# Least expensive BMS available today?



## john61ct (Feb 25, 2017)

Build it off an Arduino?


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## dtbaker (Jan 5, 2008)

john61ct said:


> Build it off an Arduino?


I am looking for something off the shelf, or with complete parts list and instructions..... I'm mechanical, not an EE.


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## MattsAwesomeStuff (Aug 10, 2017)

> because I have a few cells in my pack that are 'drifting' out of balance after a few charge cycles.



Not to beat a dead horse, but, again... you do not have cells drifting out of balance. You have pack with cells that no longer have matched capacity due to damage.

Your only solution is to make the capacities match. No amount of balancing will solve a capacity issue.

I know you keep insisting you don't want to measure the capacities because you don't have time, but, that is your only solution.

You can't "balance" a weak cell to be larger again.


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## dtbaker (Jan 5, 2008)

MattsAwesomeStuff said:


> Not to beat a dead horse, but, again... you do not have cells drifting out of balance. You have pack with cells that no longer have matched capacity due to damage.
> 
> Your only solution is to make the capacities match. No amount of balancing will solve a capacity issue.
> 
> ...


I can't change the capacity either. So, at this point it doesn't do me any good to measure either the capacity or the IR as there is nothing I can do to 'fix' the damaged cells. I cannot replace them at this time, so I am exploring ways to live with them and prevent overcharge as the short capacity cells will doubtless 'drift' higher and higher at end of charge.

I can (hopefully) catch the first cell that hits 3.7 and end the charge. Then, periodically top balance and go some more before I have to replace a whole bunch of cells.

The reason I continue to refer this as balance, or drifting, is that is an accurate description of the observed behavior I am trying to live with. I top balance all the cells, and after a couple charge cycles the damaged cells start finishing significantly ahead of the the others... i.e. 'no longer top balanced' pretty much defines the drifting voltage at the end of charge.

Why do you continue to tell me I don't understand it?

Why don't you accept that I am trying to find a way to live with the slightly mismatched cells in situ for now?


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## john61ct (Feb 25, 2017)

Either increase your skillset, reduce your expectations, or spend a lot more money. 

This is not a mature area of technology for DIY efforts.


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## dtbaker (Jan 5, 2008)

john61ct said:


> Either increase your skillset, reduce your expectations, or spend a lot more money.
> 
> This is not a mature area of technology for DIY efforts.



I think I am clearly reducing my expectations.... All I want is to know if anyone has a simple inexpensive solution off the shelf that can watch voltages on 48 cells, and send a signal to a relay I can use to cut power to the charge.

If you don't HAVE a solution, why spend your time with a meaningless unhelpful reply?


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## john61ct (Feb 25, 2017)

No one will be more pleased than I, if it turns out that what you are looking for actually exists. 

If it turns out my suggestion is the best way to go within your budget, you may find it takes just a couple of enjoyable evenings to figure out how to do it.

I am only motivated by trying to be helpful, and (constantly) climbing my own learning curves.


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## dtbaker (Jan 5, 2008)

john61ct said:


> No one will be more pleased than I, if it turns out that what you are looking for actually exists.
> 
> If it turns out my suggestion is the best way to go within your budget, you may find it takes just a couple of enjoyable evenings to figure out how to do it.
> 
> I am only motivated by trying to be helpful, and (constantly) climbing my own learning curves.


I am hoping that one of the more EE oriented people in the forum can see a way to jigger one of these guys:

https://www.ebay.com/itm/4S-12V-100A...d/152854161901

to a simple relay that would shut down the charger when the first cell hits 3.70 .


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## pm_dawn (Sep 14, 2009)

Hi !

rwaudio has his Cellog8 breakout boards for sale again.
That could be an idea.

But be aware of the unbalaced load of the Cellog8.


Regards
/Per Eklund


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## MattsAwesomeStuff (Aug 10, 2017)

dtbaker said:


> I can't change the capacity either.


Sure you can. For quite cheap you could buy some LiFePO4 18650s (or even regular lithiums if they're more available, and just cut your expectations in half) and parallel them to the weakest cells. You could build a little booster box, since presumably you already have some kind of BMS onboard, you'll already have all those sense wires broken out.



> I top balance all the cells, and after a couple charge cycles the damaged cells start finishing significantly ahead of the the others.


So, what's happening there is quite likely that the degree to which they are unballancing is directly the amount which their capacity is being murdered every single time they're used. The weak cells are continuously being amputated and permanently losing capacity an an accelerating rate.

This will continue until you add some capacity. 



> Why don't you accept that I am trying to find a way to live with the slightly mismatched cells in situ for now?


It just seems like you're labeling and attempting to fix the wrong problem the wrong way. I kinda feel shitty for you since your solutions are continuing to harm your batteries.


...

I have ghetto solution you might not like...

1 - Go to Dollar Tree and purchase 48 cell phone chargers for $48.
2 - Buy 50x TP4056 lithium battery chargers ($12.50 total).
3 - Disassemble the phone chargers and gang them up, or get a bunch of powerbars and another $48 worth of micro-USB charging cables.

That will allow blind, independent, electrically isolated charging of each cell in addition to your normal charger, at a rate of about 5 watts per cell. So, slowly, but, it will creep them higher overnight.

Or you could just guess how many are bad and only install boosters for those cells.

Err... except you're using LiFe, not LiIon, so the TP4056 might need modification of a resistor to change the termination voltage.

Well, some kind of solution like that might be what you want.

All the shunt-type BMSs will overcharge your good cells because they can't handle full charging current in the rebalancing (they can't bleed off enough of the power going to the full ones).


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## dtbaker (Jan 5, 2008)

MattsAwesomeStuff said:


> Sure you can. For quite cheap you could buy some LiFePO4 18650s (or even regular lithiums if they're more available, and just cut your expectations in half) and parallel them to the weakest cells. You could build a little booster box, since presumably you already have some kind of BMS onboard, you'll already have all those sense wires broken out.


I do not have room to create independent 'boosters' for weak cells... What I am after is how to use the pack I have a little longer without adding or replacing batteries.

Hence, just a simple sense-and-terminate system capable of sensing the first cell to 3.70 +/- .05 volts and pulling the plug on the charger regardless of the state of charge of the rest of the cells.



MattsAwesomeStuff said:


> All the shunt-type BMSs will overcharge your good cells because they can't handle full charging current in the rebalancing (they can't bleed off enough of the power going to the full ones).


exactly

All I need is something like an external box that I plug my charger into that has a relay inside that does a hard stop if any of the cells signal voltage over 3.70. I am not looking for something smart enough to shunt current around full cells and 'fill' the pack completely. I'm just trying to find the cheapest way to limp along with the pack I have and not murder any more cells from over-voltage at end-of-charge.


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## dedlast (Aug 17, 2013)

dtbaker, I am finding it rather amusing that two people with barely 200 posts between them are lecturing you on what you should do and how to do it. That seems a bit like someone trying to tell Major he doesn't know motors or Tesseract that he really should stop talking about controllers until he learns a thing or two. But that's just my opinion...


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## dtbaker (Jan 5, 2008)

dedlast said:


> dtbaker, I am finding it rather amusing that two people with barely 200 posts between them are lecturing you on what you should do and how to do it. That seems a bit like someone trying to tell Major he doesn't know motors or Tesseract that he really should stop talking about controllers until he learns a thing or two. But that's just my opinion...


I certainly do not put myself in a class with either Major or Tesseract... but I find the noise to signal ratio high in the responses to my recent posts.

I have built 2 cars, driven more than 30,000 electric miles on LiFePO4 packs and I think I have a decent understanding of their behaviour and at least the external observable actions and reactions of an aging pack that has suffered the occasional inevitable under or over charge. I chose to go without a BMS originally, just as I chose to top-balance, after much consideration... I don't need to a lecture on how those choices were wrong; and AM looking for some informed, credible, and direct input though.

I am now entering the 'how do I best deal with an aging pack' that won't stay top-balanced at end of charge without increased manual checking and tweaking. top-balanced to me is defined as when all cells end up close to the theoretical average volts/cell given your charger end-of-charge setting. 'Not balanced' to me is when one or more cells goes significantly over the pack average before the pack voltage ends the charge.

In the early stages, I found the packs stayed very well balanced for hundreds of charge cycles.... no cells 'drifting' up or down much from the pack average at end of charge. This behavior changed a little after the first under-voltage 'event' that killed one cell under load. And changed more after the second 'event' about a year later. Hence my recent posts as I try to come up with a way to deal with the issue (hopefully avoiding massive expense for at least a while).

I really don't want to argue about semantics of WHY or HOW cells get damaged, lose capacity, or increase internal resistance. All I am interested in at this point is how to keep cycling this pack and driving while minimizing manual intervention and at a minimum time and expense.

My best bet at this time appears to be to find, or build, a simple cheap way to monitor all cells during charge, and stop the charge if any cell goes above 3.70 before the pack voltage triggers a 'normal' end of charge to the charger.

so.......

If anybody has any specific answers to this, let's hear it!


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## Russco (Dec 23, 2008)

pm_dawn said:


> Hi !
> 
> rwaudio has his Cellog8 breakout boards for sale again.
> That could be an idea.
> ...


I use the CellLog8M to shut off my charger when the first cell reaches 3.500 volts.

Unfortunately, the CellLogs have been discontinued.

Too bad, they work really well. 

Dimitri has moved to California and discontinued his Clean Power BMS. A simplified version would work for you. 

Seems like products for the DIY conversion crowd are getting hard to find.

Sorry, Dan, not much to choose from. It takes an electrical smarty to design something. Probably not much market for that. 

You could build up 45 little circuit boards with an LM431, a FET opto isolator, a small transistor and 3 resistors. Daisy chain the boards like the Clean Power setup and when any cell hits your 3.7 volts, the opto opens and latches a power relay to shut off the charger. Pretty simple but sort of a PIA to make up 45 units. 

That's how I would do it.


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## Russco (Dec 23, 2008)

Russco said:


> You could build up 45 little circuit boards with an LM431, a FET opto isolator, a small transistor and 3 resistors. Daisy chain the boards like the Clean Power setup and when any cell hits your 3.7 volts, the opto opens and latches a power relay to shut off the charger. Pretty simple but sort of a PIA to make up 45 units.
> 
> That's how I would do it.


Or, take a JLD404, which every conversion should have, and set the alarm for VOLTS and set the number for (3.7) (45) 166.5 volts. Trim the volts as required a little down until the first cell hits the 3.7 volts. The alarm relay contact of the JLD can be wired to latch a power relay to shut off the charger.

And that's another way to do it.


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## nebster (Aug 15, 2017)

Zeva has an 8-cell monitor for $60 AUD. Six of those and you're good to go.


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## MattsAwesomeStuff (Aug 10, 2017)

dedlast said:


> I am finding it rather amusing that two people with barely 200 posts between them


I find it funny that, there's really nothing lamer than a pissing contest based on the number of posts they've made, or valuing anyone's opinion based on how much they've spoken. Especially on a particular forum with a particular account. Or how old their account is. Or some other clique-y but meaningless measurement. I certainly don't judge anyone that way, seems it's the entirety of how you do.

I care more about how much someone listened than how much they speak.



dtbaker said:


> I don't need to a lecture on how those choices were wrong


I haven't seen anyone do that. I don't think anyone's criticizing you for your choices. 



> I do not have room to create independent 'boosters' for weak cells.


You just need some empty space... anywhere in the car. They don't have to be side-by-side. You can locate them all in another box and just drag wires back to the pack. But, if you don't want that solution, that's okay.



> My best bet at this time appears to be to find, or build, a simple cheap way to monitor all cells during charge, and stop the charge if any cell goes above 3.70 before the pack voltage triggers a 'normal' end of charge to the charger.


I don't think your problem is on the charging side, your problem is on the discharging side. Lithiums, especially LiFes, are decently tolerable to over-charging. I've got some I've regularly thrown into normal lithium chargers, (cutting off at 4.2v instead of 3.6) and it's not doing them any harm.

What murders Lithium is the reverse charging that happens when you drain them past dead. That's what you need to be extra sure you're not doing.

The problem with doing what you want is it's probably as much effort to do that, as to actually fix it.


The easiest thing would be the relay trip, you've already got that figured. There's probably already a contactor in there to do that so just hijack it an insert another switch, or, interrupt the input side.

The difficult part is that you need 48 different voltage readings, and some brains overtop that to control it. That's 49 different wires to the cells. If you're building a circuit, that's 48 different circuits to do things to, and you seemed to indicate earlier you weren't even interested in the effort it would take to do a discharge test to find out which cells were low. So I'm trying to picture what kind of solution is cheaper than the couple hundred 18650s you could buy to fix the capacity issue, and also take less time than doing a simple capacity test. There's not much on the table with those conditions.

But in any case...

Maybe something like 48x 3.7v zener diodes, in series with a NC relay? Then when any of them conduct (when any one of their voltage cross 3.7v), it activates the coil and breaks the relay? It'll pulse and flicker, maybe add the few components to make it a one-shot-stop, like emergency pushbuttons on industrial gear?

On the discharge-side, where I think your actual problem is, would you be okay with a human solution? I.E. If you could just occasionally keep an eye on something and manually know not to keep driving?

In that case, I'd drill an 8x6 grid into some plastic and insert red LEDs. Have each of them wired to their own cell with some kind of resistor that yields barely enough current to light the LED when at your minimum voltage is.

So when you're driving you'll see the LEDs brightly lit when charged, they'll all slowly dim (but barely, it's pretty digital for most of its range), until they get near the bottom of their range and then they'll dim much more rapidly and eventually go out. If you ever see one go out, you're done, stop the car.


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## Duncan (Dec 8, 2008)

Hi
How much of your capacity do you NEED?

If you have got some degree of excess capacity the first thing to do would be to stop charging and discharging a bit earlier 

Next - you have been monitoring your cells so you should have a good idea which are the (most) dodgy ones - if you could put the dodgy ones together then you could put something like the old JLD404 across the weak sisters and set it to stop the charge and give you a warning buzzer when they get too low


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## jhuebner (Apr 30, 2010)

Have been using my homebrew monitoring system for 5 years now. http://www.diyelectriccar.com/forums/showthread.php?p=361287#post361287


BMS simply cuts out the charger when first cell reaches 3.65V. No balancing whatsoever.



Not really planning to offer it right now but just posting it as an example that a car can run just fine on a heterogeneous pack.


Some of the 40Ah cells only have 30Ah left in them because they were deep discharged. So what, still 15kWh left. As I'm fine with the range there is no need to swap cells or add booster cells in parallel.


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## dtbaker (Jan 5, 2008)

Russco said:


> I use the CellLog8M to shut off my charger when the first cell reaches 3.500 volts.
> 
> Unfortunately, the CellLogs have been discontinued.
> 
> ...


THANKS

this is the kind of info I was hopin for.  I don't really know what you are talking about with opto isolators, etc, but I have a EE friend locally I'll have translate for me. I'm thinking if these little things are daisy chainable, I might not have to build 45, just a few for the suspect cells that drift up after cycling.

This is looking like 'open source' for the few interested people to design/build their own cell sense power cutoff system. You are totally out of my wheelhouse on the design, but I sure can follow directions and solder if we can work out the details.


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## dtbaker (Jan 5, 2008)

Russco said:


> Or, take a JLD404, which every conversion should have, and set the alarm for VOLTS and set the number for (3.7) (45) 166.5 volts. Trim the volts as required a little down until the first cell hits the 3.7 volts. The alarm relay contact of the JLD can be wired to latch a power relay to shut off the charger.
> 
> And that's another way to do it.


problem is that the damaged cell(s) drift up after a number of charge cycles. I can top-balance, lower the pack EOC voltage, and after a while a few cells will be hitting 3.7 while most are chugging along at 3.5 in CA charge mode.


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## dtbaker (Jan 5, 2008)

nebster said:


> Zeva has an 8-cell monitor for $60 AUD. Six of those and you're good to go.


please provide a link?

can it shut down a charger?


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## dtbaker (Jan 5, 2008)

MattsAwesomeStuff said:


> You just need some empty space... anywhere in the car.


in my eMiata, there literally is NO space I'd want to put booster packs, have you seen the pictures of how tightly things are packed under my hood, where the gas tank was, and where the spare tire was? Nor do I want to introduce MORE complexity to the pack in monitoring and balancing. This thread is not about how to add capacity to weak or damaged cells.... it is about how to sense and stop a charge when any one cell hits a HV limit.



MattsAwesomeStuff said:


> I don't think your problem is on the charging side, your problem is on the discharging side.


While I have had 2 'problems' in 4 years on the discharge side, the issue at hand is preventing any one of the several compromised cells from further damage over-voltage during charge cycles. This is a concern with every charge cycle. charging LiFePO4 over 4.0 volts WILL damage them by the way. I am quite used to keeping an eye on pack voltage and have been successful avoiding overdischarge *most* of the time.



MattsAwesomeStuff said:


> What murders Lithium is the reverse charging that happens when you drain them past dead.


no argument there, you really don't need to interject that in this thread any more.



MattsAwesomeStuff said:


> The difficult part is that you need 48 different voltage readings, and some brains overtop that to control it.


exactly. since EE design is not my skill set, that's why I asked this group for specific solutions, either off the shelf, or open source design I can build.



MattsAwesomeStuff said:


> Maybe something like 48x 3.7v zener diodes, in series with a NC relay?


my understanding at this point is that zener diodes would not be terribly accurate, and are not available in size that could handle 10amps of current (if charger is in the middle of CA charging).

What I am looking for is not general handwaving and maybes.... I am looking for something people are using with success already. either off the shelf, or willing to document so I can fabricate myself.


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## nebster (Aug 15, 2017)

dtbaker said:


> What I am looking for is not general handwaving and maybes.... I am looking for something people are using with success already. either off the shelf, or willing to document so I can fabricate myself.


I'm using what I suggested, with success, on 96 LFP cells. But you don't seem to want to copy the words into google and look up the actual SKU and read about it? 

There is a fabrication effort required on any HVD/LVD for that many cells, so you'll have to step up and burden yourself with actual reading and some (trivial) design work.


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## dtbaker (Jan 5, 2008)

Duncan said:


> Hi
> How much of your capacity do you NEED?
> 
> If you have got some degree of excess capacity the first thing to do would be to stop charging and discharging a bit earlier
> ...


this is pretty much what I am trying to do... except I'd like an automatic stop to the charge rather than an alarm in the middle of the night.


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## dtbaker (Jan 5, 2008)

nebster said:


> I'm using what I suggested, with success, on 96 LFP cells. But you don't seem to want to copy the words into google and look up the actual SKU and read about it?
> 
> There is a fabrication effort required on any HVD/LVD for that many cells, so you'll have to step up and burden yourself with actual reading and some (trivial) design work.


I am not afraid of research, or work.... just not wanting to WASTE time on the wrong solution. Hence looking for specifics on what people have actually used lately that is still available, or DIY fabricatable.

Your reply contains little actual information of any use. If you are aware of a solution, why not post a helpful link to help (us all) get there faster?

These threads are intended to HELP each other, not scold or denigrate. If you don't have anything useful to contribute... don't bother.

I did in fact search Zeva , and they do appear to be selling some components that may be of use, and I have emailed them for more information. There may be a combination of there monitors and controller that MAY be suitable to sense and turn off a charger, but I don't have a reply from them yet. 

At first glance it looks like the 'balance' capability of the BMS module can only handle less than 1 amp of charge current. My first concern is that if I have a cell hit 3.7 while the pack is cranking along at 10amps, then the monitor gets fried.

What I will do is post what I find out from them... to see if anybody is currently using their stuff, if it is capable of individual cell sense and turning off a charger, and how it's working.


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## MattsAwesomeStuff (Aug 10, 2017)

dtbaker said:


> What I am looking for is not general handwaving and maybes.... I am looking for something people are using with success already. either off the shelf, or willing to document so I can fabricate myself.


So what you would like, is someone to sit down and engineer the entire process for you, unique to your situation, before you'll consider it without snarky remarks. You are terrified of wasting your time, so you'd like everyone else to risk wasting their time on your behalf instead.

Jeez man, I'm trying to help out here. I made a few suggestions as to the direction this could go, so you, and me, and others could take that direction if that's what was interesting to you and flesh it out.

But you don't want to put in any work to read, experiment, or consider.


So, I guess I could cut to the chase and more simply answer your question: No.

No there is nothing that fixes your problem out of the box with no effort. No one has designed an electronic product as a very-short-term kind-of-stopgap emergency solution to a crippling but preventable problem. Your constraints are too narrow.


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## nebster (Aug 15, 2017)

Wow, you are really something. Come back in a few weeks and re-read your posts, and you'll see how silly you sound.

If you actually google literally the words I wrote in my first post, you will find a different Zeva product that does exactly what you asked for. It is also, I believe, the least expensive per-cell monitoring solution available that is not a cellog8 with their well-documented drawbacks. It is not a BMS and does not charge balance. 

I gave you the single best commercial solution for your requirement, in all likelihood, and you blew it off and then went back and did some (wrong) research and forgot to mention that it looks like you might have found something else that you like better, except you're not sure it will work and you don't know how to set up a simple control line or contactor to disable your charger.

I'm not sure we can help you, sir. You are all over the map.


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## boekel (Nov 10, 2010)

dtbaker said:


> At first glance it looks like the 'balance' capability of the BMS module can only handle less than 1 amp of charge current. My first concern is that if I have a cell hit 3.7 while the pack is cranking along at 10amps, then the monitor gets fried.
> 
> 
> > buy one of these per 12 cells:
> ...


http://www.zeva.com.au/Products/datasheets/EVMS3_Manual.pdf
http://www.zeva.com.au/Products/datasheets/BMS12v3_Manual.pdf

and please don't hurt yourself, be safe.


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## prensel (Feb 21, 2010)

I can really vote for the Zeva BMS modules/system.

I have used them in several configs, with or without the Lite or core master unit.
Since the BMS modules spit out all data over CAN its pretty easy to mockup something with an arduino reading the values and act apon.
So buy 4 of these 12 channel modules, a cheap CAN enabled Arduino and you have one of the best systems you could imagine..


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## Duncan (Dec 8, 2008)

dtbaker said:


> this is pretty much what I am trying to do... except I'd like an automatic stop to the charge rather than an alarm in the middle of the night.


Hi dtbaker

The JLD operates a relay - it would be trivial (cause I did it on my first charger) to use that relay to shut off the main power

Or if you are like me use that relay to switch another mains relay to cut power


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## Kyle (Apr 28, 2015)

dtbaker said:


> I did a little poking around the web last night and found that MiniBMS is no longer sold standalone... but there are a number of chinese units that look like they MIGHT do the job except they are all intended for 100amp max thru them and I can't really tell if they would work since my Zilla controller could be pulling 1000amps.
> 
> 
> Those should work for charging. You would just have to bypass the board for discharge.


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## JoeG (Jul 18, 2010)

I have used for the last 6 years or so a Cellog8 based BMS to shut down my charger when charging based on honn1002's "Civic ER" system. He documented it really well here, http://www.diyelectriccar.com/garage/cars/281. I forget the exact cost to implement it but it was only a couple of hundred dollars. My car is here, http://www.diyelectriccar.com/garage/cars/340 in the garage. Every 6 months or so I individually top balance all the cells, which seems to keep them within 10%.


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## Adrian (Jul 26, 2007)

These cell modules are similar to the earlier version of Dmitri's MiniBMS modules:


https://evparts.com.au/ev-power-bms/bms-cell-modules.html


They will bypass up to 2A of charging current (depending on model). The headboard control module works similarly to the miniBMS version. As a bonus, you can also get a warning if a cell's voltage drops too low.

A bit spendy at $15 (AU) each - $720 for 48 modules plus $50 for the headboard. It's still less expensive than most other BMS systems (or replacing a few Li cells) and is easy to attach to DIY circuitry.


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## evric (Oct 26, 2008)

Hi,

I have looked at the Zeva 8 cell BMS module and it seems like that is exactly what you need.
It is designed for monitoring only and doesn't appear to have any balancing included.

Eric
____________________________________


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## jlgh (Mar 6, 2010)

dtbaker said:


> I'm considering adding at least simple cell level BMS to prevent over-voltage charging... because I have a few cells in my pack that are 'drifting' out of balance after a few charge cycles.
> 
> I have pack that is 48 x 130ah CALB LiFePO4 large format prismatic cells.
> 
> ...



Good question. Several things


To answer your immediate question, all I can suggest is the Orion. I've installed one. Not cheap and not easy to install, but it works and seems robust.
Are you sure that the pack is unbalanced and/or that a few cells are losing charge? There are dynamic behaviours that can cause voltage excursions on charge
If some cells are actually losing charge then I'd get them out of the pack. Significant self discharge is likely to be an internal soft short which in theory could turn nasty on you. Also if it discharges way down it will be charged backwards when the others discharge, and thus bricked. All bets are then off when you then recharge. I had 2 out of 38 CALB100s that were bad fresh out of the box and an 18650 that suddenly went bad after 140 cycles on a test rig
I have used the Cell log 8 on many test packs. Used "raw" it will unbalance your pack. There is a ladder effect: 35 microamps or so out of each cell tapping, plus a much larger drain on another means over a milliamp difference in drain top to bottom. Over a year a milliamp difference is 8 amp hours. I use my own PCB between Cell Log and test pack with voltage followers to prevent this


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## arber333 (Dec 13, 2010)

Well the best and cheapest method would be to use a single tl431 per cell with some support resistors. Its been done to balance life cells in the past. It is robust and you can either combine it with a darlington and a power resistor to shunt the cell or use one cheap opto to signal the OR circuit. That way you get 3.7V charger disconnect signal. When i get home i can load some links... It is very cheap and very analog.

EDIT: Like i said here are the links and material for simple TL balancer OR voltage trigger for stopping the charger
https://visforvoltage.org/comment/27443#comment-27443
https://endless-sphere.com/forums/viewtopic.php?t=49764
https://electronicsprojectshub.com/make-bms-for-lithium-ion-batteries-charging/

I have a friend that makes those kind of cheap 2A BMS circuits. They seem like you could open a beer bottle with them.... see the pics.
If you are interested he can make them for you. PM me.

I have used a digital BMS design in the past that you could find very customisable. It is quite cheap if you dont expand on the design too much. It is perfectly capable of shunting 1/2A which is enough. I consider 2A shunting too much and fire hazard. BMS can be programmed to disconnect charger either when single cell reaches prescribed voltage or when all cell reach some voltage. It even can monitor and signal low voltage per cell. It has a serial port that is used for data dump. It was made by Neville Harlick and expanded on by me. I made a simple program that can make xls file from BMS serial port capture, so one could make graphs of cell values while charging or discharging etc... http://forums.aeva.asn.au/viewtopic.php?f=17&t=2753&p=62101&hilit=low+cost+BMS#p62101 
I still have lots of PCB boards that fit on 160Ah Winstons or on 200Ah Sinopoly cells. I keep design files on my site as well as building instructions. Try google translate from slovenian language. https://mazdamx3ev.wordpress.com/tag/bms/
Again PM me.


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## Jerry Liebler (Feb 1, 2011)

I'm a retired EE and have made several paper tiger designs to address this type of issue. One is a simple active shunt based on a low power shunt regulating voltage reference combined with an accurate voltage divider to sense cell voltage and turn on a bipolar transistor to load that cell, it might be called a "top balance maintainer". I choose 3.5 +/- 30mv volts as the shunt threshold ( I used 0.1% resistors and the 0.5% selection of the reference) and @ 3.6 volts it shunted 2 amps that's 7.2 watts of dissipation and meant a bunch of (i used 30)quarter watt surface mount resistors. Below 3.4 volts the drain was less than 10 micro amps. Including the PWB the parts cost is about $4. The best solution is one that I'm still working on is an active balancer module which all cells would need one of. It will keep all cells within +/-30Mv of each other while charging or discharging any cell with up to 6 amps. These cell modules are 2.5" x1" x.5" and each has 3 connectors, one 2 pin to the cell and 2ea @ 3pins for a " daisy chain connection". Each cell is transformer isolated from the daisy chain so pack voltage is "don't care". The 3 wires include power, ground, balance. The power can be any DC voltage between 12 and 60 volts. the " balance" wire is where the cell to cell energy transfer takes place along with averaging the cell voltages at 3 cells above ground. The bill of material cost for each active balancer module is about $10 and in operation each will consume less than 100 milliwats in operation, with a balanced pack. When they are powered down the per cell loading is under 10 microwatts. Are either of these of interest? I'm actually out of the DIY EV interest group as I'm very happy with my 2013 plug in Prius. But I'm planning on a 48 volt LIFEPO4 pack, at least 200AH probably 800AH, as solar energy storage @ my new house (under construction).
Edit: One more note on the active balancer, it will transfer charge at about 80% efficiency.


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## [email protected] (Dec 10, 2017)

I have been using the MiniBMS in the past to equip the LiFePO4 cells of my electric vehicles and obviously was very disappointed when those were discontinued. So, I built my own which is now in operation. I also beefed it up a bit by adding bluetooth connection from the cell modules to the central network module. No more connection cable is required. Like with the MiniBMS the cell module is mounted on one cell but it can monitor and balance 4 cells. Balancing is at around 1A. The network module aggregates all voltages over bluetooth, displays them on a small OLED and switches off the charger when the first cell reaches 3.65V. In case the bluetooth signal drops, charging is immediately interrupted and resumes with active signal. So, it is very safe. For the fun of it I also developed a mobile phone interface where all voltages are displayed in a graph. Additionally, it shows you the voltage range on each cell during charging and discharging. I found this very useful as you can immediately recognize bad cells. I just produced a batch of those, so if someone is interested i can send some more info etc.


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## steveclunn (Sep 5, 2011)

if you check out BATTERY Management on my web page.. I have posted an EASY CHEAP BMS.. with a schematic and a link to my Youtube.. We have been converting 25 years now..and when the Mini BMS company went down.. I had no choice but to do my own. You can see and control from your Iphone... Greenshedconversions.com


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## [email protected] (Dec 10, 2017)

Thanks for the tip.


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## boekel (Nov 10, 2010)

This is probably the least expensive bms:

http://www.diyelectriccar.com/forums/showthread.php/outlander-bms-p1009282.html#post1009282

the Outlander bms slaves are usually thrown away by people who make solar storage systems from these batteries, so they arent expensive...just make sure you also get the cabling (both sense cables and communication) to make your life easier...

And the modules have a nice plastic case, so you can mount them easy in your battery box. If you also safe the CANbus current sensor you can use that too...ow and of course the contacters...


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