Diy ltc 6802-2 BMS?

andrew.box

100 W
Joined
Apr 15, 2011
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kansas city, USA
Hey folks, does anyone here have experience putting together something like this and setting custom voltages for cell cutoffs and balancing initiation and using higher current off chip balancing?

https://sourcelion.wordpress.com/2014/07/20/battery-management-system-ltc6802-arduino/

Eg any pcbs built up to allow using higher current balancing? Or some working code beyond what's shown here?

I'm thinking of getting a Volt EV 12 cell module to play with setting up a sort of inexpensive "power wall" type of battery backup and it seems charging above 4.1 drastically shortens the life of those cells.

One possibility is just to check balance periodically manually and not have any control circuitry for that. Then just get a CC/CV charger set to 4.05 per cell, maybe set up a LVC alarm and cut of using an arduino and a relay, and call it good.

But it would be neat to have something more like a full balancing BMS if I can get something setup that will allow changing/programming over and under charge voltages so I can stay in the middle range of the battery SOC.

Is it correct that the behavior of this chip will be to initiate balancing through the S pins for each cell that hits the user defined high voltage point? Need to re-read the spec sheet I guess. It's a long one...

For some reason I don't see any microcontroller based "open BMS" sort of projects that weren't abandoned long ago. It seems like there'd be a huge use for this sort of thing by the community here and also the DIY power wall folks and it would be worth documenting something, getting some pcbs made, etc. But perhaps that's a naive assumption. Clearly BMS projects tend to spiral out of control with complexity. It's no simple task and impossible to please all potential constituents.

Any thoughts or tips are welcome. Thanks.
 
I'm also quite eager to hear about existing open-source BMS projects around here! I feel like I've seen a couple references to LTC680x projects before, but haven't had a chance to do enough digging yet.

I'm hesitant to post because I suspect there may be better/more mature projects already out there, but I've been playing with LTC6802-2 chips and tinkering with a pet project a lot like you're describing. I think it checks most of your boxes with the exception of higher current off-chip balancing (though that probably wouldn't be difficult to add):

http://four-forty.com/product/12s-battery-monitoring-board/

I hope posting a link to a "store" page here doesn't violate the forum rules (nothing is available for sale yet and it's open-source), but I've hosted some design files there that you're welcome to look at or use if you like. I'll be making the code repo public soon too, including a little library to make interfacing with the 6802-2 easier. Please forgive the missing/rough documentation...it's an ongoing work in progress :D

To try to answer your question: yes, the 6802-2 is capable of balancing through the S-pins and internal FETs (or use these to trigger external p-channel FETs), but balancing operation can only be controlled from the host processor. Ultimately the processor needs to make the decisions of when and where to balance based on temperature and voltage data it receives from the 6802-2.
 
Oh, nice. That does seem to do most or all of what I want. Do you plan to have any boards made up? If so I might be willing to pick up a couple. Or if you have the board design files, I guess could order one or two from a PCB manufacturer. Is it at all cost effective to order low numbers of boards, pre-populated (soldering these small packages sucks), in relatively low numbers of units? Is there a company that does this for hobbiests? I'd guess there must be.

I've also contacted battery space and bestech to see if they have, or could produce, something meeting my needs. Bestech has been good in the past, I used them for a recumbent trike BMS (18S, 40A) and I'm wondering if their "BMS with communication" is basically what we're talking about. No open source but if the price is right, might be worth it. I'll post their reply if they have something. Something like one of these is what I'm thinking might work:

http://bestechpower.com/communicationbms/BMS-D270.html

http://bestechpower.com/communicationbms/BMSD338.html
 
I'm currently emailing back and forth with Greg Fordyce who has working slave modules based on that ltc chip. He plans to pickup work again with it this year. Peter Perkins and he have been working on these BMS for 10 years now on and off. I'll point him to this thread in my next email to him
 
Awesome. Yeah I know of Peter from the Insight forums. He had a lithium insight back before it was even a commercial option for most cars. That guy knows his stuff.

Would be great to see a community driven "open" BMS project become active. There are few things like this around, somewhat surprisingly given the popularity of ebikes, EVs and DIY power walls these days.
 
Yep, I've got a few on hand and the ability to produce more at smallish scale. You're right, those 44-SSOP packages are tricky without a jig and solder paste stencils. Board house assembly isn't prohibitively expensive, but I wanted to see if other people were interested before investing in the first big batch . It seems to make sense at a scale of 25+ boards. I've been testing and using them on personal projects for the last 6 months (and reaching out to beta testers to evaluate in their electric vehicles :D ). There are also matching chain-able slave boards for higher S-counts.

They aren't polished and perfect yet (still more features to add and probably bugs waiting to appear), so I don't want to give a false impression. If you or anyone else is still interested in testing a few out and doesn't mind some potential tinkering, feel free to PM me.

I haven't used Bestech before, but those look quite feature complete. Do you know what the ballpark price range is from the last unit you've purchased from them? I'd love to see more info on how to interface with it.

Totally agree about the lack of well-supported open source BMBs with the growing popularity of ebike and diy home energy storage. It seems like it's keeping the bar to entry higher than it needs to be.
 
I'm not 100% sure on interfacing with the Bestech units, the last ones I got from them weren't 'smart', but the units I linked appear to support a few things including I2C and serial UART. I think the idea is they have software that will program it through a USB to Serial adapter setup into the UART pins for this type of application. The 'dumb' BMS I got in the past was for 18S, 40A and I believe it was $55 or so for 1 unit. I bought 2 to have a spare and ended up selling it together with the bike it was working with (my KMX trike build).
 
Hey foolsday, nice work!
I've been looking at doing a similar thing. I looked at your schematics and there seem to be something weird going on with the U$3 DC DC. Shouldn't VIN be on the HV side? Also, Where is L1 currently connected to other than the DC DC converter? Shouldn't it be connected to the pack's V+ or something like that?
 
Hi eranrund, thanks!

On this board, the DC/DC is actually powering the HV isolated side. I wanted to leave the method for stepping the battery voltage down to logic levels to the end user, to keep it more flexible for different needs. The built-in 6802-2 LDO is unfortunately just shy of being able to comfortably supply its side of the digital isolator. I was concerned including the DC/DC step-down on-board because I don't know the final s-count that a user will choose. I thought this would both make it tricky to find a efficient wide-range regulator, and potentially create balance problems if a chained multi-board system were used (but were drawing unevenly from different sections of the pack!). The inductor is just filtering the DC/DC output. I'd love to hear your thoughts on if there's a better way to go about this though.

andrew- Awesome, thanks for the info! Right, I get the same impression from looking at the BMS you linked to. That seems like a very reasonable price, curious what they'll quote the version with communications.
 
FYI - I'm nearly finished building a BMS based on four-forty's 12 cell BMS board that uses the 6802 chip. I'll post a build thread soon, but overall I'm happy with how it turned out. (except that my cheap ebay 48v 1500W inverter failed short this weekend, but that's not the BMS's fault).

Here's the BMS board I used, together with a particle photon and one of the fourforty current sensor boards:

http://four-forty.com/product-tag/bms/

And my github page that has a bit more info but is a work in progress:

https://github.com/AndyinMission/PhotonBatteryManagement
 
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