Battery balancer and BMS for Li battery

Not being an expert I can only give you a modest opinion. It looks like those balancers do only half of what a BMS does. A good BMS will balance but also interact with the charger and monitor and prevent over voltage, low voltage, temperature of cells , and interact with the charger (in varying amounts depending on BMS) Some BMS are set up so you can monitor every cell or combo cells (in parallel at lowest level)
You need one of those balancers for every cell. BMS units usually come in multi cell capability so looking at price vs. BMS - it will be close to the same.

It also depends on what you are converting- If it is a car with a big cell count and expensive batteries I would go full BMS. If it is a bike conversion then taking the chance is not as critical cost wise.

There are also some that say that BMS are not needed and I don't doubt that for them it is true but for me, being lazy and less attentive to details I tend to cover my lack of knowledge and skill with spending money.

Hello PAXtss,
Thank you for your opinion.

PAXtss said:

Not being an expert I can only give you a modest opinion. It looks like those balancers do only half of what a BMS does. A good BMS will balance...

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Yes, this is exactly why I want to have everyone's opinion and experience here.
While I was searching a BMS, I ran into one article. It says same thing that "Good" BMS does balance.

But how I can know which BMS is good, which BMS doesn't balance?
Any idea what I should to check in the specification?

PAXtss said:

It also depends on what you are converting- If it is a car with a big cell count and expensive batteries I would go full BMS.

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I'm converting a car.
My first candidate is Orion BMS2. (Everybody thinks this is one of "good" ones?)
Second is these balancers and small BMS.

PAXtss said:

There are also some that say that BMS are not needed and I don't doubt that for them it is true but for me, being lazy and less attentive to details I tend to cover my lack of knowledge and skill with spending money.

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I totally agree with you.
If I'm not knowledgeable, I should pay it for somebody.
(Because I can't design BMS, I should "buy" a BMS.)

But I also don't want to buy a BMS won't do its job which I expect. Using these balancer might help me to understand what the system is doing a little better, I thought. Not for saving a cost of BMS. If I want to save money and don't know nothing, I should buy an used LEAF, correct?

However, learning is fun. That's why I'm converting a car.

Hi Gusty

The great BMS debate!

If you are using Automotive Cells - I'm using Chevy Volt - then I'm not convinced that a BMS is nessesary

If you do decide to use a BMS then be aware that BMS's have CAUSED a lot of the battery problems and battery fires

With non OEM cells there does seem to be a high failure rate
BUT a BMS will not fix that and save a dying cell
All that it will do is stop you from killing it's mates

Personaly I use a "Batt Bridge" - http://www.evdl.org/pages/battbridge.html - to give me warning when a cell has failed

Some basic functions i have found on simple BMS boards for small packs include:

cell voltage monitoring with over-charge and under-discharge cutoff
cell temperature monitoring with cutoff
cell current monitoring with cutoff
cell balancing circuit

The cutoff opens a large transistor(s) and interrupts the pack circuit. The balancing closes a small transistor to bleed off cell voltage thru small resistor.

These simple boards operate without digital commands or control--just plain analog chips on a board.

More complex boards have microcontrollers with serial or CAN communication and programs to control the functions listed above.

All of these consume some current and will eventually drain a pack that is left to sit without use.

Hello Duncan,

Duncan said:

Hi Gusty

The great BMS debate!

If you are using Automotive Cells - I'm using Chevy Volt - then I'm not convinced that a BMS is nessesary

If you do decide to use a BMS then be aware that BMS's have CAUSED a lot of the battery problems and battery fires

Click to expand...

Thank you so much for your opinion! This is exactly what I wanted!!
And thank you for baffling me in "good" way.

Now, I'm about to change my mind from buying full BMS to simpler system.

(I didn't know that BMS's have caused problems... Might be because of wrong wiring, trust too much, etc.? It's kind of complex devices.)

Coincidentally, I got Chevy Volt packs as well. But they are used. "Used" means that it's approved to run a car, but also risk of damaged.
I'll keep my eyes opened.
Don't you use even battery balancers for your car? I'm wondering balancer helps my eyes not get tired to monitor battery conditions.

Duncan said:

Personaly I use a "Batt Bridge" - http://www.evdl.org/pages/battbridge.html - to give me warning when a cell has failed

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Wow, this is another exact thing which I was dreaming! (This library is an awesome resource!)
I was thinking to use multiple voltage meter to check each module voltage. Very manual, primitive system.. But "Batt Bridge" looks a way better option.


Different question,though.
How do you stop charging without BMS when cells get close to overcharged?
I'm lazy. I want to stop charging automatically.

Thank you again, Duncan!

Hi Guys
BMS's - the problem is the load that they put on the cells
Not so much as in stealing capacity but as in what happens if you go away on holiday and your machine is left alone
The past problems have been
The BMS draws down one cell - to failure - over a month or so
When you charge your pack the BMS has died - so you end up charging the entire pack at too high a voltage - That is when people have had fires


If you can get the OEM BMS to work then you can ignore my worries but the aftermarket units at the start caused more problems than they fixed

If you decide NOT to use a BMS then you need to

Balance your cells - either Top Balance or bottom balance
This consists of charging your cells above the usual voltage (or below) and then connecting them in parallel
You need to go above the usual voltage as the voltage/charge slope is too shallow below that point

That gets them all in the same place at either the to (or bottom)

You then decide the voltage that you are going to go to - for the Volt cells I decided 4.05v
So a Volt 2 kWhr pack which is 3P12S - 36 cells arranged 3 parallel and 12 in series

That means that I would charge to 12 x 4.05 v = 48.6 Volts
I would discharge to 3.5 v = 42 v

I'm actually using 84S (6 off 2kWhr packs and 2 off 1kWhr packs) - so I charge to 340 V and discharge to 294 V

This does NOT get me the whole available capacity - but it does give me some margin at each end of the voltage

I was going to make myself a charger but in the end I bought a BRUSA - A guy on the site was selling them - I think $400US - which can be set to switch off when it gets to the right voltage

The other thing that I do is do a check to see if all 84 of my cells are at the same voltage - with a Volt pack you can modify a cheap "Cellog 8- to connect to the Volt BMS wires
Well worth reading this thread
http://www.diyelectriccar.com/forums/showthread.php/2012-chevy-volt-battery-93101.html

I checked my cells were all the same - then charged them - then used them for six months - then checked again
Another six months - checked again

Always within 13mV of each other

And I am BAAD to my battery - my controller is set to 1200 amps and when I'm on the track I use all of that

From what I have seen if a cell is going to die (I killed some Headway cells - and some just died) then the BMS won't rescue it all it will do is tell you that it's dead
And you DO need to know that it is dead before you kill your whole battery!

IMHO the Batt Bridge will tell me when a cell has died - and then I will have to react - but a BMS would do exactly the same

When I was using my Headways I used my Cycle Analyst to measure how much I had taken out of the battery in Amp Hours - that was my fuel gauge

The Chevy Volt cells have a higher voltage/charge curve than the aftermarket cells so I simply take my foot off the throttle and look at the voltage to see how much I have got left in "the tank"