Low cost BMS

[harlequin2]

A big thank you to Mark for adding the files to the wiki. It all looks pretty good!
There is one change I made recently that is not shown on those files and that is the value of R7 on the controller is reduced from 2k7 to 2k2 to give slightly better performance with the received comms. It'll still work with 2k7 but the noise immunity is improved a bit (!) with the lower value.

[sawickm]

Originally Posted by harlequin2

R7 on the controller is reduced from 2k7 to 2k2 to give slightly better performance with the received comms. It'll still work with 2k7 but the noise immunity is improved a bit (!) with the lower value.

I corrected the CellTop Master BOM (2k2) R7.

-Mark

[arber333]

Originally Posted by harlequin2

For anyone interested in building this, the relevant files should appear on the wiki soon. Its fairly straightforward, but does use all smd parts, so if you haven't worked with these before, bite the bullet and get into it! I reckon smd's are easier to assemble than thru-hole parts, once you get the hang of it.
I do have pcbs available and also can supply them with the micros mounted and programmed if you wish. Send me a PM if interested, although all the info needed to make your own is included.
A quick overview:
Each cell has its own module which bolts directly to its terminals. These modules use a PIC micro to measure the cell voltage and temperature, and the have a 6R8 shunt which can be turned on or off for balancing. The controller talks to all of the modules via a "daisy-chain" comms link and collects voltage and temperature data. It displays the max and min values on a LCD and also the total battery voltage. It also controls cell balancing via the shunts and by controlling the charger.
Have at it!

Hi

I am very interested in building your BMS, but i would like to add SOC meter for measuring current in/out of battery. That means putting shunt or hall probe on battery main cable as well as charger port input cable. That way i would know how much current passed in/out of battery at all times. Could that be done on your main board and LCD as they are? Ie, only adding some componnents and two ammeter plugs...
The problem is i have an idea, but i am mechanical engineer, not electrical. I have basic knowledge on PCB assembly, but not circuit design. Could you help me? And the rest of EV world .

TNX

Arber

[harlequin2]

Yes, one of the reasons that I didn't add current monitoring is that Freyguy's LCD meter thing does all that and I had already built one of those.
It would be easy to add current monitoring, but working out SOC is not quite so easy - but if I stick with Li cells and consequently ignore Peukert, it would work OK.
I'll have a look into what is involved.

[arber333]

yes that would be great!

Anyway i taught about measuring LIFePo cells and current from/to batteries (one sensor) and additional sensor for logging current from charger. The equation would have to deal absolute values. Adding negative attribute when current goes from battery and + when there is flow to the battery (regen and charge). Some RTC timing chip would set pace. The process would have to be continuous. Always measuring current and displaying %SOC left. So a reference has to be set. I would propose to use bottom balancing as a stage point. That way you would have 0% SOC and still some juice left for emergency crawling home.

You could just program some exponent variable koeff to account for peukert. Some assumption will be necessary though, even for LiFePo (they have some Peukert too though very small).

Arber