Lithium Battery Monitor 48 cell

Threads relating to the BMS system begun by Peter Perkins

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Re: Lithium Battery Monitor 48 cell

Postby GregsGarage » Thu Oct 11, 2012 9:28 pm

Great news, surface mount soldering fine pitch components takes a bit of practice.
Greg Fordyce

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dillond666
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Re: Lithium Battery Monitor 48 cell

Postby dillond666 » Sat Oct 13, 2012 9:54 pm

I've put the KiCad files and Gerbers etc on my webpage.

www.dillonworld.co.uk

Follow the 48 cell battery monitor link.
I'll put up instructions eventually :wink:

D

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Re: Lithium Battery Monitor 48 cell

Postby dillond666 » Sun Nov 04, 2012 10:06 pm

First test drive went well :D Car feels much lighter. I did change the front springs from 315lb to 225lb and the ride height is
the same as before. I calculated the whole car weighs 908KG with the rear 100KG heavier than the front. Pretty good I'd say 8)

I made a few edits to my firmware and will update my webpage this week.
The low voltage threshold is set to 2.8v which gives a beep under hard acceleration due to voltage sag. I have
set the controller to 450 battery amps and the weather is very cold so I expect that. I might reduce the current to
400a and might reduce the low voltage threshold to 2.6v to reduce the annoyance factor.
tomorrow I'll adjust the charger and try the charge cycle.

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Re: Lithium Battery Monitor 48 cell

Postby GregsGarage » Sun Nov 04, 2012 10:17 pm

You could also experiment with a small delay for your low voltage threshold, allow a few seconds before the warning so the brief voltage drop during acceleration doesn't trigger the alarm straight away. Glad to hear it's working great so far.
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dillond666
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Re: Lithium Battery Monitor 48 cell

Postby dillond666 » Mon Nov 19, 2012 3:35 pm

Update:

BMS has been performing flawlessly so far. I kept the low voltage threshold at 2.8v but adjusted the low voltage setting on my motor controller
to the equivalent of 2.8v per cell. This has eliminated all BMS chirps when driving. Car is running well and I'm not worrying about the cells.
BMS is also controlling the charger on/off as expected :D

D

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Re: Lithium Battery Monitor 48 cell

Postby GregsGarage » Mon Nov 19, 2012 8:38 pm

Fantastic! 8)
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Re: Lithium Battery Monitor 48 cell

Postby skooler » Mon Dec 10, 2012 1:27 pm

Hi Derek,

I've just re-read this thread, I'm very tempted now!

What was the total cost for your setup? I assume my 75cell setup will just use 2 of the 48 cell boards?

Cheers,

Mike
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dillond666
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Re: Lithium Battery Monitor 48 cell

Postby dillond666 » Thu Dec 20, 2012 12:26 pm

Hi Mike,

Sorry to be so slow to reply. The main BMS chips cost about £6 each from digikey as Farnell have removed their stock from sale. You need 8 of these. A board is circa £60 from Europe I think, bummer that I arsed up the first batch :oops: You'd be much better to order a few from ITEAD in China as it works out cheaper.
Lets say £200 would more than cover the bits? Sorry I can't be more specific, you do tend to waste much more money during the development of a project.

I hadn't given much thought to using more than 48 cells but the way to do it would be to make two totally separate BMS systems and link the charger relays in series.

If you decide to take the plunge I'm happy to help.

PS: BMS still works fine 8)

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Re: Lithium Battery Monitor 48 cell

Postby casemod » Tue Dec 25, 2012 6:45 am

Hi Derek,

I have been redesigning your board to 10*15cm, which may be usefull.
I may as well replace the optocoupler with an isolated DC/DC Converter to avoid the drain to the first 3 cells. Are you experiencing any problems with this?
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dillond666
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Re: Lithium Battery Monitor 48 cell

Postby dillond666 » Thu Dec 27, 2012 3:27 pm

Hi casemod,

Glad to see someone playing with my board :D I just noticed I didn't put the schematic in the KiCad files on my webpage so I'll get that fixed :oops:

The current drain on the first three cells is of the order of 0.6mA so I'm not overly worried.
I will be manually balancing periodically (supposedly). I am using 40aH per day from a 100aH pack so I reckon I'm within safe limits.

The dc-dc is probably a more correct solution though from a design point of view.
I tied myself in knots for ages thinking about the startup circuit, thinking there must be a way to do a capacitor isolated circuit. There must be an elegant way but in the end I just went with a way I knew would work.

I'm interested to see any refinements you make to the design, please post your progress on this thread.

Derek

EDIT: uploaded schematic into KiCad files.


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