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.

 

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.

 

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.

 

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)

 

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.

 

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.

 

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.

 

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.

 

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...rng/docs/seminar/Topic 2 - Battery 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