# Chevy Volt battery module in parallel (45v 135Ah)



## Yabert (Feb 7, 2010)

Hi

I have parallel chevy Volt module on my Smart last year with great success (first pic). Amps are shared between module whatever the internal resistance of the cells since they are mecanically connect (8 AWG wire). 
More details here: http://www.diyelectriccar.com/forums/showthread.php/smart-fortwo-ev-high-power-version-51472p31.html

Nom I'm planning to build few 45v 135Ah or 180Ah battery with few 12S modules (45v 45Ah). I hope to be able to reuse the orange original BMS connector to plug defferent module in parallel via a custom ''PCB board''.
The small wires on this connector have a external diameter just under 0,0625'', so my bet is they are 24 AWG. Of course this kind of small wires will fused instantly if large Amps pass inside. The trick is to reduce current flow with a resistor to save the small wire.
A small search show me a fusing current of 29A for 24AWG wire and I bet they can handle 4-5A for few minutes without problem. So, let me know if I'm wrong:

0,1v of imbalance between two parallel cells under load will give a current of 2A in the link between both cells with a 0.05 Ohm resistor.
0,5v of imbalance (huge and improbable imbalance) will give a current of 10A with 0.05 Ohm resistor... right?


*If you ask why parallel the cells, the answers are:
-12S BMS only
-Without BMS, manually balance 12 cells is easy. 36 or 48 cells, harder, longer, complex.
-Without BMS a lower capacity cell will cause problem at full charge or full discharge
-Without BMS a cell with a slightly different internal resistance will end with an imbalance who can cause problem at full charge or full discharge.


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## Yabert (Feb 7, 2010)

A really nice thing about the orange BMS connectors on the 2011-2015 chevy Volt is the 13 wires on the low level are for cells #1 to #12 and the 4 wires on the upper level are for temperature probe.
This mean I will be able to use a simple and super cheap connector to link the cells to the PCB.


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## Yabert (Feb 7, 2010)

...just to clarify in case isn't clear, the high current will pass by large busbars who linked the terminals of the three modules.
The interconnections on the PCB are there to parallel at cells level.


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## z_power (Dec 17, 2011)

What do you think about including ~5A resetable polyfuses in series with resistors? Just in case...


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## Yabert (Feb 7, 2010)

Interesting idea indeed. Do you have a suggestion for me who is easily weldable on a PCB board.

That let me think about this question: How much current can take a small PCB board for few seconds?


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## Yabert (Feb 7, 2010)

Is someone can confirm my resistor calcul in the first post?
Also, is someone know what is reasonable current for a regular PCB (post #5)?
Thanks


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## piotrsko (Dec 9, 2007)

The cross section in mills (Width) of the trace is more or less equivalent to the same wire size in mills. For example: a 3 mm trace might do 100 watts. (Swomag) use a circuit board trace calculator to be sure.


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## z_power (Dec 17, 2011)

Here's some short info about PTC fuses from manufacturer: http://www.littelfuse.com/about-us/education-center/fuses-vs-ptcs.aspx

Do you plan to use SMD or trough-hole elements on your PCB? I'd rather go with SMD board with thick (38 um) copper layer to reach better thermal management in extreme unbalanced state (just in case...)


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## Yabert (Feb 7, 2010)

z_power said:


> Do you plan to use SMD or trough-hole elements on your PCB? I'd rather go with SMD board with thick (38 um) copper layer to reach better thermal management in extreme unbalanced state (just in case...)


Umm! I'm mechanical designer... so the PCB job will be rough.
I would like to create a nice PCB with SMD elements and all the shiny things you can expect to see on a nicely design PCB, but the reality is I will only build an ugly PCB by hand.
Anyhow, I hope a fonctionnal and reliable ugly PCB...


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## Yabert (Feb 7, 2010)

I did a test by connecting two parallel cell together with 14 AWG wires.
First cell at 3,87v and second at 3,52v (0,35v difference). The current flowing in the wires was only 12A.
Seem good to me!

I will build the PCB board with 0,24 ohm resistances between parallel cells and the current should stay low despite if an imbalance occur between two cells.


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## Yabert (Feb 7, 2010)

The PCB with resistor is done, but something is probably wrong.
I recorded a current flow of 3A between parallel cells with only 0,3v of difference. The small resistors start to be hot (warm on skin) after a minute so I unplug a connector.

What is wrong?


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## ken wont (Jul 6, 2016)

Yabert said:


> The PCB with resistor is done, but something is probably wrong.
> I recorded a current flow of 3A between parallel cells with only 0,3v of difference. The small resistors start to be hot (warm on skin) after a minute so I unplug a connector.
> 
> What is wrong?


3A times .3 volts = .9 watts.
Are those 1 watt resistors? they look smaller.


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## Yabert (Feb 7, 2010)

Of course they are only 1/4w... out of my little electronic knowledge, 0.3v / 0.24 ohm = 1.25A x 0.3v = 0.375w. And I expect 0.1 - 0.2v of imbalance under load for few seconds.
I don't know and don't understand why I recorded 3A


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## Yabert (Feb 7, 2010)

I build another PCB with 1 ohm - 1/2w resistors.
Everything seem fine at the moment.


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## skeyes (Dec 3, 2016)

I am also considering an EV build using Chevy Volt batteries. Your idea of using the original BMS is pretty cool. Now that you have them hooked up with the PCB board, what is the next step? Are you going to hook up some kind of monitoring interface?


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## Yabert (Feb 7, 2010)

skeyes said:


> I am also considering an EV build using Chevy Volt batteries.... what is the next step?


Nothing. They are balance, so I will monitor few time during the first months and once a year after.
Chevy Volt cell are great quality and stay balance.


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