# BMS Necessary for LFP?



## Sunking (Aug 10, 2009)

If so when?

Do not want this to be a pissing match about which method is better, we can decide that for ourselves. 

My bugggie is now about 8 months old. I do Bottom Balance by choice and can easily switch to Top Balance with just a few key clicks. I do have an Orion JR I use as a battery Monitor. As of now the Orion has no control. My Motor Controller and charger have the controls. Some would argue that is a BMS. Not in my book, but OK with it. 

Further my little buggie is on 16S 100 AH LFP. No signs as of yet the batteries need re balanced.

After looking deeply into BMS an dLFP batteries for the last 1.5 years, I have noticed something which leads me to believe a BMS may not be necessary, at least on the lower voltage systems. Two things I have noticed indicates there may not be a need in smaller systems.

1. After looking at boat loads of BMS, how auto and tooling manufactures , I see a lot do not Balance at the cell level. Instead at the 2S, 3S, and 4S level., or say 6, 9, or 12 volt level. Nissan Leaf appears to be 2S. 

2. Is if you look around for 12 volt Lithium Car/RV/Marine batteries you can find hundreds of them made as Drop-In Replacements for 12 volt Pb batteries. Many if not most have no BMS built into them, or really any means to add any kind of BMS at the Cell Level. 

That leads me to believe, at least at the 12 volt level and possible 24, and 48 volt systems a BMS just may not be necessary for safe and long cycle life.


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## piotrsko (Dec 9, 2007)

Sorry to say, I believe you have lit the bms fuse.

Back in the day, when the lipo chemistry became available to the RC hobbies, there were random failures directly tied to the abuse / charge /discharge cycling. The bloody batteries were EXTREMELY expensive, so any sort of failure was preturbing and usually destroyed the plane or car or helicopter. Unfortunately, a black art evolved regarding the care and feeding of these things. This is what you have today.

The automotive industry has gotten the prices down to the point where the equipment is easy to buy and somewhat inexpensive to replace, so therefore they don't care too much about pack life. They do care about fires and venting so the packs are structurally tuned to avoid that.

In the 15 years of futzing with lithium chemistry, I have never had a single cell in a pack go bad slowly by itself. Explosively, yes. Fires, yes. The whole pack losing capacity, yes. Over / under voltage failures, yes. Internal cooking to failure, yes.

I am a BMS not needed believer.

My $0.02 USD. YMMV. Go ahead, flame away.


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## Moltenmetal (Mar 20, 2014)

Definitely not absolutely necessary, but comparatively cheap insurance.

If I had a $1500 pack, and could get replacement cells without having to hunt around for another whole $1500 pack, then sure- I'd probably bottom balance and stop charging at whatever pack voltage takes the first cell near HVC, then just not worry about it,. But knowing me, I'd be worried anyway, and I'd spend too much of my time hovering over the packs during charges. 

But I don't- I have an $8,000 pack! So a $15 BMS board for each $250 cell to avoid over-volting it during recharge is well worth it for me. If that board could drop to $10/cell or less by eliminating the shunt charging circuitry, I'd be all for that- in my pack, shunt charging adds perhaps 1-2 Ah out of 180 at most. It's worth that small extra cost to plug in and forget it entirely.

Commercial EVs tend to be quite high voltage for obvious reasons, hence a lot of cells in series and hence a desire to not bother with BMS on every single cell. They probably have the testing to show that it's not absolutely necessary in the systems they make, either, meaning that it doesn't add sufficient reliability against premature cell failure, or safety, to warrant its extra cost and its own potential reliability impact.


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## dcb (Dec 5, 2009)

piotrsko said:


> I am a BMS not needed believer.


Yup, especially if you are using OEM cells at a discount, i.e. a leaf module is ~$50 for 3.75v @60ah (~$100/module). A $15 per cell (if you can find one for that chemistry) bms would be 30% of the battery cost.

I do think monitoring a couple points in the pack makes sense though, that is simple enough. Maybe measure the bottom cell, the bottom half, and the pack, and make sure things aren't too out of wack (and don't add imbalance in the process). It is a diminishing return after that point.


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## Sunking (Aug 10, 2009)

piotrsko said:


> Sorry to say, I believe you have lit the bms fuse.


Honestly that is not my intention. I am also excluding any other Lithium chemistry need not apply here. Example you mentioned LiPo which I am very familiar with being a RC pilot. LiPo and others like LiCo are much higher energy density and far less unstable than LFP.

But here is what I am thinking if you are using LFP cells. BMS is not required for lower voltages say like 4S. Maybe even as high as 8 or 16S. 

Think about this a 4S LFP system. To charge a 4S system up to 100%, the charger has to be set to 14.6 volts, and terminate when charge current trickles down to 3% C. But if one sets charge voltage to say 13.6 volts or say 80 to 90% SOC voltage, assuming the batteries are matched up in capacity, and initially balanced either Top or Bottom, over charging becomes almost impossible.

On the Discharge side, the more destructive side we can set are Controller/Inverters LVD to say 11 or 12 volts. The danger zone does not even start until you get to 9 volts or less or more than 25% loss in voltage. 12 volt equipment will quit working long before you can even get the voltage low enough to damage LFP cells at 2 volts per cell. So setting LVD to 3 vpc eliminates any chance of over discharge.

Now on something say like a 45S many of you run at 144 volts. a missing 3 volts from a bad cell is not noticeable on a Protein Monitor, but 3 volts missing on a 12, 24, or even 48 volt system is a screaming red flag. Even on a Pb 12 volt system, when you see your battery voltage at 12 volts, you are already too low and instinctively know to stop.

Back to automotive applications. Many EV commercial applications only monitor and control at the 2S, 3S, 4S level, and many 12 volt SLI and Deep Cycle LFP batteries have no BMS in them. 

That leads me to believe a BMS is not needed at least in anything 4S or smaller, and for me I am at 16S and do not use a BMS in the traditional sense, but I do monitor cell voltages.


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## Duncan (Dec 8, 2008)

Hi
I am in two minds
(1) a Lee Hart Batt bridge or similar will immediately tell you if a cell is down for pennies
(2) I have been using Headway 16Ah cells and I have had a number of quiet - just died failures - that were neither overcharged or over discharged
(The Headway 16Ah cells were discontinued a couple of years ago - I believe they have fixed the problem with the present cells)


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## Sunking (Aug 10, 2009)

Duncan said:


> Hi
> I am in two minds
> (1) a Lee Hart Batt bridge or similar will immediately tell you if a cell is down for pennies


Interesting circuit, first time I have seen it. Really like the simplicity of it. A modern day "Idiot Light" like the ole days when you had a HOT light on the dash of a car.


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## sholland (Jan 16, 2012)

I think think too many people just equate BMS = balancing, when really the primary function of a BMS is reliable monitoring for gauging and redundant protection. 

I think all important batteries (e.g. EV traction pack) should have monitoring and gauging. Most people really do want to know with some accuracy how much range they have left anyway, and I am sure they want that done in a safe and reliable way. But, in my opinion, ALL batteries comprised of multiple cells should have at least a protector circuit. Heck, even power tool batteries have them.


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## bigmotherwhale (Apr 15, 2011)

sholland said:


> I think think too many people just equate BMS = balancing, when really the primary function of a BMS is reliable monitoring for gauging and redundant protection.
> 
> I think all important batteries (e.g. EV traction pack) should have monitoring and gauging. Most people really do want to know with some accuracy how much range they have left anyway, and I am sure they want that done in a safe and reliable way. But, in my opinion, ALL batteries comprised of multiple cells should have at least a protector circuit. Heck, even power tool batteries have them.



+1 on this, as long as you can cut off charge and discharge when any cell in the pack hits the threshold then it will work within the limits of the weakest cell without a problem correct me if i am wrong.


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## dcb (Dec 5, 2009)

Sunking said:


> Interesting circuit, first time I have seen it. Really like the simplicity of it. A modern day "Idiot Light" like the ole days when you had a HOT light on the dash of a car.


It really is, I might embellish on the half pack monitor for charge/discharge control sanity check, but otherwise it is stupid simple. The draw is a tiny bit high at 20ma, but meh.


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## Moltenmetal (Mar 20, 2014)

I have a cell level BMS for low cell detection, and even I installed a LeeHart bridge for each of my packs. Super-cheap insurance, even though the driver is part of the protection circuit.


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## Sunking (Aug 10, 2009)

dcb said:


> It really is, I might embellish on the half pack monitor for charge/discharge control sanity check, but otherwise it is stupid simple. The draw is a tiny bit high at 20ma, but meh.


Reminds me of a vary old Ground Fault Detector we use on un-grounded circuit, and is nothing more than 2 or 3 light bulbs. The bulbs are connected line-to-ground Under normal operating conditions all the bulbs are dimly lit. If any one line goes to ground, the faulted circuit bulb goes dark and the others go to full intensity alerting operators there is a ground fault .


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## Ampster (Oct 6, 2012)

I have alway been a fan of using a BMS if for no other reason I liked the data and the screen displays. My VW has one for 36 cells and it has helped me identify which ones are weak.
I recently implemented a Stationary Storage system with 16 cells. It is 3p16s 24kWh capacity and I barely use 25% on daily cycles. The inverter emfs messed with the amp measurements so I shut off the Amp readings. After reading this thread I tend to agree that a BMS is not necessary for a system like this stationary storage. Last week I installed a Lee Hart bridge and will probably fire up the BMS ocassionally to check cell voltage. My LVC is in the inverter and my HVC is a JLD5740 which shuts off the charger.


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## Tony Bogs (Apr 12, 2014)

Sunking wrote:


> 2. Is if you look around for 12 volt Lithium Car/RV/Marine batteries you can find hundreds of them made as Drop-In Replacements for 12 volt Pb batteries. Many if not most have no BMS built into them, or really any means to add any kind of BMS at the Cell Level.
> 
> That leads me to believe, at least at the 12 volt level and possible 24, and 48 volt systems a BMS just may not be necessary for safe and long cycle life.


Those cells are in enclosures. No way of being discharged or charged at different rates. 
And the cells are probably selected for matching capacity. The same probably applies to 2S,4S subassemblies as you suggested.
Finally, as PbSO4 dropins, the maximum CV charge voltage is much lower than 4 x the maximum cell voltage at full charge. 
SOC can never reach more than say 90 to 95%. 
That's very safe AFAIK LFP.

Here's a paper on CV charging at high CV voltages and imbalance in capacity and SOC:

http://focus.ti.com/download/trng/d... Cell Balancing - What to Balance and How.pdf

In par. IIIA it is stated that capacity imbalance (production spread) and CV charging at high voltages can lead to rapid cell degradation. 
Imbalance in SOC speeds it up. 

Charging at low voltage or other strategies are not discussed (probably because there's no real need for balancing when those strategies are used).

Tony Bogs NL BSEE


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