out of these 7 or 8p groups? I know you suggested that we try to stick to lower current levels but our performance characteristics are pretty demanding for the project.