BMS Balance Wire Connections

[bobbill]

I have x cells in P and I take my trusty meter and connect it to the plus (+) and minus (-) of cell group, is that reading a series measure or balance figure. IOW does the meter itself make up a circuit?

I know it is a basic question, and answer likely in Sperry directions but directions are and I am dim old elec person who is learning.

 

[HK12K]

It's measuring the voltage of that one parallel group, which should be between 3.0-4.2 volts.

To determine balance you measure each of the groups individually and compare them to one another. They should all match very very closely, as in within .05 volts or ideally less

If measuring in series you'll see the full pack voltage.

 

[E-HP]

I have x cells in P and I take my trusty meter and connect it to the plus (+) and minus (-) of cell group, is that reading a series measure or balance figure.

Your reading is for that parallel group, and should read between 3V-4.2V for 18650 cells. Not sure what you mean by balance figure.

IOW does the meter itself make up a circuit?

Yes, it's a circuit, that draws very little power/current since most meters have an internal resistance measured in megohms.

I know it is a basic question, and answer likely in Sperry directions but directions are and I am dim old elec person who is learning.

Likely not. If you bought their meter, they likely assume you know the basics, and wouldn't go into specifics of measuring battery voltages.

 

[bobbill]

HK appreciate it...figured after a bit...meter does not make it series.

.05? That is fairly close. These cells range from 3.00 to 3.6 and are grouped together, to reduce BMS labors. I think I can adjust the voltage closer on some.

Figured if cells were grouped the meter would measure sum of cell current as (nominal) series, but not so.

Thanks, again...

 

[E-HP]

HK appreciate it...figured after a bit...meter does not make it series.

.05? That is fairly close. These cells range from 3.00 to 3.6 and are grouped together, to reduce BMS labors. I think I can adjust the voltage closer on some.

If they are already paralleled, then the voltages wouldn't differ. If they aren't and you're planning to parallel them, then you want them to be within a couple of millivolts of each other before connecting them.

Figured if cells were grouped the meter would measure sum of cell current as (nominal) series, but not so.

Thanks, again...

I was assuming you were measuring voltage. If you're trying to measure current, that's another story.

 

[HK12K]

HK appreciate it...figured after a bit...meter does not make it series.

.05? That is fairly close. These cells range from 3.00 to 3.6 and are grouped together, to reduce BMS labors. I think I can adjust the voltage closer on some.

Figured if cells were grouped the meter would measure sum of cell current as (nominal) series, but not so.

Thanks, again...

A variance of .6 volts between groups is quite out of balance. Assuming that your BMS has the ability to balance, I would try to get the bms to balance that pack as well as possible which may require leaving it on the charger for quite some time. Could be hours, could be longer than a day. Once it is well balanced let it rest off of the charger for a few hours and retest to ensure it has stayed reasonably balanced. Then ride and retest again. If it goes out of balance again you have an issue with one or more cells in the pack, which would then require further investigation.

 

[docw009]

A Sperry multimeter? Does it have a needle? I had one in the past, probably purchased in the 1970's. It was OK for telling me I had a live AC outlet when I was wiring houses. I'd get a new one if I was doing batteries, The impedance of the old analog meters is surprising low.

 

[bobbill]

HK I goofed. All the cells are .6 some may be .60 some might be .69. Got my tenths and hundredths decimals messed. Still, .6 is very small...?

But, might give it aa shot to make all very equal...

Sperry is digital, Bought it couple years back.

So. How did all y'all find this stuff out; school, by trial and error, or asking here or similar...? There is very little literature out there?

And, thanks much. Sorry for the typos.

 

[E-HP]

.6 is very small...?

So. How did all y'all find this stuff out; school, by trial and error, or asking here or similar...? There is very little literature out there?

.6 is huge; .09 at the cell level is too much.

The search button in the top right works well for questions like these.

 

[markz]

Well it all depends, is it a single string like say 10S1P
or is that string connected to other strings in parallel as well like say 10S 4P
that will determine what you are measuring
if its a single string 10S1P on its own, not connected to anything then you are measuring that single string
if its a single string connected to other strings 10S4P, in parallel, then you are measuring the entire group

When connecting a string to other strings, in parallel
it should be very very close in voltage, like 0.01V difference
otherwise you get a huge drain from one string dumping into another
which could exceed the C-rate and do damage

connecting a string in series it wont matter as the voltages just add up
the problem with that is
a string at 5V and a string at 7V is that when you use it
the 5V group will drain well below the minimum voltage and damage the 5V group

Conclusion is when connecting groups of battery cans/pouches/prismatics together
is that you must be methodical and you must think
that goes for everything from the gauge of wire you are using to connect groups in parallel
to the gauge of wire you use to connect in series
what current (amps) will those wires see
what controller are you using
whats the battery cells rated for

 

[HK12K]

.60-.70 is a very very large difference. It may not seem like much to the uninitiated, but it is huge.

Bear in mind you only have 1.2 volts of usable range between fully charged and completely dead. If one group is lower than the rest by .60-.70 volts that's nearly 3/4 of a volt gone before you've even begun. Huge difference. Assuming all of those cells are in good condition you'd have lost half of your range before you've even begun riding.

I was into Vaping and RC cars long before e-bikes. Battery safety was and is paramount, especially when sticking an unregulated potential pipe bomb in your mouth. Or between your legs. It behooves you to learn as much as you can about these batteries you're working with.

 

[bobbill]

HK, Hobbies like RC-ing and e-longboarding will make the day. Forgot.

Am beginning to note voltage more carefully. And already erred in notations...the .6 used was really .61 to .65, with emphasis on the last digit or the hundredths of a volt...

Agniusm advised to get the variances down, so the BMS labored less, so I did: all cells are 3,6 and vary in 1;100 v area from ,0 to .5 which should favor BMS balancing...

I also now understand the measuring-equipment and how each functions does make this difference, also thanks to y'all and Ag.

Am picky and the heat accident have me fearing cells much.

FWIW, I am not a welder for change out reasons and a cheapo. Agniusm's (AMtech) modules are the max and a longboarder's ideal, IMO. Love how they assemble and let one make changers easier.

 

[agniusm]

Cell voltage or more precisely SOC(state of charge) is more important when assembling Parallel groups. Greater variation puts a stress on cells as charge travels from higher cell to lover one up on connection. This is where it might heat up.
If parallel groups of cells have higher variation, this puts more work on BMS.
So you need to charge cells as closely as possible for paralleling and for series connections it is not vital but it will take longer for BMS to equalize them.

 

[Blacklite]

So. How did all y'all find this stuff out; school, by trial and error, or asking here or similar...? There is very little literature out there?

Measuring the voltage of a battery or understanding series and parallel groups I learnt in high school physics. And then learnt again in first year engineering @ university. There must be tens of millions of texts that include basic electrical theory around. Let alone the internet resources.

Electricity is electricity. Ebikes and batteries are not different from anything else. All you have to do is apply the theory to the context.

 

[bobbill]

Blacklite,I dig what you are saying but my classroom learning was confined and 18650s did not exist. I know electricity, But pack building is not so common and lots of people advise from experience. Exp and sci are not the same...

Am new at his and bought into some dangerous advice and had a melt...so am cautious.

Laws are laws-science-while it seems mixing cells and powering stuff etc is less so. For example, and there are a few, the trend here is to advise doing a series first, others suggest doing parallel, a few might even suggest going back and forth. Not science.

Been a few decades since my days in physics and then it was pith balls and magnets, even though we learned about the electics generally. Spend lots of time elsewhere, sail and fish trout.

I just got into packs after crafting a number of gas bikes, makes more sense, like masking up. To me, this cell pack stuff is very picky, opinionated and scary,

Y'all have helped me.

Thanks!

Appreciate your candor.

 

[bobbill]

Finally, learned why I love Agniusm's AGtech - NESE - packs...and learned the advantage and meaning of "prismatic."

 

[bobbill]

And, I add tihis: What if your HS Physics teacher concentrated more on something besides electrics? I taught...options exist.

 

[bobbill]

I am a NooB which may for may not be obvious. I know nada.

My questions: If one places multiple (Say 7 or more cells) in a pack and all are within .5 or half a volt difference,

Will the cells balance themselves -sans BMS?

Does or will a BMS balance cells in a pack (these same cells) on its own, with no further ado?

I know this must be basic stuff, but I know little of 18650 actions.

Thanks.

 

[ZeroEm]

by bobbill » Sep 14 2021 10:00am

I am a NooB which may for may not be obvious. I know nada.

My questions: If one places multiple (Say 7 or more cells) in a pack and all are within .5 or half a volt difference,

Will the cells balance themselves -sans BMS?

Does or will a BMS balance cells in a pack (these same cells) on its own, with no further ado?

I know this must be basic stuff, but I know little of 18650 actions.

Thanks.

I would not worry about being obvious, want to know something ask. I have a bad habit of skipping the basics until they come back and bite me.

BMS are supposed to keep the battery in balance. Most (not all) only do this when the battery is fully charged. (which I don't do often enough) Think i'm going to follow the wise men here and buy me active balance boards for my bigger batteries.

Should try to get them as close as possible or .01, .5 is not close. They will balance if put in parallel but the current will rush into the lowest one at very high amp rates. This is bad.

So balance them before connecting and allow the BMS to keep them there.

 

[john61ct]

If one places multiple (Say 7 or more cells) in a pack and all are within .5 or half a volt difference,

Will the cells balance themselves -sans BMS?

Does or will a BMS balance cells in a pack (these same cells) on its own, with no further ado?

Cells that are connected directly to each other in a group - meaning in parallel, increasing Ah capacity but keeping Voltage the same at the 1S level - will when at rest and isolated, automatically without assistance, self-level to the same voltage.

They should be at close to the same voltage, say within 0.1V before being so connected.

This group will then act as a single larger unit, nothing can be detected or measured wrt the individual cells without atomizing the group, breaking the parallel connections.

 

[john61ct]

Cells or groups (as above) connected in series increase voltage but not Ah capacity.

No current can flow between each cell / group until a balancing circuit is activated between them. One such type of circuit is that incorporated into many protective BMS, whose primary function is usually HVC & LVC backup failsafes, but may also include temperature and over-current protections.

Best if each cell / group are exactly the same voltage before assembling the pack, or within a delta of at most 0.05V or 50mV

When the pack is being cycled (charged and discharged) the entire Amps current rate is "flowing through" each S-level equally, by definition.

While within the parallel intra-group connections, only a small amount of current load needs to flow, re-balancing the energy differences caused by inevitable differences in resistance between the cells.

 

[bobbill]

As I surmised!
Thank-you both...
Reality matches logic, 'cept maybe in Quantum Mechanics and MAGA minds.

And, seems to me, if so and one or so cells do exceed .5V variance, the over limit cells will (at rest) eventually balance to other cells.

Why I did not set series connections right off.

Please do not get me wrong, There is to me little logic unless one takes time to imagine end results before doing,,,

Again, thanks

 

[ZeroEm]

One thing not to over look on balancing is the cells in a pack will balance with each other but not in the next cell/pack in series. That is where the bms should balance each pack with each other.

by bobbill » Sep 15 2021 10:56am

As I surmised!
Thank-you both...
Reality matches logic, 'cept maybe in Quantum Mechanics and MAGA minds.

And, seems to me, if so and one or so cells do exceed .5V variance, the over limit cells will (at rest) eventually balance to other cells.

Why I did not set series connections right off.

Please do not get me wrong, There is to me little logic unless one takes time to imagine end results before doing,,,

Again, thanks

 

[john61ct]

And, seems to me, if so and one or so cells do exceed .5V variance, the over limit cells will (at rest) eventually balance to other cells.

Sorry if I wasn't more clear. "Natural flow balancing" only happens over parallel connections.

And nothing to do with the delta / variance, even a 0.001V difference will cause some energy to transfer, albeit at a very slow rate.

Between serially connected units, the assistance of balancing circuitry is required, and that is where the three factors / setpoints come into play, at least the way most BMS work:

1. max transfer rate allowed

2. delta trigger Start

3. delta target Stop

 

[bobbill]

Thanks, John, makes sense to this dodger!