BMS Vs LVC/HVC

geoff57

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Hi
I want to make up a battery pack that has some form of battery management on it but am not sure Which route to take, an off the shelf BMS or build a LVC/HVC setup with no balance capability’s.

The circuit I could make would have a plug in display for visual reference of the cell voltage and some way of balancing the cells with an Rc charger if needed.

One of the reasons I ask this is I just had a battery pack with BMS (from China) that worked ok cut out sooner than I would have expected as one cell had gone bad and it did not look like there was much balancing going on.

I would also be able to add or remove cells without too much difficulty unlike with the BMS I have at the moment that is cell number specific.

GEOFF
 
I have read about quite a few battery builders who have run a pack with no BMS. It is vital to use cells from a respected name-brand manufacturer who is known for a consistent product (Samsung, Panasonic, Sony, LG, etc). Then, only charge to 4.0V per cell, instead of 4.2V. Limiting your top charge to 4.1V will provide a major benefit towards helping the pack live as many months an years as possible, and then going to 4.0V will help with that a little more. However, the most important reason to do this is that it provides some headroom for one cell being slightly out of balance with the rest.

If 90% of your cells track well with each other between charging and discharging, there will still be a couple cells that are slightly out of step. One will drift a little low, and the other will drift a little high. The high cell is the problem.

If you charge all the cells the 4.2V without a BMS, one cell will end up around 4.25-4.30V, and...you only need one cell to start gassing-off and overheating to create a cascading downward spiral that often ends up with an unplanned fire.
 
what voltage and power draw do you want? what is your use case?
 
I would still use a BMS even if the balancing feature is weak. Most of the inexpensive ones can be used with fewer cells by connecting the unused balance wires at the positive end.
 
Hi
After a runaway battery without a BMS causing a fire that wrote of the bike and shed I now always use a BMS so far off the shelf from China with a non balanced charger. I have not had any problems since.

The cells I use are turnigy 20c 5ah, these were recommended to me by Jozzer who I trust. The cells were of different ages and that was probably my downfall in this case.

I normally put balance lead connectors in a place that I can get at and check the battery once a week after a ride. This battery was not made as I would normally as it was going to be changed soon, as very often is the case this temporarily time has gone on longer than I expected.

I agree with you fully charging to 4v would extend the battery life considerably as would not running it down to 3 volt per cell due to the size of the pack and the ammount I use it between it charges I never get close to 3v I do get to 4.2v as I use a BMS this is one thought to make my own, I can choose the LVC and HVC.

I use 13s, 14s,and 20s size battery packs built into cases designed for multiple 12v 12ah lead acid batteries. As for power don’t have a meter to tell me the power levels yet.

Fechter my alternative to a BMS would be an Arduino based cell monitoring device without any balancing ability’s it would have a programmable LVC and HVC limit and be as smart as I could make it within the limitations of a BMS that has no balance ability’s. The results would be a bit like the old bm6 cell monitor which coincidently used the same chip I still have one and looked, this device would have HVC capability’s and the alarm output would connect to relay switches.
The LVC would act on the throttle to cut power to the motor much as we have done before, the HVC would connect to the mains AC input to the charger as this can be disconnected with a small relay where between the charger and the battery a bank of mosfets would be needed.
Geoff
 
As long as you have individual cell voltage monitoring you have some degree of safety. The balance function is pretty useless on most BMS boards, especially if you charge to less than 4.2v. You could manually balance once in a while.

I'd be interested in seeing you you would implement a cell voltage monitor using an Arduino. It might be pretty easy for low cell counts, but it could get problematic for larger packs.
 
fechter said:
As long as you have individual cell voltage monitoring you have some degree of safety. The balance function is pretty useless on most BMS boards, especially if you charge to less than 4.2v. You could manually balance once in a while.

I'd be interested in seeing you you would implement a cell voltage monitor using an Arduino. It might be pretty easy for low cell counts, but it could get problematic for larger packs.

or just buy a smart bms that you can program to start balancing as soon as power is being put into the battery,
 
Hi
Fechter both the bm6 and the cellog8 used the Arduino chip so I’m going in the right direction.
The Nano has 8 ADC’s which the cellog8 used I’ve found a small screen and so be able to make something similar to the cellog8 but with a better buzzer and better switches I can’t duplicate the usb logging ability. It will not be as small to start with but if there is a market I could go from strip board to circuit board with through hole components eventually to SMD as I did once before (unfortunately I don’t have the contacts I did then) the end results will never be as small or as neat but will be as small as I can make it.
A cellog8 inspired device will be the first rung in the ladder to create an accurate and smart BMS.

As for buying a smart BMS there is one big problem with that most mass produced never have accurate voltage measurement anything up to 0.5v out in some cases the cellog8 did have one very good thing going for it you could calibrate each channel comparing it to a quality volt meter like a fluke to get better results.

The on board ADC’s will give a resolousion of 0.035v good enough for 8 cells in series.

As for accuracy if I do mores than one I will also make a program to calibrate a second device using a ready calibrated device as a control.

Geoff
 
Just to be clear, I DO recommend using a BMS, and pay extra for a good one. If you charge to 4.0V per cell, then most BMS's will not balance them. However, you can do a full 4.2V balance charge once a week when you are at home and awake, that way you can catch any issues before it becomes a fire.
 
I remember the CellLog used an amplifier to measure the top two cells and just voltage dividers for the first 6. They worked pretty well.
Beyond 8s, it becomes a lot more difficult to use a single chip.
 
fechter said:
I remember the CellLog used an amplifier to measure the top two cells and just voltage dividers for the first 6. They worked pretty well.
Beyond 8s, it becomes a lot more difficult to use a single chip.

Hi
I thought it was voltage dividers on all channels. Any idea how the amplifying method worked if the circuit is too complicated I’ll juststick to 6 cells with that the 6th cell will have a resolousion of 0.026 the other way is to use high resolousion ADC’s 16 bit in theory 15 in practice an Arduino can have 4 of them used each chip has 4 channels, by using4 of the on board ADC’s you could handle a 20 cell with plenty of acuratsy down to 1 dp on the highest cell probably 2. That will handle the biggest pack I work with.

The size of a system like this would be not too big
Arduino nano at 0.8”x2” 4 ADC’s at 0.5”x 1”
19 voltage dividers to keep accurate results I might have to use 1w resistors and 38 1w resistors take up quite a bit of room but no more than the size of the BMS I am using at the moment.

Things it will NOT DO, cut power to the power output lines, cut power to the charging lines, have an on board display, use all smd components, be able to monitor mor than 20 cell battery packs and be able to connect to a smart phone.

What it WILL HAVE, connections to cut the throttle power a tried and proven method as we have used before, two LVC settings with the higher one being a warning the second a cutoff, the HVC cutoff will be to an external box that the mains power to the charger goes through, an external box for monitoring the device,all settings can be set to what the user wants, once programmed with the external box it can be removed until next needed.

What the external monitor will be able to do,monitor the voltage of each cell in a battery pack a manual calibration using a quality multimeter I recommend a fluke, automatic calibration if you already have another one, alter all the settings. More things could be added if asked for if possible, the main unit will also be able to have software upgrades if needed, these can be done with a simple usb connection to a computer down loading the program to program Arduino’s Downloading the new code and uploading it via the USB port full instructions for this will be given if needed. The new code will be perfectly readable to anyone and if you want to make changes you will be able to it will be open source.

Do not expect this over night the hardware will take some time to design and the software will probably take longer.once I have the hardware design finalised I will make a schematic diagram of the circuit available.

If anyone has the schematics for cutting main or charging power on board the the circuit board using mosfets as most BMS’s do I can add that to the design but as of now I don’t have that so will be using the method I have.

Sorry this post is so long.
Geoff
 
Just had a look in my BMS from the size of the resistors I found the BMS has the same or less ability to balance cells that the old battery medic had Which is kind of ok if it is left alone for a few days attached to a trickle charger but on a ride there is no way it could keep up.

Best idea would be little or no balance ability's while cycling but have some way of letting the rider know one of the cells is starting to show sign’s of problems.

I suppose you would call this a smart BMS.

If I wanted to buy a smart BMS where would I go and how much would it cost.

Geoff
 
geoff57 said:
If I wanted to buy a smart BMS where would I go and how much would it cost.

Geoff

There are more around these days. Some are stupidly expensive.

Here's one:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=94522

Here's another:
https://www.ebay.com/itm/6s-24s-sma...:JLAAAOSwdbZa0kA~:sc:USPSPriority!94903!US!-1

Yet another:
https://bmsbattery.com/80-smart-BMS

I can't vouch for how good any of the above work but you can probably find someone on the forum that has tried them.

On most of these the amp rating could be increased by making a new FET switch or beefing up the existing one.
 
fechter said:
geoff57 said:
If I wanted to buy a smart BMS where would I go and how much would it cost.
Geoff
There are more around these days. Some are stupidly expensive.
Here's one: https://endless-sphere.com/forums/viewtopic.php?f=14&t=94522
Here's another: https://www.ebay.com/itm/6s-24s-sma...:JLAAAOSwdbZa0kA~:sc:USPSPriority!94903!US!-1
Yet another:https://bmsbattery.com/80-smart-BMS
I can't vouch for how good any of the above work but you can probably find someone on the forum that has tried them.
On most of these the amp rating could be increased by making a new FET switch or beefing up the existing one.
that ebay link is stupidly expensive.

https://www.aliexpress.com/item/Smart-Display-16S-24S-32S-Cells-300A-200A-150A-100A-70A-Lithium-Battery-Protection-Board-Balance/32826469363.html
 
Thanks for the examples.
From what I can see most of those can manage the voltage of the cells but are weak on balancing ,not to be expected good balancing is hard to do fechter do you have a link to goodrums BMS v4 thread.
These BMS’s that you’ve given me info on look like they are all ,microcontroller based like an Arduino and since the options seam to be multiples of 16 I would bet they use an AtoD converter I am familiar with.

Thanks for the help with the power switch problem fechter I will add that to my ideas. That display in the link from flippy looks interesting for the price I’ll pick one up and see if I can get it to work with the Arduino, it looks like it uses an I2C bus widely used by the Arduino and other micro controllers.

Geoff
 
geoff57 said:
Thanks for the examples.
From what I can see most of those can manage the voltage of the cells but are weak on balancing ,not to be expected good balancing is hard to do fechter do you have a link to goodrums BMS v4 thread.
Geoff

Link to the old BMS thread:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=26554&p=1384863&hilit=zephyr#p1384863

But those are not available anymore and a real chore to build the board. Some of the components used are now obsolete and no longer available.

If your cells are healthy, matched, and balanced in the beginning, the wimpy balancing feature on modern BMS boards seems to be adequate and if you have easy cell voltage monitoring, you can tell if it is getting out of balance and you could manually balance when needed.
 
Thanks for the link fechter, it’s not the whole thing I want to duplicate by any means.
The zephyr used hard wired logic with chips that did an action when certain criteria were met e.g. cell goes down to 3v.

I want to take a BMS through several stages of production the last being balancing, I may never take it that far but it’s useful to see how a balancing circuit was made.

The next thing is for me to hit the electronic forums to solve some problems building a setup like this will cause.
Geoff
 
The power stage used a gate driver chip to drive the FETs so they would have a fast turn off time. Most cheap BMS circuits just use a couple of transistors to do the cutoff.

Below is a circuit designed by member Randomly. You could use the control circuit part of this to drive a bank of FETs in parallel. Only one is shown in the drawing, but you could parallel as many as needed. The base of Q8 is held high to enable the output. When the base of Q8 goes low, the FETs will turn off. The output of an Arduino or anything can be used to pull turn Q8 off.

LVC-Randomly-02.gif
 
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