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Overkill Solar BMS Wiring???

Mike Jordan

Solar Enthusiast
Joined
Jun 15, 2020
Messages
414
I can wire the balance leads for a 4s bms in my sleep. Pack negative is named cell one. The pack main negative gets the first lead. The every positive in order, gets the next 4 leads. Pack negative then has a balance lead on the pos and neg. The other cells each get only pos.

Some vids on the non-Overkill JDS bms show this configuration

This screen shot from Overkill's manual shows different. I can't imagine this is a screw up. But I can't convince myself to do this.

Pack negative gets the first, then cells 1 and 2 get a pos. Cell 3 gets nothing. Cell 4 gets a pos and neg.

Convince me

Overkill BMS.jpg
 
It doesn't matter which terminal it is placed on when a bus bar is present. The result is the same.


1615172148475.png


For the 3 circled bus bars, I could:

move the 2nd wire from 1 (+) to 2 (-)
move the 3rd wire from 2 (+) to 3 (-)
move the 4th wire from 4 (-) to 3 (+)

and it wouldn't matter. They are effectively the same terminal.
 
It doesn't matter which terminal it is placed on when a bus bar is present. The result is the same.


View attachment 40037


For the 3 circled bus bars, I could:

move the 2nd wire from 1 (+) to 2 (-)
move the 3rd wire from 2 (+) to 3 (-)
move the 4th wire from 4 (-) to 3 (+)

and it wouldn't matter. They are effectively the same terminal.

That makes sense. But then what is the reason to buck the convention of 4s wiring?

If wire main negative, then every positive in order, then I am good to go
 
If the busbar has a high resistance connection, then putting the balance lead on the other side could plausibly be affecting the wrong cell.
 
I'm confused by what you're asking Mike. The Overkill diagram is correct and the same as every other BMS I have seen. Are you just concerned about whether a lead is placed on one bolt or another of the same bus bar? What about halfway between you say? If your bus bars are adequately sized it makes no difference where you tap the connection. The bar is considered to be equipotential at all points (as far as the BMS can tell) because the bus bar is a low resistance connection.

If you don't believe that then measure the voltage across two lugs in a single bus bar (while drawing load, otherwise you will definitely measure zero voltage difference). Keep in mind that analog electronics typically have millivolts of random offset voltage so the minimum detectable voltage difference without offset calibration is somewhere in the 3-10mV range unless you have a "low offset" or "chopper" amplifier which none of these BMSs do, or you do factory calibration which is still valid in the operating conditions, which I also doubt any of these BMSs do.

Edited a few times for clarity.
 
Last edited by a moderator:
Will just released a new vid review on the Overkill Solar BMS. He wired the conventional 4s method I described in my first paragraph in the OP above.

So still just curious why @OverkillSolar would buck the convention????
 
Will just released a new vid review on the Overkill Solar BMS. He wired the conventional 4s method I described in my first paragraph in the OP above.

So still just curious why @OverkillSolar would buck the convention????
Again, it doesn't matter if the wire is connected to the positive or negative on the bus bar. It is just a matter of preference. Pretend the - & + aren't on the battery post in the drawing.
 
Or here, even better.....is a pic where I just wired them right in the middle of the dang bus bars. Same same.

20210320_092606.jpg
 
If you measure the voltage with a volt meter, you will find the voltage at cell 3 positive terminal , the bus bar and the 4 cell negative terminal all have the same voltage. So any point from cell 3 positive to cell 4 negative will work for the balance lead. I wired one of my batteries, like pmtco2 at the mid point of my bus bars.
 
If we have beaten that horse enough, I have a question for those in the know.
I have 8 new CATL 271Ah prismatic cells thanks to @Michael B Caro . If I buy two of Overkill Solars 4S 12v 120A BMS's and hook them up as shown in the pictures on their website I will have 2- 12V, 271Ah batteries. If I take these 2 batteries and hook them up in parallel the same way as my existing 2-100Ah SLA batteries in my RV are wired I'm pretty sure this isn't a problem and it will be great on longer RV trips, giving me around 500Ah of battery vs. my current 200Ah! I'm figuring this derated capacity because I'll be charging it from the existing 120V built-in RV charger and the motorhome's alternator (with a proper DC-DC charger) and max charge will be ~14V.
I'm assuming I can rely on the BMS's to quit charging each of them at that setpoint.
I do have plans to add a 12V solar system to the RV in the near future.
Assuming there aren't any issues, here's the question! I'm not out camping in my RV much so when I'm not camping I'd like to remove these 2 big-ass batteries and hook them up in series to give me a single 24V 270Ah battery that will serve as an roughly 6.5KWh powerwall that I can attach to my existing home solar system consisting of 1.5KW of solar panels hooked to my MPP LV2424 all-in-one solar/battery controller. I have a 100A circuit breaker already in place on the battery connection to the MPP which matches with it's 2400 watt rating and won't exceed the BMS's 120A rating. Is there a problem with this plan and if so, what can I do differently to work around any problem or add protection so I don't damage the batteries or the BMS's?
Thanks in advance for your time to provide a thoughtful response.
Cheers,
 
Will just released a new vid review on the Overkill Solar BMS. He wired the conventional 4s method I described in my first paragraph in the OP above.

So still just curious why @OverkillSolar would buck the convention????
how do you feel about this set up... in your mind it isn't on either cellScreenshot_20210424-183553_Gallery.jpg
 
If I buy two of Overkill Solars 4S 12v 120A BMS's and hook them up as shown in the pictures on their website I will have 2- 12V, 271Ah batteries. If I take these 2 batteries and hook them up in parallel the same way as my existing 2-100Ah SLA batteries in my RV are wired I'm pretty sure this isn't a problem and it will be great on longer RV trips, giving me around 500Ah of battery vs. my current 200Ah!
Yup, you will have 500ah's capacity and you will be able to pull a total of 240 amps from the BMS's. This is a good solution as opposed to using one BMS. Also if one pack fails for some reason you will still be able to power some things from the good pack.

I'm assuming I can rely on the BMS's to quit charging each of them at that setpoint.
The BMS should be used as the last line of defense to protect the cells in the event a device in the system fails. The HVD of the BMS should be 3.65 volts and the LVD should be 2.50 volts and these are the default settings for the Overkill. If your DC-DC charger charges the cells to 14 volts, then this should work out well providing you parallel top balance.

Unless you are paying for Grade A capacity matched cells there will be runners and sleepers. But if you keep your voltages between the knees this will not be a problem. Parallel top balancing is always recommended to keep the delta's at the top low. In addition I would not set the float voltage to more than 3.4 volts per cell. A little less is better but it varies from user to user depending on how quickly the cells are going to be used and how often.

I have an 8S pack I am using with a UPS that floats a little less than 3.4 volts per cell. In a few months if I start to notice more than normal capacity loss then I will shut off charging once the cells are charged. The app posted on Steve's website allow charging and discharging to be turned on/off independently of each other. A few of us have tested it and it does work.

As far as the rest what Zwy posted. :)
 
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