How to charge a very large bank of 18650's? - Page 1
Forum section: electronics / projects

author: Powermax
author_rank: Regular Contributor
timestamp: December 09, 2017, 12:58:34 am
content: I am part of the Solar Car team at UVa which was recently rebooted, and our team is working towards building our first solar car. I am on the electrical team and we figured that the most optimal battery configuration is 32S-13P. That is, 32 modules wired in series, each consisting of 13 cells. (Given the batteries we choose to use, this is the configuration provides the maximum energy while remaining under the lithium ion weight limit, and the voltage range this arrangement gives is nearly perfect fit for the hub motor we plan to use. We have yet to figure out how the cells will be physically wired and arranged, however.)When the pack is fully wired up, I expect it to output between 96V to 134.4V. I am not aware of what the best way to charge this pack would be. Being able to charge the pack to 3.9V for storage would also be nice. A Orion BMS will remain connected and should be able to perform passive balancing and disconnecting the pack entirely if the voltage or temperature gets too high. Would a repurposed welder do? I have heard that more advanced welders provide a DC output with current limiting, and the output voltage is in the right range for charging, but I am concerned about this "crude" approach introducing many safety issues.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 09, 2017, 01:39:00 am
content: Rectify mains, send through a buck converter with the control loop limiting first on current, then on voltage. Also have a mechanism to disconnect the circuit if the BMS signals a fault.If you also have the Vmp of the solar panels above the voltage of the battery, you can use the same buck converter for MPPT.

author: Powermax
author_rank: Regular Contributor
timestamp: December 12, 2017, 03:48:03 pm
content: I figure it would not be difficult for building one, (I mean, I could probably use a ballast or capacitor to limit current and a full bridge rectifier as the world's simplest and probably most dangerous solution lol) but I think we would prefer a off-the-shelf solution, or something that can be made with relatively little knowledge in electronics.

author: djacobow
author_rank: Super Contributor
timestamp: December 12, 2017, 04:02:05 pm
content: I've nothing to add here except Wahoowa!Best,DaveSEAS 1995

author: Dr. Photon
author_rank: Contributor
timestamp: December 12, 2017, 04:23:28 pm
content: Hello, UT Austin solar car alum here. Most of the time, we would grid-charge with a homebuilt device that we called the "midnight sun". This was a bank of power supplies and series resistance (to somewhat emulate a solar array) that would feed into our MPPTs (Maximum Power Point Tracker) to emulate a solar array. As far as the car's electronics knew, it was a bright and sunny dayOtherwise, any sort of current limited power supply would do. Marlin P Jones (MPJA) has a fairly decent cheap bench supply that can do 0-120V at an amp. We would generally stack two of these guys in series. Any other scheme (rectify mains with isolation transformer and variac, light bulb ballast, stack a ton of power supplies, etc.) should also work, provided you can limit the current, and your BPS (battery Protection System) is functional and connected. If you're bypassing the BPS and/or charging individual cells/modules, you need to be very careful. Whatever you do, just make sure to not leave the battery unattended while charging. If you have your BPS

author: nctnico
author_rank: Super Contributor
timestamp: December 12, 2017, 05:59:20 pm
content: There are numerous off-the-shelve Li-ion chargers. Get one of those and be done with it. Just make sure the battery pack has a proper BMS with undervoltage, overvoltage, overcurrent, over temperature, etc protection. Overcharging a Li-ion cells leads to fire. Also the individual cells will need to be matched to make the pack work as good as it can. If it where up to me I'd look for a battery pack manufacturer who wants to sponsor the car by making a custom battery pack (which isn't produced in China).

author: mikerj
author_rank: Super Contributor
timestamp: December 12, 2017, 08:03:43 pm
content: 32S Charger. Might want to stand back when you switch it on the first time.

author: Powermax
author_rank: Regular Contributor
timestamp: December 18, 2017, 07:38:25 pm
content: Damn, that chinese charger!!We might consider it lolI especially like how you can choose to order it with a fiber optic "TOSLINK" cable for the DC charging end!

author: Powermax
author_rank: Regular Contributor
timestamp: December 18, 2017, 07:52:18 pm
content: Quote from: Dr. Photon on December 12, 2017, 04:23:28 pmI thought about doing something like this as well. Using a ballast or series capacitor with mains to limit current and passing that through aFULL BRIDGE RECTIFIER!which then feeds the cells. Not sure how I would shunt the power when the cells reach 4.20v. I suppose having a simple linear regulator stage following that to regulate the output voltage when the current isn't high enough to cause the voltage drop from the ballast and limit current. I don't know how much I trust my circuit knowledge to build such a large scale battery charger though.

author: hv222
author_rank: Regular Contributor
timestamp: December 18, 2017, 08:57:45 pm
content: ISL94212 looks good for charge balancer and part of fault protection in large batteries.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 18, 2017, 09:17:14 pm
content: I typically charge my LiIon packs using a standard bench supply. Set the voltage to your CV float level, set the current to your CC charge level, and let it go. It'll ride the current limit until it hits the float voltage, then sit there as the charge current dies down. Your pack is much higher voltage than what I typically work with, but the process is the same. It wouldn't take many 60V isolated bench supplies stacked in series to get to your float voltage. Or as has been brought up already, I assume your system already has MPPTs for charging from solar, so just use a high voltage supply to approximate your panels, feed it in to the MPPT input, and let them handle the charge sequence.

author: Powermax
author_rank: Regular Contributor
timestamp: December 18, 2017, 09:21:14 pm
content: The issue with this approach is that some doofus may come along and mess up the charging voltage, and I think dave mentioned in a video some traps about connecting power supplies in series and parallel. In CC mode, the voltage sharing across the power supplies may be a bit wonky. (I guess adding some resistors across the output of each one would be one workaround.) Also there is the isolation voltage to consider, too.