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Topic: my first big project - BMS - is it possible? (Read 10 times) previous topic - next topic

turok

ok! thanks for all the replies, greatly appreciated!

I don't fully understand everything, but it seems like you all agree that a method of multiplexing is the best solution.
seems like a good idea to me.
Let's see if I understand this:

Instead of reading all cells at the same time, I would connect every cell briefly to the Arduino by switching 102 relays with a separate IC?

something like this?



@ Greynomad, I don't mind buying a lot of relays (they're cheap enough), but I don't have a LOT of space in the Vectrix. Worries for later though..
@ CrossRoads, indeed :-)
@ Magician, I know there are ready-available BMS systems, but this one, I'd like to learn and understand instead..

Graynomad

If I've got this correctly you will have to switch both the analogue input and the GND. Note how your drawing has GND going to one battery, what happens for the others? So you need double-pole relays.

Also the Arduino has to be powered from a power supply that is isolated from these batteries.

Another thing to think of is when do you read these voltages. If you are going to compare readings against each other or indeed any other data you have to have a known and constant load. Meaning that I think you can't really take a reading while driving as the load will vary a lot I would think.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

turok

@Rob

Good point

If the cells are being read out in a short amount of time (a few milliseconds) there shouldn't be a problem I guess.
If the BMS gives me a warning while I'm accelerating or braking (regenerating) I could guess there might be another reason than actual imbalance.
I guess It would be quite reliable when cruising at a more or less fixed speed (even at top speed = limited to 110kph).
I can also choose/adapt the tolerance for the difference between voltages.
I could also program the arduino to give a secondary alarm if the voltage of one cell drops below a safe point.

An external power supply shouldn't be a problem (I also have a 12V circuit in the Vectrix).
I'd make the circuit so I can put it on when I want (not all the time)

CrossRoads

This is the system I was discussing.
The MAX6921 might be used, or the other demux chips discussed above.
Note how each battery is connected to nothing until a relay is closed.
A change is needed in the the Gnd for the battery WOULD NOT the GND for the Arduino, demux, & relay coils.
The two would be isolated so that the measuring electronics only connect to the battery under measurement +/- terminals.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

a.d

I'm reluctant to be negative towards the relay idea because I don't have a better one to replace it, but how much room is 102 relays going to take up? I mean if it was a car, then yeah just stick it in the boot, but it's a scooter, and there isn't much storage room on a bike...

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