# Where to buy BMS or PCM system (what ones to choose)



## Vitoc~ (May 19, 2016)

Hi folks
Building a 120cell Lithium battery pack for my ebike.

What kind of BMS or PCM or both do I need. 
Should I put a BMS "" on each pack (6) or one on the whole battery (72v 55A.H)
Thanks


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## Moltenmetal (Mar 20, 2014)

Hi:

Are the 120 cells in series, or a series/parallel arrangement?

What sort of cells?

You should have a BMS voltage sensor channel or board on every cell, or every group of cells in parallel (which act simply as a single cell with large capacity).

There are a number of vendors, but if you have 120 cells in series you're looking at a substantial spend.


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## Vitoc~ (May 19, 2016)

Moltenmetal said:


> Hi:
> 
> Are the 120 cells in series, or a series/parallel arrangement?
> 
> ...



Hi I'm looking at 6 packs of 12volts, all excess cells in parallel.


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## dcb (Dec 5, 2009)

"should" is a funny word, really you should know what the options and limitations and level of difficulty/cost are as it suits your situation.

There are plenty of folks who have no bms and just leave a safety cushion on the extremes of charge and discharge with some occasional probing sanity check, there are folks like me who just monitor parts of the pack (i.e. top half vs bottom half) and look for imbalance at a high level, and there are folks who monitor every cell, and there are folks who monitor and shunt every cell.

And some of these interface with the charger, or the controller, or both, and some rely on bottom balancing, or top balancing, or will (eventually) do balancing for you, or will cause an imbalance, or both.

55ah on a bike will have a large margin of safety IMHO, I wouldn't think you need much more than to make sure the top and bottom haves of the pack aren't too far apart. But 20 bms nodes isn't too cost prohibitive either.


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## Moltenmetal (Mar 20, 2014)

There are those who get away without a BMS, but no OEM does. It's risky. Doesn't mean you can't do without one, but yeah, you SHOULD use a BMS with lithium ion cells.

Unless you want to be monitoring your pack manually, a lot, buy a BMS. Cheap insurance for an expensive pack.

20 BMS channels isn't cost prohibitive. Elithion, miniBMS etc. are some reasonable options.


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## dcb (Dec 5, 2009)

Moltenmetal said:


> Unless you want to be monitoring your pack manually, a lot


It's like glancing at the fuel guage, no big deal.


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## Moltenmetal (Mar 20, 2014)

dcb: I would agree, if the only issue was running into the bottom of your pack during driving. Since I want my pack to last, I stay well away from the bottom of my pack by watching my real fuel gauge, i.e. my Ah meter. But the real issue is during charging, and not too many people want to be sitting there during an entire charge "glancing at the fuel gauge".


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## dcb (Dec 5, 2009)

terminating charge based on the highest pack half voltage (and indicating a balance problem in the process) gets you an incrementally huge amount of imbalance protection vs whole pack voltage, it is diminishing returns after that. Likewise for terminating discharge based on the lowest pack half voltage, and these are fairly trivial circuits to implement. Probably should terminate on any significant difference for best protection though.

I mean look at the odds, one (or an odd number of) cell failure is bound to be covered. 2 cells are more likely to be in close proximity and on the same pack half, the odds of an even number of identical failures evenly distributed on opposite halves is pretty small.


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## Duncan (Dec 8, 2008)

The other part of the BMS/No BMS debate is that we keep hearing about people whose BMS actually caused their battery failure

A full blown factory BMS - a great idea!

Some dubious system that is being sold to the DIY market??? 

I normally disagree with dcb but I think we are on the same page on this


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## WolfTronix (Feb 8, 2016)

If you are manually monitoring the cells, then you are BMS.


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## Sunking (Aug 10, 2009)

For an ebike, there is no real need for a BMS. Just perform an Initial Balance and use cut-off points on the charger and controller to eliminate the knee curves. Lithium cells do not go out of balance unless you use BMS or monitors. 
For such a low voltage system, your controller is going to know right away something is wrong if you loose a cell.

Example if you are running 16S aka 48 volt nominal set low voltage cut-off for 48 volts or 3.0 vpc. The real danger zone is below 44 volts and death is 40 volts. At 48 volts LVD, you are far away from problems.

On the charge side, charge to 3.4 vpc or 54.4 volts. A BMS would not even activate until you reach 57 volts. At 3.4 vpc nominal you are at roughly 90% SOC and staying far away from 57 volts where you would need a BMS. 

Finally periodically check cell voltages to make sure none are going bad. Check when either charged up, or near the bottom. What you are looking for is the weak cell. If near the Bottom, the weak cell will be the lowest voltage. If the pack is charged up the weakest cell will be the highest voltage. Look for the Queer, it will stand out if there is a problem cell. If you find one, replace it.


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## Moltenmetal (Mar 20, 2014)

Look- we can discuss how minimal the BMS can be while remaining tolerably safe, but the typical strategy used by people doing unattended charges without a BMS seems to rely generally on terminating charge based on whole pack voltage. Doing so is dangerous, in my opinion. Break the pack into halves and terminate on highest voltage or also on imbalance- definitely less dangerous, but that IS a BMS strategy, just a primitive one, similar to acting as your own BMS. That strategy works too- until you get lazy or complacent, or forget. Just ask Jack Rickard, champion of the no BMS crowd. Not worth it!

Break it down to the cell level, and the danger during charge is now reduced to the risk of failure on demand of your BMS channels or boards. That of course goes up with the number of cells in series. While it is definitely a consideration, there is no way you're going to convince me that charging terminated solely based on total pack voltage is adequately safe, or that charge terminated by a cell level BMS is less safe. The only case I see where BMS is truly risky is for months of storage without charging. Personally I'll take some bulged damaged cells over a fire due to overcharging any day, thanks. I realize LFP greatly reduces the risk of that vs other higher energy density chemistries, but by no means does it eliminate that risk. 

Want to argue about the risk- benefit of shunt charging as a BMS feature? No contest- I think it is potentially risky to pack health. I feel it's a waste of time with LiFePO4 chemistry because of how little additional useful capacity it gives you in practice, in return for holding cells at high voltage for longer than necessary- something known to hasten capacity loss.

Some folks think that the whole concept of a DIY EV is insane, so opinions may of course vary! There is no one right answer which fits all. Many in the bottom balance and no BMS camp have long lasting happy packs without incident. But a fire might not just destroy your property- it might hurt others. It's not worth the risk from my perspective.


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## Sunking (Aug 10, 2009)

Moltenmetal riddle me this.

Let's say for argument sake I have a 16S (48 volt nominal) LiFeP04 battery pack. I set my CCCV charger to 54 volts. I have a BMS and it does not trigger until a cell reaches 3.6 volts or a pack voltage of 57.6 volts. 

The BMS will never turn on.. That is pretty much how OEM EV manufactures work. They never allow the customer to fully charge a pack and only use the BMS if needed to rebalance when the controller determines cells are out of balance. 

Second point is LiFeP04 cells are fairly tolerant of over charge voltages. That is basically why they are made. That do not hit the thermal stress point to roughly 4.2 volts. So over charge is not so much a concern as say a Lithium Cobalt cell which is very sensitive to over charge voltages and thermally unstable above 3.8 volts and run into Thermal Runaway or great balls of fire.

On the flip side LifeP04 is very sensitive to over discharge like all lithium batteries. Manufactures say 2.5 vpc is the cut-off, but the real danger zone is 2.0 volts before you run into polarity reversal and damage. If you set the LVD to 3.0 vpc leaves you a heck of a lot of room. On a 16S pack would 48 volt LVD. The real danger zone is 32 volts assuming the cells are fairly well balanced and equal capacity. That is 16 volts of safety margin. You eliminate all possibility of an over discharge by using a different strategy that does not cost a penny, or some failure of an automated BMS. 

Granted on say a 35S to 100S pack, you can loose a few cells, and the pack voltage is still good. But with 16S or less that is not the case. 

Look all I am saying is for lower voltages like 16S or less, and you know what to look for, a BMS is not really required. All you need to do is make sure you perform a good initial balance from the start. The cells do not go unbalanced if you do not have cell monitors or balance boards on them

Personally I have done it both ways, and I prefer to Bottom Balance. When you BB on the first charge watch cell voltages. One cell near the end of charge will get to 3.6 volts sooner than all the rest of the cells. That will be the weakest cell. When you see it get to 3.6 volts, not pack voltage and use that as a starting point to determine what voltage to set the charger too. For the first few charges lower and raise the voltage accordingly, so that wen rested, the weakest cell reads roughly 3.45 volts. All the rest will be slightly lower. So at the Top when charged up, the cell voltage will vary a little bit. But when discharged all cell voltages are EQUAL, and if you set LVD to 48 volts, you will never over discharge. 

I agree with you OEM has to use a BMS for liability reasons and have to shun Bottom Balance as that would be a conflict of interest. But for DIY who is knowledgeable, does not require a BMS. Choose whatever makes you more comfortable.


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## Moltenmetal (Mar 20, 2014)

Sunking- I have a bad cell in my pack. Mechanical damage, poor mfg, whatever- nobody knows. It has dropped from 180 Ah to near 130 Ah and the only reason I know this is because of my BMS lvc warning alarm. Other than capacity loss, it's perfect- no apparent excessive voltage droop on high current draw from my pack, no bulging- just a smell of electrolyte in my trunk with no one cell looking like the culprit...

If I were counting on bottom balance to protect my pack, I would almost certainly have reversed that cell before my Lee Hart bridge told me something was wrong. The bridge would have acted like the helpful LED on the fuel gauge of my 1980 Lada- which basically turned on after the car had run out if gas and left me stranded. Dcb's voltmeters would have been more useful.

My charge terminates every time as a result of one cell going above lvc. Any cell losing capacity could rapidly become that cell, without warning, dropping the max safe total pack charge voltage.

Yes LFP is harder to wreck catastrophically than the other chemistries. And yes, to each their own- we agree there. But I wouldn't be able to plug my car in and walk away without those little alarm trip modules sitting there watching my pack during charge- and I was damned glad for the warning they gave me when I took an extra long spring trip and discovered that bad cell. Worth the money I paid for the 32 miniBMS boards and the header board? Absofrickinglutely!


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## frodus (Apr 12, 2008)

The MiniBMS mentions OPTIONAL shunting. Does anyone know if the load resistor on MiniBMS can be disabled so it never shunts current? That way you just use the HVC and LVC on the boards for over and undervoltage conditions, but balancing is disabled. I see mention on it in places, but nowhere does it say how to disable, or how to order without shunt balancing.

I know you can do it on the Elithion so it'll never turn on (adjustable shunt turn-on voltages), and I would assume with the Orion, but I'm not sure about the non-programmable minibms.

Seems like everyone agrees that Cell level monitoring is a non-issue, and won't do anything to hurt the cells, but it seems people come out of the woodwork to comment about BMS, when in reality they're warning against shunt BMS systems.


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## Sunking (Aug 10, 2009)

frodus said:


> The MiniBMS mentions OPTIONAL shunting. Does anyone know if the* load resistor* on MiniBMS can be disabled so it never shunts current?


Stop and think about what you just said. If in fact it uses a Load Resistor; How would you remove it from the circuit and prevent it from Shunting?

I would use a pair of Flush Cuts...


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## Sunking (Aug 10, 2009)

Moltenmetal said:


> Sunking- I have a bad cell in my pack. Mechanical damage, poor mfg, whatever- nobody knows.


Replace it, that is what I would do.

Look on a 32S pack you have a point. But 16S or less, you will know if you have a bad cell from pack voltage being 3 volts low. Bet you a dollar if you see 9-11 volts on a 4S, or 21-23 volts on a 8S will get your attention real quick. Besides I use a Powerlab 8 for diagnostics and that will run circles around your BMS


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## Moltenmetal (Mar 20, 2014)

I'm replacing the bad cell- it just takes a while to get one, and right now I can do my daily commute and still have ~ 60 Ah left so there's no problem leaving it there. But if I didn't have a BMS, I would have taken it below 2V during driving before I'd noticed, and I'd likely have been stranded at the side of the road. I always carry a pack jumper and tools and it is possible to pull the rear pack at the side of the road, but the rear cells aren't accessible without pulling the whole pack- it'd be a pain to do.

I have Lee Hart bridges on the front (22S) and rear (10S) packs, with the six LEDs lined up in a pretty string on my dash. They've been tested and definitely work. When my 2.5 V LVC alarm was going off on that bad cell in the rear pack, the Lee Hart bridge didn't tell me anything because the rest of my cells were all still around 3.2 V. The Lee Hart bridge will switch states and give you indication around what, 1.5V? Maybe 1.2V? Certainly not 0.7 V... But it's charging that worries me most. Had my capacity-losing cell been in a bottom balanced pack which hadn't been bottom balanced for a while, it could easily have been fried up real good by a total pack voltage terminated charge. 

But if that's the way you want to roll, best of luck to you. If you want to be your own BMS, be my guest- I've got better things to do.

I don't think shunt charging is worth the risk, but I won't be taking flush cutters to the shunt resistors on my miniBMS boards to get rid of it. I merely terminate when the first cell hits HVC, which happens before shunting has gone on for very long at all. I'm not at all worried about the heat from the boards cooking my cells. As to what Dimitri has up his sleeve to turn it on or off or to order without it, he'll have to chime in himself. Apparently he's now offering boards with different LVC/HVC points for chemistries other than LFP, but beyond that I have no idea.

I too have no idea why you guys are phobic of BMS when it is used merely as cell by cell HVC/LVC alarm trip modules. I can't see how it can itself be the cause of imbalance, and the parasitic loss from the celltop boards is so low it's not any kind of major concern to me. $500 worth of protection for an $8,000 pack is a no brainer in my opinion- granted, you might think twice about that for a $2,000 salvaged OEM pack, but there the chemistry is more dangerous so it's probably even more necessary. I use my DC/DC to keep my FLA 12V battery charged full time, and the two LEDs for the two L-H bridges pull probably a watt continuous total (I don't have those switched on and off). I just check my Ah meter once every week or two, charge about once a month during winter storage to keep the pack between 30 and 70% DOD and otherwise don't worry about it.


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## dtbaker (Jan 5, 2008)

Duncan said:


> The other part of the BMS/No BMS debate is that we keep hearing about people whose BMS actually caused their battery failure
> 
> A full blown factory BMS - a great idea!
> 
> ...


a quality BMS to manage the end-of-charge is *probably* advisable for a pack with lots of cells like the OP is talking about ... but I'd take a half step back and ask why the design is using 120 cells rather than fewer larger capacity cells to reduce complexity.

I have had very good luck with no BMS, but on 38 cell and 48 cell series packs that I did manual top balance on and check about every 200 charge cycles. I have found very little cell drift over time once you have a good initial balance at end of charge.

This approach is very risky at 90+%DOD though... so you have to stay away from the bottom or have a controller that prevents use when pack voltage drops too low.


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## dcb (Dec 5, 2009)

re:120 cells


Vitoc~ said:


> Hi folks
> Building a 120cell Lithium battery pack for my ebike.
> ...Should I put a BMS "" on each pack (6) or one on the whole battery (72v 55A.H)


I took this to mean that he had 120 cells but they were 6P, which means 20S and 72v/20 = 3.6v.

But yah discharge is easy to monitor, like not running out of gas. charging is usually when any damage makes itself known, at the highest energy state, and is long enough to not want to watch it. So if I had to prioritize I would sort out charging protection first too, though discharge protection might prevent damage in the first place (if you aren't paying attention while driving that is)...


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## Moltenmetal (Mar 20, 2014)

We sorted out that the OP is planning a 20S6P pack.

The notion that well matched cells don't drift may well be right most of the time, and seems to be near enough right for the moment for 31 of my 32 cells, but I've already observed significant capacity loss in one cell in a pack with only 18 calendar months of operation and only twelve of driving.

If and when a cell loses capacity the way mine did, you will be glad to have a BMS. And I know I am not the only one this ever happened to. And no, there is no reason at present to suspect that the BMS caused the failure.


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## Moltenmetal (Mar 20, 2014)

Btw I have never been below 75% DOD, don't push temperature extremes for driving or charging, and most drives end around 50-60% DOD, so over discharge isn't the culprit either. My normal charges terminate in the 110-112V range or 3.44-3.5 V/cell average, but always with one or another of the cells in my rear pack tripping HVC. The top balance isn't perfect but is pretty good.


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## SWF (Nov 23, 2007)

I used a miniBMS system on my ATV project and the LV cutoff saved me when one of my cells had a high self-discharge. The ATV is not a daily driver and I find the cells (CALB) do become unbalanced over time.

I know this is a DIY site and the majority of drivers will be aware of what to pay attention to, but there are also some people who want builds that are user friendly so that anyone can drive it and not worry about the possibility of damage.


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## Zimnismoboy34 (Sep 27, 2016)

The age old debate of BMS and non bms usage .
So I'm going to chime in here as unexperienced as I am which I might add might be enough to change the thought pattern of most of you and side with Sunking .
Never the less lets change the thought pattern slightly . Instead of a bms which is battery management systems lets go with a Battery monitoring system .
So we not throwing caution to the wind entirely . We are using some safety measures of HVC and LVC as well and temp Cut off should there be a internal short in a cell .
So why recommend this, well simply put we all know low voltage below 2.5v kills cells. We all know high voltage charge to 3.65 stretches cells and cramming in every last bit of energy is nonsense and only shortens calendar life in lifepo4 cells . I know for fact that you can over charge a cell to 3.8v terminating voltage but don't try and go to 4.2 volts as you will shorten its life and for what purposes.... top balancing non which doesn't help anything at all.
So why stress the cells to get extra energy in ... don't worry about the extra 20% using a Li ion cell between 3.5v and 4.1v is 80% and ive tested this myself seen it and believed it, knowing that I'm not going to the edges means less chance of problems.
All the BMS systems in the world with technical information and feed back and wireless and network access are brilliant, they are very good for what they do but they all do the simple things at the end of it all.
HVC protection , LVC protection and thermal protection .
Complex bms systems that use Shunts and measure coulombs in and out are great but when you can predict usage on a pack wether Li ion or Lifepo4 then why pay extra ?

So just go for a basic battery monitor bottom balance to ensure all cells reach the same basic cut off voltage 3.1 on Lifepo4 very conservative and very safe indeed making sure you have 3000 cycles on the table .
And for lack of a better expression charge to a useable soc that gives you the mileage you require 3.5vpc !!!!

So where does that put all the users out there, some divided some wondering why BMS systems are available and want to sleep at night, simply its your decision you must choose no amount of convincing will change your mind but we can educate you to make the best decision instead you can decide for yourself what makes sense and what doesn't .
To repeat whats said before shunt and bleed resistors cannot keep up with a charger banging in major amps into a pack !!!! and why because they aren't made for such imbalance .. but why charge to 3.65vpc when you only getting like 5-10% extra out the cell from 3.5v or 3.55v .

Take it as you will peace out !


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