Help DIY Arduino BMS (48v 13S) E-Bike Battery Balancer Project

Projects General Guidance
1

Hi, first of all my primare language isn't english and i'm not an electronic guy so please try to understand me.

i want to make a seteable BMS 13 series with 8 cells on parallel = 13s8p Battery

Normal operation for Litio Ion batteries are arround 3.0 to 4.2V

i used recicled bateries, i was measured the diferents mAh values cell by cell (monts of work), i will put 4 amper wire as fuses by cell (tesla cars use that way)

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The problem:

Chinese BMS are just crap and i do not know where to find a good one. i be measured some cells on my batteries at 4.6v on chinese BMS o_O wtf, they kill it!!!

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i have new recicled batteries but i don't trust on chinese BMS any more...

also i would like to set minimun and maximun voltaje cut to protect and extend the life time of the cells, example i want to charge at max 4.1V and alarm when one serie is under 3.2 or other... so i have to know how to test every voltaje of that

when i think on 13 rails to test voltaje from indepent series of cell, i can´t figurate very well how to do it

My Solution to measure de voltage: (please tell me if i can do it)
Voltage divider using resistors and 13 conections whit a ADC (i have never use an ADC)

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so, i think voltage will be not very acurate and have to be calculated by summation cel by cel

Discharge to cut the picks voltages:
i pretend to use a tip122 conected to a resistor by every series of cell

if my charger has 2A at 54.6v
2000/13=154 mA at charguing current by serie, to stop it ill turn on the TIP122 feeding a resistor...

V/I=R => 4.2/0.16 = 26 ohm resistor (is that ok?) may be 20ohm to drop voltage at 4.1v

Cuting the current (shutdown the load and crarge)

my ebike has 1000W motor,
39v Minumun
54.6V Maximun
1000w / 39v = 26 amper aprox

i need 30A relay

maybe i will need 3 or more atmega328p on my sistem... i do not know i feel is too much.

Other ideas:
use isolated transformes to chrage by independet serie by serie

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1 Like
2

Hi,

  • Can you tell us your electronics, programming, Arduino, hardware experience?
  • Are you connecting in SERIES or PARALLEL, you picture shows SERIES yet you quote parallel?
  • What will be the final output voltage and current you are expecting to draw from these batteries?

Thanks.. Tom.. :slight_smile:

3

chamilonst:
i'm not an electronic guy so please try to understand me.

What your proposing would be an advanced project for an experienced Electronic and Arduino person.

Start with something simpler for your first electronic project ?

4

For starters, a 4067 muxer won't work on a 54.6volt string.
Absolute max for that chip is 18volt.

Not sure about the B1205S. Seems unregulated. Bad for LiPo.

The other (USB) chargers also won't work, because they rely on USB common ground
Leo..

5

TomGeorge:
Hi,

  • Can you tell us your electronics, programming, Arduino, hardware experience?
  • Are you connecting in SERIES or PARALLEL, you picture shows SERIES yet you quote parallel?
  • What will be the final output voltage and current you are expecting to draw from these batteries?

Thanks.. Tom.. :slight_smile:

Hi Tom i'm Glad to read you,
i know how to make PCB, and work with arduinos, i made a automatic soil measure to water my tomatos, and other thinks like sensor reading and 3g sms notifier using arduino sleep to save batteries.

this cells are 13 series of 8 parallels LI-ION 18650 batterys = Max output 54.6V to feed 1kw brushless motor

6

Wawa:
For starters, a 4067 muxer won't work on a 54.6volt string.
Absolute max for that chip is 18volt.

Not sure about the B1205S. Seems unregulated. Bad for LiPo.

The other (USB) chargers also won't work, because they rely on USB common ground
Leo..

you have to mix it, use the inestable 5v ISOLATED to power up the usb charger with diferent grounds

that is my most easy way because is so simple, but i prefer to forgot it for know, i cant regulate maximun V of charge and B1205S has really bad eficiency (71%) and low max current (200ma to charge 12000 mAh aprox)

7

Wawa:
For starters, a 4067 muxer won't work on a 54.6volt string.
Absolute max for that chip is 18volt.

Not sure about the B1205S. Seems unregulated. Bad for LiPo.

The other (USB) chargers also won't work, because they rely on USB common ground
Leo..

Dear Wawa, that is the question, if i add volt divider on each terminal acording at maximun volt must to be a summation or matematical algoritm to calculate the independent voltage of the series of cells, volt reading has to be calibrated. for example

if i know the firt cell is on 3.8v and you put a volt divider on the second at 9v => 5V you can know the second cell volt using math

8

Problem with voltage dividers is uneven discharge.
The bottom cell has to supply the current for 13 dividers, and the top cell for only one.
And resolution goes down with the ratio of the voltage divider.
With an Arduino, the 13th cell resolution drops to about 0.1volt.
Leo..

9

Wawa:
Problem with voltage dividers is uneven discharge.
The bottom cell has to supply the current for 13 dividers, and the top cell for only one.
And resolution goes down with the ratio of the voltage divider.
With an Arduino, the 13th cell resolution drops to about 0.1volt.
Leo..

humm. what can i do to read that voltage...

10

Hi,
This is a concept I drew up a while ago for 12V batteries that are series, and need each battery to be monitored.
It uses relays and capacitors to transfer battery voltage to the UNO.

Multibatt.jpg
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The idea was that you could expand the number of batteries to be monitored and not loose any AtoD resolution.
Tom... :slight_smile:

11

Many battery stack monitor projects on this site.
Use the search box on top of this page.
Made a "flying cap" one a while ago for 24 lead-acid cells.
Pictures in post#49
Leo..

12

Do you really need a voltage divider on 18650 cells that have a maximum of 4,2 volt?
Just have each cell on a analog input, and you're done..
Or am i wrong again?
Either way i guess you wont need much more than 0,1 volt as Wawa is talking about.

I am on the other hand going to follow this, becouse i guess i need help with exactly the same thing i am starting of trying out myself.

13

Raptor50-73:
I am on the other hand going to follow this, becouse i guess i need help with exactly the same thing i am starting of trying out myself.

Make sure you have a lot of experience in electronics and a full understanding of the principles of charging lithium batteries.

Lithium battery charging can be dangerous.

Even large companies like Boeing and Samsung have got it very badly wrong.

14

Raptor50-73:
Do you really need a voltage divider on 18650 cells that have a maximum of 4,2 volt?
Just have each cell on a analog input, and you're done..
Or am i wrong again?
Either way i guess you wont need much more than 0,1 volt as Wawa is talking about.

I am on the other hand going to follow this, becouse i guess i need help with exactly the same thing i am starting of trying out myself.

Hi, it may take a while, that posting was 12 months ago.
Check out @Wawa suggestion in post #10.
Tom.. :slight_smile: